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LIBRARY 

UNIVERSITY  OF  CALIFORNIA 
DAVTS 


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THEORIES  OF 

ORGANIC   CHEMISTRY 


BY 

DR.  FERDINAND  HENRICH 

Professor  in  the  University  of  Erlangen 

Translated  and  Enlarged  from   the  Revised   Fourth 
German  Edition  of  1921 

BY 
TREAT    B.   JOHNSON 

Professor  of  Organic  Chemistry,  Yale  University 

AND 

DOROTHY  A.  HAHN,  PH.D. 

Professor  of  Organic  Chemistry,  Mount  Holyoke  College 


NEW  YORK 

JOHN  WILEY   &   SONS,  Inc. 

LONDON:  CHAPMAN  &  HALL,  LIMITED 
1922 


Copyright  1922, 


BY 


TREAT  B.  JOHNSON  AND  DOROTHY  A.  HAHN 


PRESS  OF 

BRAUNWORTH   4.  CO. 

BOOK   MANUFACTURERS 

BROOKLYN,    N.    Y. 


PREFACE  TO  THE  AMERICAN  EDITION 


THE  general  theory  of  Organic  Chemistry  is  at  present  in  a  state 
of  violent  transformation.  While  the  old  hypothesis  of  valency  with 
the  accepted  division  into  single  valencies  on  the  atoms  has  not  yet 
by  any  means  been  abandoned,  it  is  nevertheless  no  longer  looked  upon 
as  a  noli  me  tangere.  On  the  contrary,  an  ever-increasing  number  of 
observations  would  seem  to  indicate  that  the  division  of  the  original 
valency  force  present  on  the  atom  is  in  reality  an  unequal  one  and  may 
even  vary  under  different  conditions. 

This  view  already  discussed  by  A.  Glaus  in  1881  has  received  experi- 
mental support  from  facts  discovered  in  the  course  of  investigations  on 
free  radicals  (Triphenylmethyl,  etc.).  A  more  detailed  study  of  the 
physical  properties  of  organic  compounds  (such  as  the  relation  between 
color  and  constitution)  has,  moreover,  served  to  connect  conceptions 
regarding  the  inner  structure  of  the  molecule  with  the  "  Electron 
Theory  of  the  Constitution  of  the  Atom."  For  example,  observations 
as  to  what  parts  of  the  molecule  are  especially  influenced  by  light  may 
be  made  the  basis  for  discussion  in  regard  to  the  mechanism  of  molecular 
formation  in  all  its  phases  and  may  ultimately  lead  to  the  discovery  of  a 
theory  which,  based  upon  the  new  views  in  regard  to  the  constitution  of 
the  atom,  will  afford  a  satisfactory  explanation  for  all  physical  and 
chemical  phenomena. 

American  chemists  have  played  a  considerable  role  in  the  newer 
developments  and  have  contributed  many  fundamental  researches. 
Unfortunately  foreign  periodicals  in  Germany  have  become  such  a 
rarity  that  it  is  no  longer  possible  to  cover  the  original  literature  com- 
pletely. 

I  am,  therefore,  very  grateful  to  Professor  Treat  B.  Johnson  for 
filling  in  some  of  these  gaps.  It  is  to  be  hoped  that  the  book  will  be  of 
use  to  research  chemists  as  well  as  to  the  teaching  profession  and  thus 
lead  to  an  ever  greater  perfection  of  our  beautiful  science. 

A  Q>  \  F.  HENRICH. 

ERLANGEN,  August,  1920. 


111 


TRANSLATOR'S  PREFACE 


THE  foreign  text  by  Ferdinand  Henrich,  upon  which  the  present 
volume  is  based,  has  long  been  familiar  to  students  matriculating  for 
the  M.S.  and  Ph.D.  degrees  in  the  Graduate  School  of  Yale  University. 
It  has  served  not  only  as  a  means  of  stimulating  a  greater  interest  in  the 
literature  of  organic  chemistry,  but  it  has  also  proved  a  valuable  source 
and  guide  for  advanced  seminar  discussions.  Since,  however,  the 
many  problems  which  it  represents  are  interwoven  with  the  develop- 
ments of  all  modern  chemical  theory  it  seemed  desirable  to  undertake 
its  translation  into  English  and  in  this  way  bring  it  to  the  attention  of 
a  much  larger  number  of  our  young  readers.  The  task  of  doing  this 
successfully  has  been  somewhat  complicated  by  the  fact  that  it  has 
often  been  necessary  to  rewrite  and  even  to  rearrange  completely 
portions  of  the  material  in  the  original  text  to  render  it  acceptable  to 
the  American  reader.  Thus,  for  example,  the  development  of  a  given 
conception  in  the  original  German  frequently  involves  a  suspension  of 
the  thought  through  many  pages  while  involved  phases  of  the  subject 
are  considered  before  the  actual  objective  becomes  apparent.  This 
procedure  would,  it  was  felt,  find  little  favor  in  translation  since  the 
average  American  prefers  to  be  enlightened  as  to  the  particular  object 
in  view  before  venturing  upon  a  long  course  of  speculative  reasoning. 

The  construction  of  the  English  text,  therefore,  has  been  a  very 
laborious  task,  and  the  writer  wishes  to  emphasize  here  the  fact  that  the 
present  translation  would  not  have  been  undertaken  without  the 
assistance  and  cooperation  of  an  efficient  co-worker  in  organic  chemistry. 
It  is,  therefore,  my  very  pleasant  duty  to  record  here  my  indebtedness 
to  Professor  Dorothy  A.  Hahn  of  Mount  Holyoke  College  for  her  able 
collaboration  with  me  in  the  translation  and  construction  of  the  text. 

One  of  the  conspicuous  weaknesses  of  our  present-day  graduate 
students  majoring  in  chemistry  is  the  lack  of  what  may  be  called  the 
historic  sense.  There  are  so  many  interesting  and  practical  problems 
of  research  being  placed  before  them  for  solution  that  their  attention 


vi  TRANSLATOR'S  PREFACE 

is  naturally  diverted  from  the  older  works  and  focused  chiefly  upon 
present-day  achievements.  In  fact,  in  the  present  age  the  opportuni- 
ties for  great  and  rapid  success  are  so  alluring  and  the  appeal  of  the 
present  and  future  is  so  strong  that  most  of  us  forget  what  we  owe  to 
the  past.  And  because  we  take  so  little  time  to  think  of  what  our 
past  leaders  in  chemical  science  have  given  us,  we,  too,  often  in  our 
rush  and  haste  have  an  exaggerated  notion  of  our  own  immediate  work 
as  compared  with  that  of  our  forefathers.  One  of  the  chief  purposes 
of  this  translation  is  to  stimulate  in  our  graduate  students  during  their 
period  of  research  training  a  desire  to  follow  more  closely  the  historical 
development  of  organic  chemical  theory  and  gain  thereby  an  apprecia- 
tion of  the  work  of  the  old  masters. 

In  presenting  the  work  to  an  American  public  it  has  seemed  desirable 
to  stress  wherever  possible  the  contributions  of  American  investigators. 
With  this  end  in  view  the  chapter  on  " Molecular  Rearrangements" 
has  been  completely  rewritten  and  a  large  amount  of  additional  material 
has  been  introduced  (covering  the  work  of  Stieglitz  and  co-workers  at 
Chicago  University  and  of  Wheeler  and  Johnson  at  Yale). 

Several  entirely  new  chapters  have  been  added  to  those  already 
included  in  the  German  text.  The  chapter  on  "The  Theoretical 
Speculations  of  John  Ulric  Nef "  represents  Nef's  own  summary  of  his 
particular  theoretical  speculations  as  published  in  the  Journal  of  the 
American  Chemical  Society,  and  has  been  inserted  without  change  or 
abbreviation  through  the  courtesy  of  that  journal.  The  chapter  on 
"The  Electron  Conception  of  Valency"  has  been  written  entirely  by 
my  colleague,  Professor  Arthur  J.  Hill,  for  whose  assistance  in  the 
correction  of  the  final  text  I  wish  to  express  my  sincere  thanks.  This 
particular  chapter  is  also,  for  the  most  part,  new  material  and  embodies 
the  work  of  Lewis,  Fry,  Falk,  Nelson  and  others  of  the  American  school 
of  investigators.  This  chapter  is  necessarily  incomplete  since  limitation 
of  time  and  space  have  prevented  the  inclusion  of  the  work  cf  Langmuir 
and  other  prominent  American  and  English  workers  in  the  newer  fields 
of  atomic  chemistry. 

With  a  subject  like  organic  chemistry,  the  literature  of  which  is  so 
broad  and  comprehensive,  it  is  not  possible  to  give  an  exhaustive  review 
or  summary  of  all  important  chemical  literature.  The  aim  has  been, 
therefore,  to  incorporate  into  the  text  a  record  of  the  major  lines  of 
development  with  explanations,  and  to  refer  the  reader  to  the  original 
literature  for  the  special  details  and  data  that  may  be  required.  It 
is  hoped  that  the  book  will  prove  a  valuable  adjunct  of  every  library 
of  chemistry.  Criticism  of  the  text  and  any  additional  information 
pertaining  to  the  subject  matter  treated  will  be  welcomed. 


TRANSLATOR'S  PREFACE  vii 

In  conclusion  I  wish  to  thank  all  my  colleagues  of  the  Department 
of  Chemistry  for  suggestions  and  criticisms  which  have  been  offered, 
and  also  to  express  my  appreciation  of  the  assistance  of  Mr.  Lawrence 
W.  Bass  of  the  Yale  Graduate  School,  who  has  been  of  great  help  to 
me  in  checking  up  and  confirming  literature  references. 

TREAT  B.  JOHNSON. 

YALE  UNIVERSITY, 
New  Haven,  Conn. 
May  1,  1922. 


TABLE  OF  ABBREVIATIONS  EMPLOYED  IN  THE 

REFERENCES 


Abbreviated  Title 
Abh.  d.  Naturf .  Ges.  zu  Halle 

Am.  Chem.  Jour. 
Annalen  der  Chemie 
Annales  Chimie  et  Phys. 
Annales  Chim. 
Annalen  Physik 
Ber. 

Berzelius  Jahresb. 
Bioch.  Zeitschr. 
Bull.  soc.  chim. 
Chem.  Centralbl. 
Chem.  Zeitschr. 
Chem.  Zeitung 
Chemie  der  Jetztzeit 
Chem.  Abstr. 
Compt.  rend. 

Drude's  Annalen 
Gazz.  chim.  ital. 
Helv.  chim.  Acta 
Jahrb. 

Jahrb.  der  Radioaktivitat 
Jour.  Am.  Chem.  Soc. 
Jour.  Chem.  Soc. 
Jour,  de  chemie  phys. 


Jour.  Ind.  Eng.  Chem. 

Jour,  prakt.  Chemie 

Jour,  physikal.  Chemie 

Jour.  Russ.  Physikal.  Chem.  Ges. 


Journal 

Abhandlungen  der  Naturforschenden  Gesell- 

schaft  zu  Halle 
American  Chemical  Journal 
Justus  Liebig's  Annalen  der  Chemie 
Annales  de  Chimie  et  de  Physique 
Annales  de  Chimie 
Annalen  der  Physik 
Berichte  der  Deutschen  Chemischen  Gesell- 

schaft 

Berzelius  Jahresberichte 
Biochemische  Zeitschrift 
Bulletin  de  la  Societe  chimique  de  France 
Chemisches  Centralblatt 
Chemische  Zeitschrift 
Chemische  Zeitung 

Chemical  Abstracts 

Comptes  rendus  hebdomadaires  des  Seances 
de  FAcademie  des  Sciences 

Gazzetta  chimica  Italiana 

Helvetica  chimica  acta 

Jahresberichte    iiber    die    Forts  chritte    der 

Chemie 

Jahresberichte  der  Radioaktivitat 
Journal  of  the  American  Chemical  Society 
Journal  of  the  Chemical  Society  (London) 
Journal  de  chimie,  physique,  electrochimie, 

thermochimie,      radiochimie,      mecanique 

chimique,  stoechiometrie 
Journal     of     Industrial     and     Engineering 

Chemistry 

Journal  fiir  praktische  Chemie 
Journal  physikalische  Chemie 
Journal    of    the    Physical    and    Chemical 

Society  of  Russia 

ix 


X    TABLE  OF  ABBREVIATIONS  EMPLOYED  IN  THE  REFERENCES 


Abbreviated  Title 
Monatsh.  Chemie 

Ostwald's  Klassiker 
Philos.  Mag. 

Philos.  Trans. 

Philippine  Jour.  Sci. 
Physikal   Zeitschr. 
Poggendorf's  Annalen 

Proc.  Chem.  Soc. 

Rec.  trav.  chim.  des  Pays-Bas 

Rev.  gener.  chim. 
Rep.  Brit.  Assoc. 

Schweigger's  Jour. 

Sitzungsber.  d.  Berl.  Akad.  d.  Wiss. 

Trans.  Faraday  Soc 
Wiener  Monatsh. 

Wiedem.  Annalen 
Zeitschr.  angew.  Chemie 
Zeitschr.  anorg.  Chemie 

Zeitschr.  Elektrochemie 
Zeitschr.  physikal.  Chemie 

Zeitschr.  f.  Chemie 

Zeitschr.  f.  Farben  u.  Textilchemie 

Zeitschr.  f.  Physik 

Zeitschr.  f.  Physiol.  Chemie 

Zeitschr.  wiss.  Phot. 

Zeitschr.  f.  Chemie  u.  Ind.  d.  Kolloide 


Journal 

Monatshefte  fiir  Chemie  und  verwandte 
Theile  anderer  Wissenschaften 

Philosophical  Magazine  (The  London,  Edin- 
burgh and  Dublin) 

Philosophical  Transactions  of  the  Royal 
Society  of  London 

Philippine  Journal  of  Science 

Physikalische  Zeitschrift 

Poggendorf's  Annalen  der  Physik  und 
Chemie 

Proceedings  of  the  Chemical  Society 
(London) 

Recueil  des  travaux  chimiques  des  Pays- 
Bas  et  de  la  Belgique 

Revue  generate  de  chimie  pure  et  appliqu^e 

Report  of  the  British  Association  for  the 
Advancement  of  Science 

Schweigger's  Journal  fur  Chemie  und  Physik 

Sitzungsberichte  der  kaiserlichen  Akadamie 
der  Wissenschaften  (Berlin) 

Transactions  of  the  Faraday  Society 

Wiener  (Monatshefte  fiir  Chemie  und  ver- 
wandte Theile  anderer  Wissenschaften) 

Wiedemann  Annalen  der  Physik  und  Chemie 

Zeitschrift  fur  angewandte  Chemie 

Zeitschrift  fiir  anorganische  und  allgemeine 
Chemie 

Zeitschrift  fiir  Elektrochemie 

Zeitschrift  fur  physikalische  Chemie,  Stochi- 
ometrie  und  Verwandtschaftslehre 

Zeitschrift  fiir  Chemie 

Zeitschrift  fiir  Farben  und  Textilchemie 

Zeitschrift  fur  Physik 

Hoppe-Seyler's  Zeitschrift  fiir  physiologische 
Chemie 

Zeitschrift  fiir  wissenschaftliche  Photo- 
graphic, Photophysik  und  Photochemie. 

Zeitschrift  fiir  Chemie  und  Industrie  der 
Kolloide  (Kolloid-Zeitschrift) 


CONTENTS 


CHAPTER  I 

THE  HISTORICAL  DEVELOPMENT  OF  THE  THEORY  OF  ORGANIC  CHEMISTRY 
UP  TO  THE  PERIOD  OF  THE  THEORY  OF  TYPES 

PAGE 

Lavoisier's  speculations.  The  electrochemical  theory  of  Berzelius.  Phenom- 
enon of  substitution  as  interpreted  by  Dumas  and  Laurent.  The  unitary 
theory  of  Dumas  versus  the  dualistic  electrochemical  theory  of  Berzelius. 
The  earlier  Type  Theory  and  the  conception  of  the  chemical  radical 1 


CHAPTER  II 
THE  EARLY  HISTORY  OF  STRUCTURAL  CHEMISTRY 

The  new  theory  of  types  and  the  early  speculations  of  Kekule.  Development 
of  the  ideas  of  saturation,  atomicity  and  basicity  (Frankland  and  Kolbe). 
Kekule's  conception  of  valency,  atomic  and  molecular  compounds.  The 
use  of  graphic  formulas,  the  idea  of  maximal  saturation  capacity  and  the 
assumption  of  changing  valencies.  Theories  in  regard  to  the  condition  of 
unsaturation.  The  development  of  a  graphic  formula  for  benzene. 
Physical  isomerism ,,,,,, , 10 

CHAPTER  III 
LATER  DEVELOPMENTS  IN  STRUCTURAL  CHEMISTRY 

Van't  Hoff's  hypothesis.  The  application  of  the  idea  of  tetrahedral  carbon 
to  structural  chemistry  by  Baeyer.  The  theory  of  tension  in  organic 
compounds.  Objections  to  Kekule's  formula  for  benzene.  Bamberger's 
speculations  in  regard  to  ring  structure 18 

CHAPTER  IV 
JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES 

The  conception  of  double  bonds  and  systems  of  double  bonds.  Examples  of 
1^4  and  1-2  addition  to  so-called  conjugate  and  crossed  systems.  The 
speculations  of  Hinrichsen,  Borsche,  Staudinger  and  Meisenheimer. 
Application  of  Thiele's  conceptions  to  benzene,  naphthalene,  anthracene, 
phenanthrene,  etc.  The  investigations  of  Willstatter  and  others 


xii  CONTENTS 

CHAPTER  V 
THE  THEORY  OF  ALFRED  WERNER 

PAGE 

New  conceptions  of  valency.  Speculations  in  regard  to  the  distribution  of  the 
total  energy  of  the  carbon  atom.  Werner's  modification  of  Thiele's  concep- 
tion of  partial  valencies.  The  ideas  of  principle  and  residual  valencies, 
coordination  number,  inner  and  outer  spheres  of  influence.  Differentia- 
tion of  reactions  involving  simple  additions  (molecular  compounds)  and 
those  where  such  additions  are  accompanied  by  infiltrations  of  the  atoms 
of  one  molecule  into  the  other  molecule.  The  application  of  Werner's 
conceptions  to  the  problems  of  structural  chemistry 75 

CHAPTER  VI 
RECENT  THEORIES  IN  REGARD  TO  VALENCY 

Kaufmann's  theory  regarding  atomic  relationships,  valence  fields  and  chemical 
activity.  Hypothesis  of  O.  Hinsberg.  The  speculations  of  J.  Stark. 
Development  of  the  electron  theory  of  valency.  Electro  valency.  Electro- 
magnetic theory  of  Beckenkamp  and  kinetic  theory  of  von  Weniberg 90 

CHAPTER  VII 
THE  ELECTRON  CONCEPTION  OF  VALENCY 

The  more  recent  speculations  of  Nelson,  Falk  and  Fry.  Development  of  the 
corpuscle  theory  of  J.  J.  Thomson.  Specific  applications  of  the  electronic 
conception  of  valency.  The  onium  theory.  Classification  of  organic 
compounds  from  the  electronic  standpoint.  Positive  and  negative 
valency.  The  speculations  of  Lewis,  Stieglitz,  Jones,  Noyes,  Bray  and 
Branch.  Valence  number.  Polar  and  non-polar  compounds.  The 
Beckmann  Rearrangement.  The  recent  speculations  of  Ramsey  and 
Bohr.  The  magneton  theory  of  Parsons 107 

CHAPTER  VIII 
THE  SO-CALLED  NEGATIVE  NATURE  OF  ATOMIC  GROUPS  OR  RADICALS 

Investigations  of  V.  von  Meyer,  Haller,  Henrich,  Vorlander  and  Hinsberg. 
The  influence  of  positive  and  negative  atoms  and  radicals  upon  the  course 
of  organic  reactions 143 

CHAPTER   IX 

RECENT  THEORIES  IN  REGARD  TO  THE  MECHANISM  OF  CHEMICAL  REACTIONS 

Kekul6's  conception  of  addition  reactions  followed  by  rearrangement  and 
possible  decompositions.  Modern  methods  for  detecting  addition 
products.  Theories  of  P.  Pfeiffer  and  Reddelien  in  regard  to  the  forma- 
tion and  subsequent  transformations  of  addition  products 168 


CONTENTS  xiii 

CHAPTER  X 
THE  QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE 

PAGE 

The  speculations  of  Debye  and  Scherrer.  Willstatter's  discovery  of  cyclo-octate- 
traene,  and  the  grounds  for  his  objections  to  Thiele's  formula  for  benzene. 
The  work  of  Reddelien  and  his  support  of  the  Thiele  formula.  Conceptions 
in  regard  to  the  mechanism  of  substitution  in  the  benzene  ring  by  Wieland, 
Fischer  and  Werner.  Review  of  the  speculations  of  Hiibner,  Noelting, 
Brown,  Gibson,  Armstrong,  and  Vorlander  in  regard  to  orientation  phe- 
nomena together  with  the  more  recent  investigations  of  Holleman,  Chat- 
taway  and  Orton,  Flurscheim  and  Obermiller.  A  review  of  the  work  of 
Holleman  on  mechanism  of  substitution  in  benzene  compounds,  and  the 
recent  work  of  Pauly  and  Kauffmann.  Stark's  valence-electron  hypothesis 
and  its  application  in  the  interpretation  of  the  mechanism  of  benzene 
reactions.  The  development  of  the  Stark-Pauly  electrochemical  formula  for 
benzene  on  the  basis  of  recent  electro-atomic  theory.  Vorlander's  interpre- 
tation of  chemical  reactions,  which  is  based  on  new  conceptions  of  the 
positive  and  negative  character  of  atoms  and  radicals.  The  investiga- 
tions of  Dimroth,  Kurt  Meyer,  and  Karrer ,  175 


CHAPTER  XI 
TAUTOMERISM  AND  DESMOTROPISM 

The  development  of  Baeyer's  conception  of  pseudoforms  in  connection  with  the 
phenomenon  of  lactam-lactim  rearrangements.  Isolation  of  desmotropes 
by  Claisen  and  Wislicenus,  and  a  discussion  of  the  properties  and  trans- 
formations common  to  such  substances.  Equilibrium  isomerism.  Knorr's 
system  of  nomenclature.  Speculations  of  Dimroth  and  Michael.  The 
structure  of  ethyl  acetoacetate.  The  mechanism  of  rearrangement  of 
tautomeric  substances 237 

CHAPTER  XII 
IONIZATION  ISOMERISM 

Pseudo-acids  and  pseudo-bases.  Desmotropism  in  fatty  aromatic  nitro- 
compounds  and  its  detection  by  means  of  conductivity  measurements. 
Changes  in  constitution  during  salt  formation  and  the  discussion  of  criteria 
by  means  of  which  such  changes  may  be  recognized ,  277 

CHAPTER  XIII 
THE  APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  TO  ORGANIC  CHEMISTRY 

Historical  introduction.  The  relation  of  physics  to  organic  chemistry.  Addi- 
tive arid  constitutive  properties.  Molecular  volume,  spectrochemistry, 
specific  and  molecular  refractions.  Lorentz-Lorenz  formulas.  Dispersion. 
Formulas  of  Aiiwers  and  Eisenlohr.  Landolt's  tables  of  atomic  refractions. 
Optical  exaltation.  Later  researches  of  Bruhl,  Atiwers  and  Eisenlohr.  Rela- 


xiv  CONTENTS 

PAGE 

tion  between  constitution  and  optical  rotation,  heat  of  formation  and 
heat  of  combustion.  Optical  refraction  and  dispersion.  Internal  molec- 
ular energy.  The  thermochemistry  of  isomers  possessing  conjugated  and 
non-conjugated  double  bonds.  Investigations  of  Aiiwers  and  Roth  on 
spectro-chemical  effects.  The  investigations  of  Walden,  Haller,  Hilditch, 
Rupe,  and  Ackermann  on  optical  rotation 288 


CHAPTER  XIV 

THE  THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEP  l 

The  variability  of  valency.  Bivalent  carbon.  Development  of  the  theory  of 
addition- reactions.  Relationship  between  dissociation  of  organic  mole- 
cules and  ionization 334 

CHAPTER  XV 

CONCEPTIONS  IN  REGARD  TO   THE   INDEPENDENT  EXISTENCE   OF  FREE 
ORGANIC  RADICALS 

Nef's  Theory  in  regard  to  the  independent  existence  of  radicals  substantiated 
following  the  discovery  and  identification  of  free  triarylmethyls  by  Gom- 
berg,  Schlenk  and  Weickel.  Speculations  in  regard  to  the  nature  of  metal- 
ketyls  by  Schlenk  and  Weickel.  The  addition  of  alkali  metals  to  double 
bonds.  Wieland's  investigations  of  free  radicals  containing  nitrogen. 
The  significance  of  the  conception  of  free  organic  radicals  in  the  future 
development  of  organic  chemistry 361 

CHAPTER  XVI 
THE  RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION 

Absorption  phenomena  from  the  point  of  view  of  pure  physics.  The  visible 
and  invisible  spectra.  Absorption  in  ultraviolet.  The  specific  influences 
of  auxo-,  diminu,  batho-,  and  hypsochrome  groups.  Continuous  and 
selective  absorption.  Laws  governing  absorption.  Electron  oscillations 
and  selective  absorption.  The  measurement  of  absorption  in  solutions. 
Researches  on  absorption  spectra  of  organic  compounds  by  Hartley,  Baly, 
Stark,  Henri,  Weigert  and  Schaefer.  Beer's  law  and  its  applications. 
Piccard's  so-called  colorimetric  law  of  dilution.  Graphic  methods  for 
representing  absorption  spectra.  Comparison  of  the  merits  of  the  methods 
of  Bartley-Baly,  V.  Henri  and  F.  Weigert.  The  quantitative  investigations 
of  V.  Henri  and  J.  Bielecki.  The  work  of  Hantzsch  on  absorption  and 
its  relation  to  organic  structures.  O.  N.  Witt's  chromophore  theory  and  its 
subsequent  extension  and  amplification  by  Kauffmann,  Lifschitz  and  other 
investigators.  Discussion  of  chromophore,  chromogen  and  auxochrome 
groups.  Kauffmann's  auxochrome  theory.  The  decentralization  of 
chemical  functions.  Criticism  of  Kauffmann's  views  by  Staudinger. 
Triphenylmethane  dyes.  Speculations  in  regard  to  the  chemical  consti- 

1  Reprinted  from  the  Jour.  Am.  Chem.  Soc.,  26,  1549  (1904)  through  the  courtesy  of  the  editor. 


CONTENTS  xv 

PAOB 

tution  of  these  dyes.  Early  theories  of  E.  and  O.  Fischer.  Quinoidal 
structure.  The  application  of  the  conception  of  the  triphenyl  free-radical 
to  the  solution  of  this  problem.  Interpretation  of  the  phenomena  based 
upon  halochromism.  Chromoisomerism,  pantochromism,  polychromism, 
chromotropism.  homochromotropism  and  valence  isomerism.  The  latter 
interpretations  of  Kauffmann.  Color  as  conditioned  by  differences  in 
molecular  weight.  The  researches  of  Willstatter  and  co-workers  on  quinoids 
and  quinoid-imides.  Recent  theories  in  regard  to  relation  between  color 
and  constitution 382 


CHAPTER  XVII 
THE  THEORY  OF  INDICATORS 

The  earlier  speculations  of  Ostwald,  and  the  more  recent  views  of  Stieglitz 
and  Hantzsch  in  regard  to  intramolecular  rearrangements  and  the  chromo- 
isomerism  of  indicators 479 

CHAPTER  XVIII 

THE  RELATIONSHIP  BETWEEN  FLUORESCENCE  AND   CHEMICAL  CONSTITUTION 

A  consideration  of  the  laws  governing  the  phenomena  of  fluorescence  from  the 
point  of  view  of  pure  physics.  Fluorescence  and  absorption.  The  appli- 
cation of  the  phenomena  to  a  determination  of  the  chemical  constitution 
of  the  molecule.  The  speculations  of  E.  Wiedermann,  C.  G.  Schmidt,  R. 
Meyer,  H.  Kauffmann,  J.  Stark  and  H.  Ley.  Discussion  of  luminophore, 
fluorogens,  bathoflors,  hypsoflors,  auxoflors  and  diminuflors 485 

CHAPTER   XIX 
MOLECULAR  REARRANGEMENTS 

Classification  of  molecular  rearrangements.  Types  of  rearrangements. 
Cyanic  and  thiocyanic  acid  rearrangements.  Rearrangements  involving 
migration  of  groups  or  radicals  from  carbon  to  carbon,  from  oxygen  to 
carbon,  from  nitrogen  to  carbon,  from  nitrogen  to  nitrogen,  from  nitro- 
gen to  oxygen,  from  oxygen  to  nitrogen,  from  oxygen  to  oxygen,  from 
oxygen  to  sulphur,  from  iodine  to  carbon,  and  from  carbon  to  nitrogen. 
The  Beckmann  rearrangement  and  closely  related  transformations.  The 
Hofmann  rearrangement.  The  Curtius  rearrangement.  The  conception 
of  free  radicals  and  their  possible  formation  in  intramolecular  transfor- 
mations. The  pinacone-pinacoline  rearrangement  and  the  views  of 
Erlenmeyer,  Montagne,  Tiffeneau,  Merrwein  and  others  regarding  the 
mechanism  of  this  change.  Imidoether  rearrangements.  The  speculations 
of  Lapworth  and  Tiffeneau  regarding  the  mechanism  of  intramolecular 
change.  Stieglitz 's  interpretation  of  the  Beckmann  and  Hoffmann  rear- 
rangements and  the  conception  of  free  radicals.  The  recent  work  of 
Schroeter,  Montagne,  Henrich  and  others.  The  benzilic  acid  rear- 
rangement    4^4 


xvi  CONTENTS 

CHAPTER  XX 
THE  BASIC  PROPERTIES  OF  OXYGEN 

PAGE 

A  consideration  of  the  quadrivalence  of  oxygen  and  the  early  speculations  of 
Collie,  Tickle  and  F.  Kehrmann.  Werner's  discovery  of  xanthonium  salts 
and  Decker's  discovery  of  pyryllium  salts.  Speculations  of  Baeyer, 
Villiger,  Gomberg,  Kendall  and  others,  in  regard  to  the  constitution  of 
so-called  oxonium  salts 552 

CHAPTER  XXI 

THE  THEORETICAL  SPECULATIONS  OF  ARTHUR  MICHAEL 

The  importance  of  addition  reactions  in  organic  chemistry.  The  application 
of  the  principle  of  entropy  to  the  study  of  chemical  reactions.  Neutraliza- 
tion of  affinity  inside  the  molecule.  The  chemical  plasticity  of  the  carbon 
atom  and  rules  governing  the  positive  and  negative  influence  of  organic 
radicals.  The  law  of  distribution  of  affinity.  Speculations  of  Michael 
and  Derick  in  regard  to  the  influence  of  position  upon  chemical  properties. 
Intramolecular  rearrangement  of  alkyl  halides 56 

CHAPTER  XXII 
RECENT  ELECTROCHEMICAL  THEORIES 

The  work  of  Lob,  Elbs  and  Haussermann.  The  reduction  of  nitrobenzene  as 
interpretated  by  Haber.  The  ionic  theory  and  speculations  of  Nernst. 
Electro-chemical  speculations  of  Faraday  and  the  work  of  Thomson  and 
Helmholtz.  Abegg  and  Bodlander's  electro-valence  theory.  Electro- 
affinity 585 

INDEX.  .  .  597 


THEORIES  OF  ORGANIC  CHEMISTRY 


CHAPTER  I 

THE  HISTORICAL  DEVELOPMENT  OF  THE  THEORY  OF 
ORGANIC  CHEMISTRY  UP  TO  THE  PERIOD  OF  THE 
THEORY  OF  TYPES1 

THE  fundamental  conceptions  out  of  which  the  present  theories 
of  organic  chemistry  have  developed  originated  in  the  discoveries 
of  the  French  chemist,  Lavoisier,  in  the  latter  part  of  the  eighteenth 
century.  As  soon  as  this  ingenious  investigator  recognized  the 
importance  of  oxygen  in  nature  he  undertook,  in  co-operation  with  his 
students,  a  far-reaching  investigation  of  all  substances  which  were 
known  to  contain  this  element,  and,  as  a  result,  oxygen  became  the 
point  about  which  the  chemistry  of  the  age  centered.  What  was 
combined  with  oxygen  aroused  little  interest  and  was  referred  to  in 
general  as  the  base  or  radical.  Thus  even  at  that  early  period  com- 
pounds were  believed  to  consist  of  two  parts,  oxygen  and  the  residue 
or  radical,  so  that  the  very  beginnings  of  chemical  theory  may  be  said 
to  have  been  dualistic.  The  simplest  compounds  of  oxygen  were 
divided  into  two  groups  possessing  opposite  properties, — viz.,  acids 
and  bases.  Since  salts  are  formed  as  a  result  of  mixing  acids  and 
bases,  this  third  group  of  compounds  was  believed  to  represent  a 
chemical  combination  of  the  first  two.  According  to  Lavoisier,  the 
real  difference  between  inorganic  and  organic  compounds  was  to  be 
found  in  the  fact  that  the  former  contained  a  simple,  while  the  latter 
contained  a  complex  radical.  The  immediate  development  of  this 
theory  was  linked  with  a  study  of  the  simpler  types  of  compounds. 

The  Atomic  Theory  was  advanced  to  explain  the  laws  of  Simple 
and  Multiple  Proportion,  and  interpreted  chemical  action  as  resulting 

1  Compare  Edv.  Hjelt,  "  Geschichte  der  organischen  Chemie  von  altester  Zeit 
bis  zur  Gegenwart."  Braunschweig,  Friedr.  Vieweg  &  Sohn,  1916. 


2  THEORIES  OF  ORGANIC  CHEMISTRY 

from  the  union  of  a  number  of  atoms.  Chemical  compounds  were 
supposed  to  consist  of  aggregates  of  atoms  which  were  represented  in 
terms  of  a  system  of  concrete  symbols.  In  harmony  with  Lavoisier's 
conceptions  the  Atomic  Theory  started  as  pre-eminently  dualistic  in 
character.  Of  the  many  hypotheses  advanced  to  explain  the  cause 
of  chemical  combination,  the  one  which  received  the  most  immediate 
as  well  as  general  acceptance  was  the  Electrochemical  Theory  of 
Johann  Jakob  Berzelius.  Its  vogue  at  that  time  was  due  to  the  fact 
that  it  permitted  the  full  development  of  the  prevailing  conceptions 
as  to  the  cause  and  effect  of  chemical  action,  in  such  a  way  as  to  allow 
a  convenient  and  comprehensive  classification  of  both  elements  and 
compounds. 

The  close  relation  which  exists  between  electrical  and  chemical 
phenomena  was  observed  early  in  the  history  of  chemistry,  and  atten- 
tion had  been  drawn  in  particular  to  the  fact  that  chemical  decomposi- 
tion and  combination  often  accompany  electric  and  galvanic  processes. 
Volta's  discovery  that  the  mere  contact  of  unlike  substances  was 
sufficient  to  generate  electricity  had  aroused  especial  interest  and  this 
was  increased  by  Sir  Humphrey  Davy's  further  discovery  that  many 
elements  and  compounds  (among  others,  acids  and  alkalies)  were 
also  oppositely  electrified  by  contact,  and  that  the  electrical  tension 
thus  generated  was  stronger  in  those  cases  where  the  chemical  differ- 
ence between  the  two  substances  was  most  marked.  It  was  observed, 
further,  that  in  cases  where  contact  resulted  in  chemical  combination 
all  signs  of  the  presence  of  electricity  disappeared.  Because  the  energy 
which  is  set  free  by  the  neutralization  of  elective  charges  may  be 
sufficiently  intense  to  give  rise  to  the  phenomena  of  heat  and  light,  the 
question  naturally  arose  in  these  early  days  as  to  whether  the  forces 
at  work  in  electrical  neutralization  might  not  be  analogous  to  those 
associated  with  chemical  neutralization,  since  the  latter  phenomenon 
is  also  frequently  productive  of  the  same  results.  In  both  cases  it  was 
assumed  that  opposing  forces  counteract  each  other,  and  it  therefore 
seemed  reasonable  to  interpret  chemical  combination, — as  for  example, 
the  neutralization  of  acids  and  bases, — as  dualistic  in  the  same  sense 
as  electrical  processes  were  regarded  as  undoubtedly  dualistic.  Further- 
more, since  it  was  the  custom  at  that  tune  to  assume  that  similar 
phenomena  were  due  to  similar  causes,  the  question  was  also  raised  as 
to  whether  the  phenomenon  of  flame  (light)  was  not  directly  due  to 
electrical  discharge  in  the  case  of  chemical  combination  just  as  in  the 
case  of  the  union  of  positive  and  negative  electricity.  Berzelius  weighed 
all  these  questions  very  shrewdly  and  cleverly,  and  came  to  the  con- 
clusion that  in  every  chemical  combination  a  neutralization  of  positive 


THE  HISTORICAL  DEVELOPMENT  3 

and  negative  electricity  takes  place,  and  that  during  this  process 
the  phenomena  of  flame  (light)  may  be  produced  in  exactly  the  same 
way  as  in  the  discharge  of  Leyden  jars,  galvanic  piles,  etc.,  the  only 
difference  being  that  in  the  latter  case  the  phenomenon  is  not  accom- 
panied by  chemical  union.1 

Davy  had  already  transferred  the  seat  of  both  positive  and  nega- 
tive electricity  (i.e.,  chemical  affinity)  to  the  atoms  of  the  elements, 
but  he  assumed  that  the  atoms  became  oppositely  electrified  only 
upon  their  approach  to  each  other.  Berzelius,  on  the  other  hand, 
supposed  that  the  electric  charge  was  inherent  in  the  individual  atom, 
and  to  a  certain  extent  concentrated  at  definite  points  or  poles.  He 
also  supposed  that,  just  as,  in  general,  bodies  which  are  oppositely 
charged  with  electricity,  neutralize  their  charges  on  contact  with  each 
other,  so  also  oppositely  charged  atoms  can  discharge  their  electricity 
and  in  so  doing  unite  to  form  chemical  compounds.  Thus,  for  example, 
oxygen  combines  with  sodium  because  the  atoms  of  the  former  element 
possess  free  negative  electricity  which  attracts  the  free  positive 
electricity  of  the  sodium  atoms.  Sodium  oxide,  which  is  formed 
in  this  way,  may  still  possess  free  positive  electricity,  and  the  reason 
for  this  is  to  be  found  in  the  fact  that  the  quantity  of  free  negative 
electricity  present  on  the  oxygen  atom  is  not  sufficient  to  completely 
neutralize  all  of  the  free  positive  electricity  present  on  the  sodium 
atom.  Thus  the  assumption  was  made  that  the  atoms  of  the  different 
elements  possess  different  amounts  of  free  electricity  and,  in  con- 
formity with  this  idea,  Berzelius  arranged  a  series  of  elements  in  which 
oxygen  stood  first  as  the  most  electro-negative  element,  followed  by 
sulphur,  nitrogen,  and  other  transitional  elements  over  to  the  most 
electro-positive  elements  sodium  and  potassium. 

In  this  series  of  Berzelius',  sulphur  came  immediately  after  oxygen, 
and  yet  it  was  observed  that  these  two  strongly  negative  elements  showed 
a  pronounced  chemical  attraction  for  each  other.  In  order  to  explain 
this  apparent  anomaly,  Berzelius  assumed  that  each  atom  of  an 
element  possesses  two  opposite  electrical  poles,  provided  with  different 
quantities  of  positive  and  negative  electricity,  so  that  in  any  given 
atom  there  is  always  present  a  surplus  of  one  or  the  other  kind  of  elec- 
tricity, and  the  atom  appears  either  positive  or  negative  according 
to  its  predominating  polarity,  or,  in  other  words,  it  is  specifically 
unipolar.  It  was  by  virtue  of  their  opposite  polarities  that  the  atoms 
of  elements  combined.  The  simple  combinations  of  positive  and 

1  J.  J.  Berzelius  "  Versuch  liber  die  Theorie  der  chemischen  Proportionen  imd 
iiber  die  chemischen  Wirkungen  der  Electrizitat, "  etc.  Translated  by  K.  A.  Blode, 
pp.  75  and  79,  Dresden,  1820. 


4  THEORIES  OF  ORGANIC  CHEMISTRY 

negative  elements  furnished  compounds  of  the  first  order.  Since 
the  electrical  charges  in  these  compounds  were  not  necessarily  neutral- 
ized, there  might  remain  a  surplus  of  positive  or  negative  charges  which 
would  enable  them  to  enter  into  further  combination,  forming  com- 
pounds of  the  second  order.  Thus,  for  example,  sodium  oxide,  formed 
by  the  union  of  sodium  and  oxygen,  is  a  compound  of  the  first  order 
possessing  free  positive  electricity,  while  SOs,  formed  from  sulphur  and 
oxygen,  is  a  compound  belonging  to  the  same  order  but  possessing 
free  negative  electricity.  It  is  obvious  that  these  two  compounds 
may  in  turn  combine,  by  virtue  of  their  opposite  polarities,  to  form 
sodium  sulphate  (SOsNaO).1  It  follows  according  to  Berzelius2 
that  sodium  sulphate  is  not  formed  from  sodium  and  sulphuric  acid 
but  from  sodium  oxide  and  sulphur  trioxide,  which  in  turn  may  be 
decomposed  into  electro-positive  and  electro-negative  components. 
The  positive  and  negative  electricity  in  sodium  sulphate  might  still 
remain  unneutralized,  and  it  could  thus  combine  with  other  similar 
substances  of  opposite  polarity  to  form  compounds  of  the  third  order 
(double  salts). 

As  soon  as  Berzelius  discovered  that  the  fundamental  laws  which 
govern  the  combination  of  elements  to  form  inorganic  compounds 
apply  also  to  the  formation  of  organic  compounds,  he  gave  his  atten- 
tion to  the  task  of  interpreting  the  chemistry  of  animal  and  plant 
products  in  terms  of  the  dualistic  electrochemical  theory.  He  reasoned 
that  the  only  difference  between  inorganic  and  organic  substances 
was  to  be  found  in  the  fact  that  the  former  consist  of  simple  radicals 
while  the  latter  consist  of  complex  radicals  in  union  with  oxygen.3 
The  same  laws  of  cause  and  effect  governing  chemical  combination 
hold  for  both  classes  of  bodies.  This  conception  was  called  upon  to 
give  the  first  successful  impulse  to  the  interpretation  of  the  structure 
of  organic  compounds  and  has  led  without  interruption  to  the  enor- 
mously complex  theoretical  development  with  which  organic  chemistry 
confronts  us  at  the  present  time. 

The  first  convincing  evidence  of  the  existence  of  a  compound  radical 
was  brought  forward  by  Liebig  and  Wohler  4  in  their  classical  memoir 
entitled  "  Experiments  on  the  Radical  of  Benzoic  Acid."  This  recounts 
the  investigation  of  a  large  number  of  substances  in  which  the  residue 
Ci4Hio02  remains  unchanged,  as  for  example  benzoic  acid,  Ci4HioO2- 
0-H20,  benzoyl  chloride,  C^HioC^Ck,  benzoyl  cyanide,  sulphide, 


1  Old  atomic  weights. 

2  Versuch  iiber  die  Theorie,  etc.,  pp.  103  (1820). 

3  Kekule,  "  Lehrbuch  der  organ.  Chemie,"  Vol.  I,  62. 

4  Annalen  der  Chemie,  3,  249  (1832). 


THE  HISTORICAL  DEVELOPMENT  5 

amide,  etc.  This  was  followed  by  the  discovery  of  other  radicals, 
and  just  as  benzoyl  came  to  be  recognized  as  the  radical  of  benzoic 
acid  and  its  derivatives,  so  ethyl  became  identified  with  alcohol  and  its 
derivatives  while  cyanogen  was  seen  to  be  the  common  constituent 
in  the  various  cyanogen  compounds.  In  short  it  became  possible  in 
the  light  of  these  new  conceptions  to  consider  a  whole  series  of  separate 
phenomena  from  the  vantage  ground  of  a  common  point  of  view. 

Soon  after  this,  Dumas  made  a  remarkable  discovery.  He  found 
that  when  chlorine  acted  upon  certain  organic  compounds  such  as 
wax,  turpentine,  etc.,  hydrogen  was  removed  from  the  substance  and 
an  equivalent  amount  of  chlorine  was  substituted.  In  1835  Laurent 
asserted  that  the  chlorine  atom  assumed  the  same  position  in  the  mole- 
cule as  had  been  occupied  by  the  hydrogen,  playing  to  a  certain  extent 
the  same  role,  and  that,  therefore,  a  chlorinated  compound  should  be 
similar  in  its  properties  to  the  body  from  which  it  was  derived.1 
This  idea  was  distinctly  incompatible  with  the  dualistic  electro- 
chemical theory  and  in  open  contradiction  to  its  fundamental  hypoth- 
esis How  could  the  strongly  electro-negative  chlorine  take  the 
place  of  the  electro-positive  hydrogen  or  play  the  same  role?  The 
violent  protestations  of  Berzelius  induced  Dumas  to  refute  the  asser- 
tion of  Laurent  that  chlorine  took  the  place  of  the  hydrogen  in  the 
molecule,  but  he  was  unable  to  maintain  his  contention.  The  phenom- 
enon of  substitution  was  soon  discovered  to  be  a  very  general  and 
widely  disseminated  law  of  nature  and  further  investigation  by 
Dumas,  Laurent  and  others  showed  that  hydrogen  was  replaceable 
not  only  by  chlorine  but  also  by  bromine,  iodine,  oxygen  and  even  by 
carbon.  Indeed  the  marked  similarity  between  trichloracetic  acid 
and  acetic  acid  led  Dumas  himself  to  assert  that  in  this  case  chlorine 
occupied  the  place  of  hydrogen  in  the  molecule  and  played  the  same 
role.  Furthermore,  he  now  made  the  absolutely  general  assumption 
that  compounds  which  are  formed  as  the  result  of  substitution,  still 
possess  the  same  fundamental  or  type  groupings  as  the  compounds 
from  which  they  are  derived.  This  conception  represents  the  begin- 
ning of  the  so-called  "  Theory  of  Types." 

Dumas  suggested  an  analogy  between  the  parts  composing  a 
chemical  compound  and  the  parts  of  a  planetary  system,  assuming 
that  in  both  cases  the  components  are  held  together  by  the  exercise 
of  mutual  attraction.  In  the  constitution  of  the  compound  the 
parts  may  be  simple  atoms  or  composite  radicals  and  play  the  same 
roles  respectively  as  Venus  and  Mars,  or,  as  the  Earth  with  its  Moon 
and  Jupiter  with  its  satellites.  If  in  such  a  system  one  part  is  replaced 
1  Annales  chimie  et  phys.  (2)  53,  384  (1833). 


6  THEORIES  OF  ORGANIC  CHEMISTRY 

by  another  of  a  different  kind,  equilibrium  is  still  maintained  and  if 
the  replaced  and  substituting  elements  resemble  each  other,  the  new 
compound  has  properties  similar  to  those  of  the  compound  from  which 
it  is  derived.  If,  on  the  other  hand,  the  elements  are  different,  the 
two  compounds,  while  belonging  to  the  same  mechanical  system, 
show  little  resemblance  to  each  other  in  their  chemical  properties. 
Thus  in  1839  Dumas  fully  subscribed  to  a  doctrine  which  was  in 
absolute  contradiction  to  Berzelius'  theory  of  a  binary  arrangement 
of  parts,  and  which  completely  ignored  electricity  as  the  force  uniting 
the  atoms  of  a  compound.  This  doctrine  was  frequently  referred 
to  as  the  "  Unitary  Theory  "  to  distinguish  it  from  the  Binary  or  "Dual- 
istic  Theory  "  of  the  Swedish  chemist. 

It  is  to  be  noted  that  only  two  years  previous  to  this  time,  in  1837, 
all  European  chemists  of  prominence,  and  among  them  Dumas,1  had 
signified  their  desire  to  make  the  electrochemical  radical  theory  the 
foundation  for,  and  the  guiding  principle  of,  all  future  observations 
in  organic  chemistry.  It  is  therefore  pertinent  to  consider  the  attitude 
of  the  chemists  of  other  nations  after  discord  had  been  introduced  into 
the  international  concert  by  members  of  the  French  school. 

Berzelius  was  not  at  all  inclined  to  subscribe  to  the  views  of  Dumas, 
since  they  signified  for  him  the  overthrow  of  the  entire  structure  of 
chemical  knowledge,  and  he  even  went  so  far  in  his  opposition  as 
to  deny  vehemently  the  possibility  of  substituting  chlorine,  iodine, 
or  even  oxygen,  for  the  electro-positive  hydrogen.  In  regard  to 
oxygen  derivatives  he  wrote:  "  A  radical  cannot  be  an  oxide.  The 
very  meaning  of  the  word  radical  indicates  that  it  represents  a  body 
which  is  in  union  with  oxygen.  To  regard  a  radical  as  an  oxide  would 
be  equivalent  to  supposing  that  sulphurous  acid  (862)  is  the  radical 
of  sulphuric  acid,  and  manganese  dioxide  (MnO2),  the  radical  of 
manganic  acid."2  To  apply  this  conception  consistently  it  was 
necessary  for  Berzelius  to  eliminate  chlorine  as  well  as  oxygen  and  to 
include  only  carbon,  hydrogen  and  nitrogen  as  the  constituents  of  an 
electro-positive  radical.  Thus  benzoyl,  Ci4HioO2,  the  radical  of 
benzoic  acid  originally  accepted  by  Berzelius,  was  now  replaced  by 
the  expression  CuHio,  and  the  oxide  and  acid  became  respectively 
CuHioOa  and  CuHioOa+HO.  Analogous  changes  were  made  in 
the  case  of  acetyl  and  other  acid  radicals.  A  difficulty  was  presented  by 
bodies  which  contained  both  chlorine  and  oxygen,  and  in  such  cases 
it  became  necessary  to  double  and  sometimes  to  treble  the  original 
molecule.  Thus  benzoyl  chloride  was  written  as: 


Compt.  rend.,  6,  567  (1837).  2  Lehrbuch,  5th  Ed.  Vol.  I,  p.  674. 


THE  HISTORICAL  DEVELOPMENT  7 

According  to  this  conception  of  constitution  all  such  radicals  were 
supposed  to  be  capable  of  an  independent  existence,  and  might  there- 
fore be  isolated.  It  is  not  necessary  to  point  out  further  how  funda- 
mentally different  were  these  radicals  and  the  role  which  they  played 
from  our  present  conceptions  in  regard  to  them. 

The  followers  of  Berzelius  and  of  the  "  Radical  Theory  "  were  by  no 
means  unanimous  in  subscribing  to  his  individual  interpretations. 
Liebig  in  particular  gave  repeated  expression  to  his  dissatisfaction 
as  to  the  manner  in  which  Berzelius  forced  all  the  phenomena  of  organic 
chemistry  into  his  very  rigid  system.1  In  connection  with  the  mutual 
substitution  of  electrically  opposite  atoms  he  pointed  out  most  con- 
vincingly that  in  inorganic  chemistry  the  manganese  in  permanganic 
acid  may  be  replaced  by  chlorine  to  give  perchloric  acid,  and  that  the 
new  compound  shows  very  little  difference  from  the  old  in  its  reaction 
with  bases.  He  saw  no  reason  for  refusing  to  assume  the  presence 
of  oxygen  in  organic  radicals  and  therefore  held  to  the  benzoyl  radical 
Ci4HioO2  in  interpreting  the  transformations  of  benzoic  acid 
CuHioOa+Aq.,  and  its  derivatives.  In  1843,  in  his  textbook,2 
Liebig  denned  organic  chemistry  as  the  chemistry  of  the  compound 
radical.  In  describing  the  properties  of  acid  radicals,  he  says,  that  they 
combine  with  oxygen  and  with  sulphur  to  form  acids  and  even  with 
hydrogen  in  many  cases  to  form  hydrogen  acids.  Among  acid-forming 
radicals  he  includes  certain  compounds  of  carbon  with  oxygen  (the 
oxides  of  carbon),  cyanogen,  benzoyl,  cinnamyl,  salicyl,  acetyl,  formyl, 
etc.,  and  among  the  base-forming  radicals,  ethyl,  methyl  and  others.3 

The  great  majority  of  German  chemists  of  this  period  accepted 
Liebig' s  definition  of  organic  chemistry  and,  further,  assumed  that 
radicals  were  present  as  separate  individual  components  in  the  mole- 
cules of  the  various  compounds  containing  them.  Although  the 
statement  was  made  in  regard  to  radicals  that  only  a  few  actually 
existed  and  that  they  were  for  the  most  part  hypothetical,  it  was 
made  in  the  sense  that  only  a  few  radicals  had  as  yet  been  isolated.4 
As  a  result,  chemists  were  occupied  for  many  years  in  an  effort  to  obtain 
free  radicals.5  These  attempts,  although  they  failed  to  attain  their 
immediate  objects,  were,  nevertheless,  valuable  in  leading  to  the  dis- 
covery of  many  interesting  substances. 

While  Liebig  and  his  followers  had  emancipated  themselves  from 
many  of  the  speculations  of  Berzelius,  they  continued  to  hold  definitely 

1  Annalen  der  Chemie,  31,  119;  32,  72  (1839). 

2  Handbuch  der  organischcr  Chemie,  1S43.  p.  1. 

3  Ibid.,  p.  8. 

4  H.  Kopp.  "  Die  Entwickelung  der  Chemie  der  neuen  Zeit,"  p.  581. 

5  Kolbe,  Annalen  der  Chemie,  69,  257  (1849)  and  Frankland,  ibid.,  71,  171  (1849). 


8  THEORIES  OF  ORGANIC  CHEMISTRY 

to  his  dualistic  electrochemical  theories,  and  at  the  beginning  of  1840 
were  still  in  arms  against  the  theories  of  substitution  and  the  "  Type 
Theory  "  as  represented  by  the  French  School.  Not  until  1845  did 
Liebig  himself  accept  the  unitary  conception  of  the  constitution  of 
the  molecule.1 

So  much  bitterness  arose  as  the  result  of  the  discussions  which 
engaged  the  attention  of  chemists  of  the  opposing  schools  that  each 
of  the  contending  parties  ended  by  completely  ignoring  the  views 
of  the  other.  Since,  however,  there  was  something  fundamentally 
true  in  the  conceptions  for  which  each  was  fighting,  what  actually 
resulted  was  that  each  side  unconsciously  assimilated  the  ideas  of  the 
other,  and  developed  them  to  meet  its  own  needs.  Thus,  as  has  been 
seen,  Liebig  incorporated  the  idea  of  substitution  into  the  Radical 
Theory  of  Berzelius.  The  theory  of  the  opposing  school  underwent 
a  corresponding  modification;  the  adherents  of  the  Type  Theory  had 
ceased  using  the  dualistic  method  of  writing  formulas  (corresponding 
to  the  Electrochemical  Radical  Theory)  and  had  substituted  empirical 
formulas  for  them,  the  latter  being  based  upon  Dumas'  conception  of 
the  stellar  grouping  of  atoms  in  the  molecule  and  corresponding  in 
essentials  to  the  formulas  in  use  at  the  present  time.  Following 
this,  however,  Gerhardt  discovered,  as  a  result  of  a  study  of  double 
decomposition  reactions  in  organic  chemistry,  that  only  certain  atoms 
and  groups  of  atoms  present  in  the  molecule  take  an  active  part  in 
these  processes,  and  that  the  "  residue  "  passes  unchanged  from  one 
molecule  into  another.  Thus  the  idea  of  radical  was  resurrected  under 
a  new  name,  which  was  free  from  all  dualistic  electrochemical  taint 
and  which  eliminated  the  conception  of  the  independent  existence  of 
the  residual  group. 

As  new  classes  of  organic  compounds  were  discovered,  as  for  example 
the  polybasic  acids,  the  chlorinated  and  brominated  anilines,  and  the 
substituted  amines,  and  as  the  effort  was  made  to  interpret  all  new 
phenomena  in  terms  of  a  single  theoretical  conception,  it  became  evident 
that  the  Type  Theory,  used  in  conjunction  with  the  idea  of  Residues, 
afforded  a  simpler  and  more  natural  instrument  than  the  Dualistic 
Electrochemical  Theory.  In  the  course  of  time  the  older  Type  Theory 
merged  with  the  Radical  Theory  stripped  of  its  dualistic  trappings, 
and  from  the  elements  of  this  combination  Gerhardt  developed  a  new 
Type  Theory  by  means  of  which  it  was  not  only  possible  to  systema- 
tize all  known  phenomena,  but  also  to  discover  new  compounds  and 
even  in  many  cases  to  predict  properties. 

Gerhardt  arranged  organic  compounds  in  four  groups  which  were 
1  Kopp,  p.  626. 


THE  HISTORICAL  DEVELOPMENT 


9 


referred  to  four  simple  inorganic  types,  namely,  water  (H20),  hydro- 
gen (H2),  hydrochloric  acid  (HC1),  and  ammonia  (NH3).  In  the 
following  scheme  each  member  of  a  vertical  series  is  derived  from  the 
type  by  replacing  the  hydrogen  by  radicals: 


H\ 

H-HType 

HC1  Type 

HOH  Type 

H-)N  type 
H/ 

Ethyl  hydride, 

Ethyl  chloride, 

Ethyl  alcohol, 

Ethyl  amine, 

C2H5-H 

C2H5-C1 

C2H5-OH 

C2H5NH2 

Diethyl, 

Acetyl  chloride, 

Ethyl  ether, 

Diethvlamine. 

C2Hs  •  C2H5 

C2H3OC1 

C^j-HsOC^Hs 

(C2H5)2NH 

Aldehyde, 

Benzoyl  chloride, 

Acetic  acid, 

Triethylamine, 

C2H3O-H 

C8H5OC1 

C2H30-OH 

(C2H5)-N 

To  these  four  original  types  of  Gerhardt's,  Kekule  later  added  a 
fifth,  methane.  Thus,  for  quite  a  period  of  time,  chemists  abandoned 
themselves  to  the  amusing  task  of  assigning  newly  discovered  compounds 
to  their  proper  places  as  representatives  of  one  or  another  of  these 
types.  It  was  possible  to  determine,  by  means  of  double  decomposi- 
tion reactions,  the  fundamental  type  from  which  a  given  substance 
might  be  regarded  as  derived  and  also  the  radical  which  supposedly 
replaced  the  hydrogen  in  the  original  molecule  representing  the  type. 
In  this  way  the  properties  of  the  substance  could  be  derived  genetically 
from  the  properties  of  the  ancestral  form. 

Williamson's  conception  of  "Polyatoir.ic  Radicals"  played  an  impor- 
tant part  in  the  later  development  of  the  type  theory.  These  were  res- 
idues capable  of  replacing  two  or  more  hydrogen  atoms  in  a  given  type. 
When  the  hydrogen  atoms  replaced  by  such  radicals  were  present  in 
different  molecules,  "Multiple  or  Condensed  Types"  and  "Mixed  Types" 
were  formed.  According  to  this  conception,  for  example,  sulphuric  acid, 
HO — SC>2 — OH,  may  be  regarded  as  formed  by  the  condensation  of 
two  molecules  of  water  and  sulphur  trioxide,  while  oxamic  acid, 
NH2 — C2O2 — OH,  represents  a  mixed  water  and  ammonia  type. 
Many  other  cases  might  be  mentioned  to  illustrate  Williamson's 
conception  of  organic  combinations. 


CHAPTER  II 
THE  EARLY  HISTORY   OF   STRUCTURAL   CHEMISTRY 

IN  the  course  of  the  development  of  the  Type  Theory,  attention 
was  completely  diverted  from  the  atoms  and  the  forces  acting  between 
atoms,  and  became  absorbed  in  the  construction  of  images  representing 
the  various  forms  of  combinations  of  the  elements  in  the  different 
compounds.  Formulas  were  used  to  express  not  the  relative  position 
of  the  atoms  in  the  molecule,  but  merely  the  reactions  of  the  molecules 
and  their  relations  as  developed  in  the  processes  of  double  decompo- 
sition.1 It  was  thought  that  the  nature  of  a  substance  was  to  be  inferred 
from  a  study  of  classes  and  kinds  of  substances,  and  the  conception 
of  atoms  received  very  little  support  from  chemists  in  general.  In 
other  words,  the  original  idea — developed  by  Berzelius  and  founded 
upon  Dalton's  hypothesis — that  compounds  were  built  up  primarily 
from  oppositely  electrified  atoms,  was  either  neglected  or  rejected  by 
chemists.  There  were,  of  course,  strenuous  protestations  from  some 
quarters,  to  the  effect  that  the  highest  aim  of  science  could  never 
be  to  derive  the  properties  of  chemical  compounds  from  their  positions 
in  a  mechanical  system,  and  that  true  insight  into  these  properties 
was  to  be  gained  only  from  a  study  of  the  atoms  themselves.  Frank- 
land,  and  later  Blomstrand,  were  both  instrumental  in  redirecting 
attention  to  the  electrochemical  character  of  the  atoms,  but  it  was 
not  until  1858  that  any  substantial  progress  was  made. 

It  has  been  noted  that  with  the  passage  of  time  the  conception  of 
the  radical  changed,  that  it  ceased  to  represent  a  group  capable  of  an 
independent  existence  and  came  to  signify  merely  a  residue  which 
usually  passed  unaltered  from  one  substance  to  another,  but  which 
in  certain  reactions  might  even  suffer  slight  modifications  in  its  com- 
position. When  it  was  finally  discovered  that  individual  radicals 
could  be  completely  broken  down  as  a  result  of  more  drastic  reactions, 
interest  was  aroused  once  more  in  the  elements  out  of  which  the  radicals 
were  built  up.  "  In  the  present  state  of  chemical  knowledge  I  regard 
it  as  both  necessary  and  possible  to  explain  the  properties  of  compounds 
from  the  nature  of  the  elements  composing  them.  I  do  not  consider 
1  Kekule,  Textbook,  Vol.  I,  p.  92. 
10 


THE  EARLY  HISTORY  OF  STRUCTURAL  CHEMISTRY  11 

that  the  most  important  task  of  the  age  is  to  demonstrate  the  presence 
of  certain  groups  of  atoms,  or  radicals,  in  new  compounds  and  in  this 
way  to  relate  all  compounds  to  a  few  types  which  have  little  signifi- 
cance beyond  that  of  serving  as  models.  Investigations  should  be 
extended  to  a  study  of  the  constitution  of  the  radicals  themselves. 
The  relation  of  one  radical  to  another  should  be  carefully  determined 
and  the  nature  both  of  radicals  and  of  the  compounds  which  they 
constitute  should  then  be  referred  back  to  the  character  of  the 
elementary  atoms."  With  these  words  Kekule 1  redirected  the 
attention  of  chemists  to  the  fundamental  conception  of  the  atom.  He 
was  able  to  hold  their  attention  by  further  developing  the  conception 
of  "  Basicity  "  or  "  Atomicity." 

Even  in  the  time  of  Berzelius  it  was  customary  to  speak  of  the 
"  saturation  capacity  "  of  acids  and  bases,  understanding  by  this  the 
ability  of  these  substances,  under  quite  definite  conditions,  to  com- 
bine with  each  other.  Frankland  2  extended  this  idea  to  the  atoms 
of  the  elements,  and,  in  co-operation  with  H.  Kolbe  and  others,3 
amplified  it  somewhat.  The  elements  N,  P,  As,  and  Sb,  for  example, 
form  chemical  compounds  in  which  either  three  or  else  five  equivalents 
of  other  elements  are  always  found.  The  affinity  of  a  single  atom  of 
the  elements  in  question  is  thus  always  satisfied  by  a  specific  number 
of  other  atoms.  The  ability  of  an  atom  to  combine  with  a  definite 
number  of  other  atoms  or  groups  of  atoms  was  referred  to  as  the 
"  Atomicity  "  or  "  Basicity  "  of  the  element,  and  is  fundamentally 
the  same  as  the  term  "  Quantivalence  "  recently  suggested  by  A.  W. 
von  Hofmann. 

Kekule  adopted  this  conception  and  amplified  it.  In  1857  he 
pointed  out  that  H,  Cl,  Br,  K  and  others  belong  to  the  monatomic 
elements;  S  and  O  to  the  diatomic;  N,  P,  As,  etc.,  to  the  triatomic 
elements.  A  year  later  he  extended  his  speculations  to  carbon.  In 
working  out  the  application  of  the  conception  of  atomicity  to  this 
element  he  laid  the  foundations  for  future  important  developments 
in  the  theory  of  Organic  Chemistry.4  He  wrote  as  follows:  "  If  the 
simpler  compounds  of  carbon  are  examined  as,  for  example,  CH4, 
CHC13,  CH3C1,  CCU,  CO2,  COC12,  CS2,  HCN,  etc.,  it  is  obvious 
that  one  atom  of  carbon  always  combines  with  four  monatomic  or 
with  two  diatomic  atoms.  In  other  words,  the  sum  of  the  units  of 

iAnnalon  der  Chemie,  106,  136-137  (1858). 

2  Ibid.,  86,  368  (1858). 

3  See  E.  von  Meyer,  "  Geschichte  der  Chemie,"  3d  Edition  1905,  p.  286,  and  Hjelt 
1.  c.,  p.  287. 

4  Annalen  der  Chemie,  106,  153-154  (1858). 


12  THEORIES  OF  ORGANIC  CHEMISTRY 

affinity  by  means  of  which  carbon  combines  with  other  elements  is 
always  four,  and  this  leads  to  the  belief  that  carbon  itself  is  tetra- 
atomic  (tetrabasic)."  In  relation  to  the  three  groups  of  other  elements 
which  have  been  mentioned,  carbon  takes  its  place  as  the  representa- 
tive of  a  fourth  group. 

It  should  be  noted  that  in  accepting  the  idea  of  atomicity  and 
applying  it  to  carbon,  Kekule  used  the  term  in  a  sense  essentially 
different  from  his  predecessors — that  is  to  say,  he  assumed  that  atom- 
icity was  an  absolute  constant.  At  that  time  he  had  good  reasons  for 
making  this  assumption.  Dalton's  Atomic  Theory,  taken  by  itself, 
was  able  to  explain  only  the  Law  of  Definite  Proportions,  and  afforded 
no  adequate  interpretation  as  to  why  the  different  elements  always 
combined  in  definite  multiple  numbers  and  not  irregularly.  In  order 
to  explain  logically  the  Law  of  Multiple  Proportion,  it  was  necessary 
to  add  to  Dalton's  original  theory  the  further  assumption  of  Atomicity. 
This  fact  led  Kekule*  to  conceive  that  the  property  of  constant  atomicity 
was  a  fundamental  property  of  the  atoms,  and  as  unchangeable  as  the 
atomic  weights  themselves. 

Thus  in  1858  a  new  theory  in  regard  to  the  nature  of  chemical 
combinations  had  its  rise,  and  by  1864,  Kekule* 1  had  formulated 
this  theory  in  a  way  which  allowed  of  its  very  general  application. 
In  the  course  of  time  a  great  number  of  theoretical  speculations  which 
had  been  advanced  by  other  chemists — as,  for  example,  Frankland, 
Kolbe  and  Erlenmeyer,  Sr. — were  first  modified  and  then  merged  into 
Kekule's  theory,  so  that  it  became,  as  it  were,  the  cradle  of  structural 
chemistry.  It  may  be  briefly  summarized  as  follows: 

The  molecules  of  chemical  compounds  consist  of  atoms  which  are 
held  together  by  a  special  kind  of  attraction.  Many  atoms  have 
only  one  center  of  attraction  while  others  have  several  such  centers, 
and  they  may,  therefore,  be  classified  as  mono-  or  multi-atomic. 
In  all  combinations  of  atoms  the  units  of  affinity  of  a  given  atom  are 
saturated  wholly  or  in  part  by  an  equal  number  of  units  belonging 
either  to  one  or  to  several  other  atoms.  Atoms  may  combine  with 
other  atoms  of  the  same  kind  or  of  different  kinds.  By  means  of  these 
assumptions  it  is  possible  to  explain  why  many  elements  seem  to  show  a 
variation  in  valency  toward  others.  In  the  case  of  substances  which 
contain  several  atoms  of  carbon,  for  example,  it  is  necessary  to  assume 
that  the  carbon  atoms  are  mutually  bound  to  each  other  and  that, 
therefore,  only  a  part  of  the  affinity  of  each  is  available  for  holding 
other  kinds  of  atoms  within  the  molecule.2 

1  Compt.  rend.,  58,  510  (1864). 

2  Annalen  der  Chemie,  106,  154  (1858). 


THE  EARLY  HISTORY  OF  STRUCTURAL  CHEMISTRY  13 

The  compounds  in  which  the  atoms  are  mutually  held  together  by 
means  of  their  affinities  are  called  "  atomic  "  or  true  chemical  com- 
pounds, and  their  molecules  are  stable  in  the  gaseous  state.  Kekule 
distinguished  between  atomic  and  molecular  compounds.  The  latter 
may  be  obtained  from  the  former  in  the  following  manner:  When  the 
molecules  of  different  atomic  compounds  react,  the  atoms  constituting 
one  molecule  may  exercise  an  attraction  for  the  atoms  constituting  the 
other.  This  mutual  affinity  causes  the  two  molecules  to  draw  together 
and  to  unite.  The  process  is  often  preliminary  to  chemical  decompo- 
sition, in  which  case  the  addition  of  the  two  molecules  is  only  tem- 
porary and  is  accompanied  by  a  rearrangement  of  the  atoms.  Where 
rearrangements  of  the  atoms  are  impossible  because  of  the  nature  of 
the  two  molecules,  the  union  may  be  permanent  and  the  two  mole- 
cules may  cling  together  forming  a  more  or  less  stable  group.  The 
relative  stability  of  such  unions  must  necessarily  be  less  than  in  the 
case  of  atomic  compounds,  and  this  explains  why  "  molecular  "  com- 
pounds do  not  vaporize  as  such,  but  on  heating  decompose  into  the 
simpler  compounds  from  which  they  were  formed.  Ammonia  and 
phosphorus  trichloride  were  regarded  by  Kekule  as  examples  of  atomic 
compounds,  while  NHs  •  HC1  and  PCls  •  Cl2  were  classed  as  molecular 
compounds. 

The  theory  of  Kekule  was  subject  to  vehement  attack  even  in  the 
very  beginning  of  its  development.  The  students  and  followers  of 
Berzelius  held  firmly  to  the  view  that  chemical  combinations  were 
due  to  the  action  of  positive  and  negative  electrochemical  forces, 
and  that,  therefore,  the  valency  of  the  elements  was  not  a  constant 
but  a  variable  quantity.  Hermann  Kolbe  stood  foremost  among 
the  opponents  of  Kekule's  views.  He  assailed  them  to  the  very  end 
of  his  life  with  the  full  force  of  his  impressive  personality  and  with 
powers  of  discernment  which  were  far  greater  than  those  usually 
exercised  in  scientific  controversy.  But  in  seeking  to  establish  the  law 
of  electrochemistry  as  the  causal  factor  in  promoting  chemical  change, 
he  actually  defeated  his  own  ends  by  his  very  vehemence;  for  Kekule" 
and  his  followers  ceased,  as  it  were  by  common  agreement,  to  reply 
to  his  charges,  and  the  law  of  electrochemistry  was  almost  completely 
ignored.  Kekule's  theory  of  the  constancy  of  valency  was  not  accepted, 
however,  without  other  challenge. 

In  1856  Gerhardt  concluded  from  a  study  of  the  compounds  of 
nitrogen  that  this  element  exercises  sometimes  three  and  sometimes 
five  valencies.  Variation  in  the  valencies  of  other  elements  had  been 
discovered  by  Frankland,  Williamson,  Couper,  Kolbe  and  others, 
and  the  opinion  had  even  been  expressed  that  carbon  functions  as  a 


14  THEORIES  OF  ORGANIC  CHEMISTRY 

bivalent  element  in  ethylene,  acrylic  acid  and  other  compounds.  As 
a  result  of  these  investigations  the  Theory  of  Maximum  Saturation 
Capacity  had  its  rise,  and  it  was  in  part  at  least  in  opposition  to 
Kekule's  hypothesis.  In  terms  of  this  new  conception,  the  atoms 
of  all  the  elements  possess  a  definite  limiting  number  of  affinities  by 
means  of  which  they  may  attract  the  atoms  of  other  elements.  The 
greatest  number  of  valencies  which  one  atom  of  an  element  may 
bring  into  play  is  called  the  "  Maximum  Saturation  Capacity  "  of 
that  element.  In  the  case  of  nitrogen  and  of  carbon,  this  number 
is  represented  by  five  and  four  respectively.  Compounds  in  which 
two  elements  function  to  the  limit  of  their  powers  (NEUCl,  CHCls,  etc.), 
were  regarded  by  Erlenmeyer,  Sr.,  as  saturated  compounds.  In  many 
compounds,  however,  the  elements  do  not  exercise  their  maximal  powers 
of  affinity  (NHs,  C2KU,  C2H2,  etc.)  and  such  compounds  were  regarded 
by  Erlenmeyer,  Sr.,  as  "  unsaturated."  Kekule  opposed  the  Theory 
of  Maximum  Saturation  Capacity  1  and  held  firmly  to  his  assumption 
in  regard  to  the  absolute  constancy  of  valency,  attaching  to  it  the 
importance  of  a  fundamental  law  of  nature. 

In  the  subsequent  historical  developments  neither  conception  gained 
complete  acceptance.  The  idea  of  maximum  saturation  capacity  was 
more  or  less  neglected  until  quite  recently,  when  it  was  resurrected 
again;  but,  on  the  other  hand,  variation  in  the  basicity  of  at  least 
some  elements  (as,  for  example,  nitrogen)  came  to  be  more  and  more 
generally  accepted.  Kekule  did  succeed  in  impressing  the  great 
majority  of  his  fellow  chemists  with  the  belief  in  the  constant  valency 
of  carbon.  He  accomplished  this  by  reason  of  his  influence  on  the 
development  of  structural  chemistry. 

After  the  distinction  between  atom  and  equivalent  had  been  clearly 
demonstrated  by  Cannizarro,  an  attempt  was  made  by  Butlerow, 
Erlenmeyer,  Kekule  and  others  to  gain  deeper  insight  into  the  struc- 
ture of  chemical  compounds.  Butlerow  was  the  first  investigator 
to  use  the  expression  "  the  structure  of  a  chemical  compound " 
although  Erlenmeyer,  writing  at  about  the  same  time,  used  the  term 
"  constitution  "  to  express  the  idea  of  the  mutual  relation  existing 
between  the  atoms  in  a  molecule.  Butlerow  may  indeed  be  said  to 
have  outstripped  Kekule,  in  so  far  as  he  discerned  in  1860,  that  the 
future  task  of  chemists  was  to  be  that  of  determining  "  the  nature  and 
the  manner  of  the  mutual  union  of  the  atoms  in  the  molecule." 

In  1861  Kekule  began  to  use  graphic  formulas.  He  was  able 
to  attack  successfully  the  problem  of  representing  the  relations  of  the 
atoms  in  the  molecule  by  assuming,  first,  the  tetravalency  of  carbon 
iKekule's  "  Lehrbuch,"  I,  pp.  160  and  162;  Compt.  rend.,  58,  512  (1864). 


THE  EARLY  HISTORY  OF  STRUCTURAL  CHEMISTRY  15 

in  all  of  its  compounds,  and  second,  the  ability  of  carbon  atoms  to 
combine  with  each  other  by  means  of  one  or  more  units  of  affinity. 
In  amplifying  the  atomic  pictures  used  by  Berzelius,  atoms  of  different 
basicity  (atomicity,  valency)  were  represented  as  of  different  size. 
Thus  hydrogen  chloride,  water,  ammonia,  methane  and  hydrogen 
cyanide  were  represented  by  the  following  graphic  formulas  :l 


These  symbols,  although  long  since  abandoned,  signified  a  very 
important  step  forward  at  that  time,  since  they  made  it  possible  to 
picture  the  exact  relation  of  the  atoms  in  the  molecule,  and  they 
represent  the  direct  transition  from  the  theory  of  types  to  the  theory 
of  structural  chemistry.  Soon  after  this  the  letters  B,  C,  N,  O,  H, 
etc.,  were  used  by  Erlenmeyer,  Sr.,  to  denominate  the  different 
elements,  while  dashes  were  added  to  signify  the  number  of  units  of 
affinity  (valency)  in  any  given  case.  This  method  of  representation 
gradually  took  the  place  of  the  graphic  formulas.  Thus,  by  about 
1870,  structural  or  constitutional  formulas  began  to  be  used  in  much 
the  same  form  as  at  the  present  time.2 

Kekule  was  able  to  demonstrate  the  constant  valency  of  carbon 
in  saturated  compounds  without  much  difficulty,  but  the  matter  was 
not  quite  so  simple-  in  the  case  of  the  unsaturated  compounds.  At 
first  he  merely  assumed  that  the  carbon  atoms  present  in  compounds 
of  the  latter  type  were  in  much  closer  union  than  was  the  case  in 
saturated  compounds,  and  made  no  attempt  to  explain  the  nature  of 
this  union.  But  later,  when  he  had  become  convinced,  as  a  result 
of  experimental  investigations  in  the  field  of  unsaturated  acids,  that 
ethylene  and  its  derivatives  really  show  the  properties  of  unsaturation 
and  are  able  to  add  H2,  Br2,  etc.,  with  great  ease,  he  evolved  the  hypoth- 
esis "  that  two  units  of  affinity  of  the  carbon  are  not  saturated, 
and  are  therefore,  in  a  sense,  present  in  the  molecule  in  a  free  state."3 
Or  again:  "  At  those  positions  in  the  molecule  where  hydrogen  atoms 
a  e  lacking,  the  two  units  of  affinity  of  the  carbon  atoms  remain 
unsaturated  and  in  a  sense  there  is  a  void.  It  is  not  difficult,  there- 
fore, to  understand  why  substances  of  this  type  add  hydrogen  and 

1  Lehrbuch,  I,  pp.  160-162. 

2  For  a  history  of  the  development  of  graphic  formulas  see  Anschutz,  Zeitschr. 

.  Chemie,  27,  323  (1914). 

3  Lehrbuch,  II,  p.  251. 


16  THEORIES  OF  ORGANIC  CHEMISTRY 

bromine  with  such  ease.  The  free  units  of  affinity  have  a  tendency 
to  become  saturated  and  so  to  fill  this  void."1 

Kekule  soon  abandoned  these  assumptions,  and  the  very  volume  of 
his  textbook  from  which  these  quotations  have  been  taken,  contains 
suggestions  which  form  the  nucleus  for  theories  which  are  held  at  the 
present  time.  Since  these  theories  are  intimately  connected  with 
Kekule's  conception  in  regard  to  the  constitution  of  benzene,  a  con- 
sideration of  this  subject  is  now  in  order. 

Kekule  started  with  the  assumption  that  aromatic  compounds  in 
general  are  to  be  regarded  as  derivatives  of  benzene,  since  in  even  the 
most  drastic  reactions  one  product  is  always  aromatic  and  contains 
at  least  six  carbon  atoms.  To  explain  the  aromatic  structure  of  these 
substances  on  the  basis  of  the  tetravalence  of  carbon,  it  was  necessary 
to  evolve  an  essentially  new  hypothesis,  and  he  conceived  that  the  six 
carbon  atoms,  common  to  all  such  compounds,  were  bound  together 
not  in  an  open  chain  but  in  the  form  of  a  closed  ring.  He  conceived, 
further,  that  they  were  held  together  in  this  way  by  the  mutual 
exercise  not  of  two,  but  of  three  affinity  units  (valencies)  each.  The 
following  graphic  formula,  published  in  1865,  pictures  for  the  first  time 
double  bonds  and  a  ring  of.  six  carbon  atoms : 


By  means  of  this  hypothesis  it  was  obviously  possible  to  express  the 
mutual  relations  of  the  atoms  in  the  molecule  without  abandoning 
the  idea  of  the  tetravalence  of  carbon. 

From  this  time  "  structural  "  or  "  constitutional  "  formulas  came 
into  more  and  more  general  use,  and  they  were  found  to  be  quite  as 
serviceable  as  the  formulas  of  the  "  type  theory  "  in  correlating  and 
systematizing  the  innumerable  organic  compounds  which  were  being 
discovered.  Indeed,  the  new  formulas  did  more,  in  that  they  afforded 
an  insight  into  the  mutual  relationships  of  the  atoms  within  the  mole- 
cule, or,  in  other  words,  into  its  internal  structure. 
1  Lehrbuch  II,  p.  259. 


THE  EARLY  HISTORY  OF  STRUCTURAL  CHEMISTRY  17 

Structural  formulas  were  especially  helpful  in  interpreting  numerous 
instances  of  isomerism,  since  almost  all  were  explicable  on  the  assump- 
tion of  differences  in  the  arrangement  of  the  atoms  in  the  molecule. 
In  the  case  of  one  relatively  small  group  of  isomeric  substances,  how- 
ever, structural  formulas  proved  to  be  inadequate  to  explain  the 
observed  differences  in  properties.  This  was  the  group  of  "  physical 
isomers,"  so  named  by  Kekule  because,  while  corresponding  very 
closely  as  to  chemical  properties,  these  substances  were  often  almost 
exactly  opposite  in  physical  properties.  Thus,  while  one  isomer 
turned  the  plane  of  polarized  light  a  definite  amount  to  the  right,  the 
other  either  turned  the  plane  of  polarized  light  exactly  the  same 
amount  to  the  left  or  else  had  no  effect  whatever  upon  it.  To  this 
group  of  substances  belonged  tartaric  acid,  aspartic  acid,  malic  acid 
and  others.  Toward  the  solution  of  the  problem  offered  by  these 
substances,  the  classical  researches  of  Pasteur  had  already  yielded 
results  of  fundamental  importance.  In  1869  Johannes  Wislicenus 
added  lactic  acid  to  this  series  of  "physical  isomers,"  demonstrating 
beyond  the  shadow  of  a  doubt  that  the  lactic  acid  obtained  from  sour 
milk  and  the  so-called  "  sarco-lactic  acid  "  were  both  a-hydroxypro- 
pionic  acid,  and  therefore  corresponded  to  the  structural  formula 

CHsCHOHCOOH. 

"  The  isomeric  lactic  acids  offer  the  first  authentic  case  where 
the  observed  number  of  isomeric  substances  exceeds  the  structural 
possibilities,"  and  Wislicenus  goes  on  to  observe  that  "  Facts  of  this 
sort  can  be  explained  only  by  supposing  that  the  difference  in  the 
isomeric  molecules  is  to  be  accounted  for  by  a  difference  in  the  spacial 
arrangement  of  their  atoms."1  Thus,  eight  years  after  Butlerow  (1861) 
had  emphatically  denied  that  his  conception  of  the  term  "  structure  " 
included  the  idea  of  space  relationships,  J.  Wislicenus  and  Blomstrand 
were  able  to  show  the  necessity  for  this  further  extension  of  the  theory 
of  atomic  relationships.  The  problems  of  physical  isomerism  were  not, 
however,  solved  at  this  time  but  were  temporarily  neglected,  because 
for  the  moment  the  concentrated  effort  of  the  majority  of  chemists 
was  bent  upon  the  solution  of  the  more  immediate  problems  of  struc- 
tural chemistry,  as,  for  example,  those  involved  in  the  chemistry  of 
benzene.  Nevertheless,  by  1874,  the  phenomena  of  physical  isomerism 
had  found  a  mastei  interpreter  in  the  person  of  Jacobus  Henricus 
van't  Hoff. 

1  Annalen  der  Chemie,  167,  343  (1873). 


CHAPTER  III 
LATER  DEVELOPMENTS  IN  STRUCTURAL  CHEMISTRY 

IT  was  Jacobus  Henricus  van't  Hoff  who  finally  solved  the  problems 
presented  by  the  phenomena  of  physical  isomerism.  He  was  able  to 
do  much  more  than  meet  the  immediate  difficulties  of  the  times  in 
which  he  lived,  for  his  researches  in  the  field  of  Thermodynamics  led 
him  to  conceive  the  matter  in  terms  so  fundamental  and  far-reaching 
that  their  full  significance  has  only  recently  been  recognized.  Van't 
Hoff  gained  his  first  knowledge  of  the  theory  of  structural  chemistry 
at  Bonn  under  the  leadership  of  Kekule  himself,  while  later  in  Paris 
he  became  conversant  with  the  classical  researches  of  Pasteur  on  the 
isomerism  of  the  tartaric  acids.1  Thus  his  theoretical  conceptions 
were  from  the  beginning  subject  to  these  two  particular  influences. 

"  Modern  chemical  theory  has  two  weak  points;  it  fails  to  consider 
the  relative  position  which  the  atoms  occupy  in  the  molecule  and  also 
the  nature  of  their  motion."  This  is  the  opening  sentence  of  the 
German  translation  of  van't  HofTs  classical  treatise  entitled  "  Die 
Lagerung  der  Atome  in  Raume."  The  conceptions  set  forth  in  this 
text  are  in  a  certain  sense  more  fully  and  exhaustively  treated  in  the 
book  entitled  "  Ansichten  iiber  Organische  Chemie,"2  by  the  same 
author.  In  both  books  van't  Hoff  emphasized  the  two  fundamental 
weaknesses  in  chemical  theory  which  have  just  been  mentioned.  He 
was  the  first  to  feel  the  necessity  of  representing  the  four  valencies  of 
carbon  as  directed  toward  the  four  corners  of  a  tetrahedron  at  the 
center  of  which  the  carbon  atom  was  assumed  to  be  located.  It  must 
be  noted  that  this  conception  of  the  distribution  of  the  valencies  of 
carbon  is  quite  different  from  the  original  conception,  which  supposes 
that  all  four  valencies  of  carbon  lie  in  the  same  plane.3  On  the 
assumption  of  van't  Hoff  it  is  possible  to  explain  the  phenomena  of 
physical  isomerism  observed  in  the  cases  of  lactic,  malic,  tartaric  acids, 
etc.,  by  supposing  that  the  difference  in  the  properties  of  the  isomeric 

1  E.  Cohen,  "  Jac.  Henr.  van't  Hoff,"  p.  12,  Leipzig,  Engelmann,  1899. 

2  Braunschweig,  Friedr.  Vieweg  und  Sohn,  1878. 

3  Kekule  also  made  use  of  atomic  models  to  illustrate  atomic  constitution  as 
early  as   1867.     See   Zeitschr,  f.  Chemie,  N.  F.  3,  218. 

18 


LATER  DEVELOPMENTS  IN  STRUCTURAL  CHEMISTRY    19 

substances  is  due  to  a  difference  in  the  arrangement  of  the  atoms  in 
space.  The  science  of  stereochemistry  l  was  thus  founded  upon  van't 
Hoff's  conceptions  in  regard  to  the  asymmetry  of  carbon,  and  following 
this  time  it  developed  steadily  although  not  rapidly.  It  can  be  referred 
to  here  only  in  so  far  as  it  is  related  to  the  evolution  of  structural  theory 
in  general. 

In  the  year  1885  A.  von  Baeyer  succeeded  in  establishing  a  logical 
basis  for  many  previously  unfounded  assumptions  in  regard  to  the 
constitution  of  organic  compounds  by  interpreting  them  in  terms 
of  the  new  conception  of  the  carbon  atom.  His  work  had  a  directing 
and  most  fruitful  influence  upon  research,  and  demonstrated  what 
real  progress  might  be  made  on  the  basis  of  van't  Hoff's  assumption. 

Kekule  explained  the  constitution  of  benzene  by  assuming  that  the 
six  carbon  atoms  were  joined  together  in  the  form  of  a  ring,  and  this 
idea  was  later  extended  to  explain  the  constitutions  of  pyridine,  quino- 
line,  naphthalene,  thiophene,  pyrrol,  tri-,  tetra-,  penta-,  and  hexa- 
rnethylene,  and  other  cyclic  combinations.  The  assumption  of  ring 
structure  in  these  cases,  while  offering  an  illuminating  interpretation 
of  the  relative  stability  of  the  substances,  was  nevertheless  arbitrary. 
There  was  no  really  logical  ground  for  supposing  ring  formation,  and 
because  of  this  fact  numerous  objections  were  raised  against  Kekule's 
formula  for  benzene. 

A.  von  Baeyer,2  in  logically  developing  his  " Theory  of  Tension" 
for  ring  compounds  on  the  basis  of  the  new  space  model  for  the  carbon 
atom,  succeeded  in  meeting  some  of  these  objections.  He  reasoned 
that,  on  the  basis  of  van't  Hoff's  assumption,  the  angle  formed  by  the 
mutual  union  of  the  valencies  of  any  two  carbon  atoms  must  be  109°  28'. 
If  other  carbon  atoms  are  now  imagined  as  joined  successively  to  the 
first  pair,  in  such  a  way  thatrtheir  centers  of  gravity  all  fall  within  the 
same  plane,  it  will  be  found  that  these  atoms  must  arrange  themselves 
either  in  a  zig-zag  line  or  a  ring  form,  supposing  only  that  the  fixed 
angle  be  maintained.  By  referring  to  a  model  constructed  by  joining 
five  carbon  atoms  in  a  chain  of  this  sort,  it  becomes  apparent  that  a 
valence  of  the  first  atom  in  the  series  may  be  readily  united  to  a  valence 
of  the  fifth  to  form  a  closed  ring.  It  is  also  obvious  that  rings  of  three 
and  four  carbon  atoms  can  be  formed  only  as  the  result  of  deflecting 
the  direction  of  the  combining  valencies  away  from  their  normal 
angles  of  109°  28'.  The  resistance  offered  by  the  valencies  of  carbon 
atoms  to  deflection  from  their  fixed  positions  around  the  atom  shows 

1  Compare  Textbooks    of    Stereochemistry  by  van't  Hoff,  Hantzsch,  Bischoff- 
Walden,  A.  Werner,  E.  Wedekind  and  others. 

2  Ber.,  18,  2278  (1885). 


20  THEORIES  OF  ORGANIC  CHEMISTRY 

itself  in  the  form  of  a  strain,  and  is  comparable  to  the  tension  observed 
in  the  case  of  a  bent  metal  spring.  On  the  basis  of  such  a  conception 
it  is  easy  to  understand  why  rings  of  five  carbon  atoms  are  stable, 
while,  on  the  other  hand,  rings  of  three  or  four  are  unstable. 

Baeyer  calculated  the  degree  of  the  deflection  of  carbon  valencies 
in  the  case  of  various  ring  structures,  when  the  following  results  were 
obtained.  The  degree  to  which  each  valence  is  bent  toward  the  center 
of  the  ring  is 

24°  44'  for  the  trimethylene  ring; 

9°  44'  for  the  tetramethylene  ring; 

0°  44'  for  the  pentamethylene  ring; 

while  the  degree  of  angular  distortion  away  from  the  center  in  the  case 
of  the  higher  cycles  is 

5°  16'  for  the  hexamethylene  ring; 
9°  33'  for  the  heptamethylene  ring. 

According  to  this  conception  the  pentamethylene  ring  should  be  the 
most  stable  of  all,  and  this  conclusion  is  supported  by  the  observation 
that  it  is,  in  fact,  the  most  easily  formed  of  all  rings.  It  must  be 
noted,  however,  that  while  hexamethylene  rings  are  formed  less  readily 
than  pentamethylene  rings,  they  show  a  greater  stability  than  might 
be  expected  on  the  basis  of  the  theoretical  considerations  represented 
in  the  above  table.  This  stability  may  be  explained,  on  the  other  hand, 
by  supposing  that  the  centers  of  gravity  of  the  six  carbon  atoms  lie  in 
different  planes,  in  which  case  a  model  may  be  constructed  without 
necessitating  the  deflection  of  any  valence  of  carbon  from  the  normal 
angle  of  109°  28'.  It  should,  nevertheless,  be  added  that  the  stability 
of  carbon  rings  in  general  depends  not  only  upon  the  number  of  atoms, 
but  also,  and  to  a  very  great  extent,  upon  the  nature  of  the  substituents 
vwhich  are  in  union  with  these  atoms. 

According  to  Baeyer's  "Tension  Theory/'  a  parallel  should  exist 
between  the  ease  with  which  rings  are  formed  and  the  stability  of  the 
resulting  compound.  The  number  of  instances  where  the  facts  are  in 
direct  contradiction  to  this  assumption  have  been  greatly  augmented 
in  recent  years,  so  that  the  whole  question  as  to  the  correctness  of 
Baeyer's  conception  has  been  opened  up  anew.  For  example,  Harries 
and  his  students  1  have  succeeded  in  preparing  a  number  of  ozonides. 
These  bodies  represent  products  which  are  formed  by  the  addition  of 

1  Annalen  der  Chemie,  374,  303  (1910);  Compare  also  H.  and  C.  Thieme,  Ber., 
39,  2849  (1906)  and  H.  and  L.  Tank,  Ber.,  41,  1701  (1908) ;  also  Berger  "Zur  Kenntniss 
des  l-methylcyclopentanon-3  und  l-methylcyclopentene-2, "  Inaugural  Dissertation, 
Kiel,  1914. 


LATER  DEVELOPMENTS  IN  STRUCTURAL  CHEMISTRY         21 

ozone  to  the  unsaturated  carbon  atoms  present  in  the  following  types 
of  ring  structure: 

CH  CH=CH 

CH=CH  CH    CH2  CH2  CH2 

II  II  II 

CH2  CH2  CH2  CH2  CH2  CH2 

\/  V"  V 

CH2  CH2  CH2 

Cyclopentene  Cyclohexene  Cycloheptene 

As  is  well  known,  ozonides  of  this  class  may  be  hydrolyzed  by  water 
when  they  yield  oxidation  products  which  contain  open  carbon  chains, 
as  for  example  aldehydes  and  acids.  It  would  seem  natural  on  the  basis 
of  Baeyer's  conception  of  ring  structure  to  suppose  that  the  ozonide 
of  cyclopentene  would  be  the  most  stable  of  these  derivatives  and  that 
of  cycloheptene  the  least  stable.  As  a  matter  of  fact,  however,  this 
is  not  true,  for  it  has  been  observed  that  the  ring  actually  opens  most 
readily  in  the  case  of  the  ozonide  of  cyclopentene. 

Analogous  phenomena  have  been  observed  by  J.  v.  Braun  1  in  con- 
nection with  a  study  of  heterocyclic  compounds.  A  comparison  of  the 
following  types  shows  that  the  pyrrolidine  ring  is  formed  most  readily. 

CH2 

CH2  CH2  H2C— CH2 

CH2  CH2'  H2C     CH2 

V  V 

NH  NH  NH 

Tetra-hydroquinoline  Piperidine  Pyrrolidine 

It  should,  therefore,  in  the  sense  of  Baeyer's  theory,  be  the  most 
difficult  to  open,  but  such  is  not  the  case.  J.  v.  Braun  has  succeeded 
in  modifying  A.  W.  v.  Hofmann's  method  of  exhaustive  methylaticn 
in  such  a  way  as  to  make  it  of  general  application  in  opening  lings 
of  this  type,2  and  a  comparison  of  the  behavior  of  the  above  and  other 
substances  has  demonstrated  the  fact  that  the  pyrrolidine  ring  opens 
much  more  readily  than  the  piperidine  ring,  and  that  this  in  turn 
opens  somewhat  more  readily  than  the  tetra-hydroquinoline  ring. 
In  arranging  ring-bases  in  the  order  of  their  relative  stability  both 

1  Chem.  Zeitung,  1911,  374;  Ber.,  44,  1253  (1911). 

2Ber.,  33,  36,  36,  37,  38,  39,  40,  41,  42,  See  particularly  49,  2630  (1916),  and 
"  Uber  die  Entalkylierung  und  Aufspaltung  organischen  Basen  mit  Hilfe  von 
Bromcyan  und  Halogenphosphor."  Wallach — Festschrift,  p.  313  and  following. 


22  THEORIES  OF  ORGANIC  CHEMISTRY 

with  reference  to  the  effects  of  exhaustive  methylation  and  the  action 
of  cyanogen  bromide,  J.  v.  Braun  discovered  a  somewhat  remarkable 
parallelism.  In  both  series  stability  increases  from  left  to  right  in  the 
following  order: 

Pyrrolidine  >  Piperidine  >  Tetra-hydroquinoline 

The  fact  that  these  substances  behave  analogously  under  the  influ- 
ence of  such  distinctly  different  types  of  reagents  must  find  its  explana- 
tion in  something  fundamental  in  the  nature  of  such  ring  systems. 
While  it  is  not  yet  possible  to  say  just  what  this  is,  it  must,  neverthe- 
less, be  acknowledged  that  this  action,  which  obviously  proceeds  with 
fairly  definite  regularity,  cannot  be  explained  on  the  basis  of  the  theory 
of  internal  tension  or  strain. 

In  the  case  of  heterocyclic  rings  which  are  formed  by  the  addition 
of  ketene,  R2C==C=O,  to  substances  possessing  the  unsaturated 
group  C  ;  N,  H.  Staudinger  1  has  observed  that  the  stability  of  the  ring 
depends  not  only  upon  the  number  of  atoms  present  in  the  ring,  but 
also  upon  the  character  and  the  interrelationships  of  these  atoms. 
Thus,  for  example,  certain  /3-lactams  of  the  general  formula 

CO 


are  practically  as  stable  as  the  7-  and  6-lactams  of  5  and  6  members 
respectively,  i.e., 


=C—  CO 


It  should  be  noted,  however,  that  the  foregoing  and  other  excep- 
tions to  the  assumptions  of  Baeyer's  "  tension  theory  "  have  only  very 
recently  come  to  the  attention  of  chemists.  In  1885  Baeyer's  con- 
ception of  the  mechanism  of  ring  formation  was  very  generally 
accepted.  Indeed,  it  seemed  to  follow  as  a  logical  consequence  of  the 
assumptions  in  regard  to  the  nature  and  chemical  behavior  of  the 
carbon  atom  which  were  prevalent  at  that  time.  These  assumptions 
Baeyer2  was  able  to  condense  into  the  form  of  seven  fundamental 
principles  which  may  be  stated  as  follows: 

1  H.  Staudinger  "  Die  Ketene,"  p.  59,  Stuttgart,  Enke,  1912. 
2Ber.,  18,  2278  (1885). 


LATER  DEVELOPMENTS  IN  STRUCTURAL  CHEMISTRY         23 

1.  Carbon  is  usually  tetravalent. 

2.  Its  four  valencies  are  equivalent,  as  shown  by  the  fact  that  there 
is  no  isomerism  in  the  case  of  the  monosubstitution    products    of 
methane. 

3.  These  valencies  may  be  represented  as  directed  toward  the  corners 
of  a  regular  tetrahedron,  constructed  around  the  carbon  atom  as  a 
center,  and  are,  therefore,  equidistant  from  each  other  in  space. 

4.  The  atoms  or  groups,  joined  to  carbon  by  the  exercise  of  its  valen- 
cies, occupy  definite  and  permanent  positions  both  with  reference  to 
the  central  carbon  atom  and  with  reference  to  each  other.    This  is 
shown  by  the  fact  that  isomerism  occurs  in  the  case  of  derivatives 
of  methane  where  the  substituting  atoms  or  groups — a,  b,  c,  and  d — 
are  all  different.     (The  law  of  Le  Bel  and  van't  Hoff.) 

5.  Carbon  atoms  are  capable  of  entering  into  combinations  with 
other  carbon  atoms  by  means  of  1,  2  or  3  valencies. 

6.  The  resulting  compounds  form  either  open  or  closed  carbon 
chains.1 

7.  The  four  valencies  of  carbon,  operating  from  the  central  atom 
in  the  direction  of  5he  corners  of  a  circumscribing  regular  tetrahedron, 
diverge  from  each  other  by  an  angle  of  109°  28'.     This  angle  may  be 
increased  or  diminished,  but  such  a  change  is  always  accompanied  by  a 
condition  of  strain  or  tension  within  the  molecule. 

These  principles  follow  as  logical  deductions  from  the  assumptions 
of  van't  Hoff,  and  were  accepted  by  the  majority  of  chemists.  With 
them  as  a  basis  Baeyer  now  proceeded  to  subject  the  much  debated 
question  of  the  constitution  of  benzene  to  a  systematic  experimental 
investigation.  Up  to  this  time  benzene  and  its  derivatives  had  been 
investigated  and  discussed  principally  with  reference,  first,  to  the 
question  of  the  relative  position  of  the  groups  substituting  in  the  ring, 
and  second,  the  question  of  decomposition  products.  These  investi- 
gations had  led  to  no  definite  choice  between  the  several  equally 
probable  formulas  which  had  been  advanced  to  explain  the  various 
phenomena.  Baeyer  turned  his  attention  to  an  experimental  study 
of  the  gradual  transformation  of  fatty  into  aromatic  compounds,  and 
of  the  corresponding  reverse  processes.  The  results  of  these  researches 
have  become  classical,  and  form  the  basis  for  new  theoretical  con- 
ceptions which  are  among  those  most  widely  discussed  at  the  present 
time. 

Kekule's  formula  for  benzene  found  more  general  acceptance  than 
any  other  proposed  at  that  time.  It  is  true  that  the  assumption  of 
alternate  double  and  single  bonds  between  the  carbon  atoms  of  the 
i  Compare  G.  Gruttner  and  E.  Krause,  Ber.,  49,  2666  (1916). 


24 


THEORIES  OF  ORGANIC  CHEMISTRY 


ring  presupposes  the  existence  of  is  omeric  or^/io-disubstitution  products 
as  for  example, 

a  a 


and 


and  as  yet  no  isomerism  of  this  kind  had  been  observed.  This  objection 
has  been  met  in  part,  however,  by  Kekule's  "Oscillation  Hypothesis."1 

It  was  to  be  assumed  from  the  presence  of  three  pairs  of  unsaturated 
double  bonds  in  the  molecule  that  benzene  would  show  highly  unsatu- 
rated properties — i.e.,  that  it  would  add  hydrogen,  hydrogen  chloride, 
halogen,  etc.,  and  that  it  would  reduce  permanganate  readily — and 
the  facts  of  the  case  are  in  formal  agreement  with  these  assumptions. 
One  of  the  first  and  most  important  tasks  of  Baeyer  was  to  prove 
beyond  the  shadow  of  a  doubt  that  hexahydrobenzene  and  hexamethyl- 
ene  were  identical.  He  was  careful,  however,  to  point  out  a  very 
important  difference  between  aliphatic  and  aromatic  compounds  as 
shown  in  the  ease  with  which  they  react  respectively  with  hydrogen, 
halogen,  etc.  Thus  while  the  great  majority  of  unsaturated  compounds 
of  the  aliphatic  series  form  addition  products  readily  at  ordinary 
temperatures,  aromatic  compounds  react  slowly  and  require  the  presence 
of  heat  or  sunlight.  Further,  benzene  derivatives,  having  no  readily 
oxidizable  side  chains  show  for  the  most  part  very  little  reactivity  to 
permanganate  and  at  best  are  only  incompletely  oxidized,  while  unsatu- 
rated aliphatic  compounds  are  immediately  attacked  by  this  reagent 
even  in  the  cold. 

Differences  in  the  behavior  of  benzene  derivatives  as  compared 
with  unsaturated  aliphatic  compounds  were  very  carefully  investi- 
gated by  Baeyer  in  connection  with  a  study  of  the  phthalic  acids. 
These  acids,  for  example,  react  successively  with  two,  four,  and  then 
six  hydrogen  atoms,  giving  along  with  others  the  following  compounds: 


COOH 


COOH 

Terephthalic  acid 


HOOC 


HOOC 


pdrotere- 


A^Tetrahydrotere- 
phthalic  acid 


inyc 
phthalic  acid 

1  Annalen  der  Chemie,  162,  86  (1872). 


Hexahydrotere- 
phthalic  acid 


LATER  DEVELOPMENTS  IN  STRUCTURAL  CHEMISTRY         25 

Terephthalic  acid  itself,  in  spite  of  its  three  pairs  of  doubly-bound 
carbon  atoms,  is  very  stable  toward  permanganate  and  adds  halogen 
only  with  great  difficulty.  Upon  reduction  these  properties  abruptly 
change  and  the  resulting  dihydro-  and  tetrahydro-  derivatives  not 
only  react  readily  with  bromine,  adding  two  and  four  atoms  respect- 
ively, but  are  also  so  unstable  in  the  presence  of  permanganate  as  to 
be  mistaken  for  unsaturated  aliphatic  compounds. 

These  typically  unsaturated  properties  vanish  again  when  the  proc- 
ess, which  has  just  been  described,  is  reversed  and  the  hydro-deriva- 
tives are  oxidized  to  phthalic  acid.  In  this  case  the  change  from 
tetrahydro-  to  dihydrophthalic  acid  is  accompanied  by  an  increase  in 
unsaturated  properties  such  as  would  normally  be  expected  in  passing 
from  a  substance  containing  one  pair  of  doubly-bound  carbon  atoms 
to  a  diethylene  compound.  But  with  the  next  step  in  the  process, 
i.e.,  with  the  formation  of  a  third  pair  of  doubly-bound  carbon  atoms, 
the  substance,  instead  of  showing  increased  unsaturation,  suddenly 
and  abruptly  loses  all  of  the  characteristics  commonly  associated  with 
a  condition  of  unsaturation  in  the  molecule.  In  other  words,  its  ability 
to  add  hydrogen,  halogen,  etc.,  has  vanished,  and  its  general  stability 
has  become  comparable  to  that  usually  attributed  to  saturated  com- 
pounds. These  generalizations  were  arrived  at  "by  Baeyer  as  the  result 
of  a  purely  chemical  investigation  of  the  substances. 

Stohmann  came  to  the  same  conclusions  in  a  different  way, 
but  in  order  to  understand  the  significance  of  his  investigations,  it 
will  be  necessary  to  make  a  few  preliminary  remarks.1  It  is  usually 
assumed  that  a  certain  amount  of  energy  is  stored  up  in  every  molecule 
that  is  capable  of  maintaining  an  independent  existence.  This  is 
always  the  same  in  quantity  for  molecules  of  the  same  kind,  whether 
of  elements  or  of  compounds.  In  other  words,  all  molecules  possess 
a  definite  amount  of  potential  energy  which  is  changed  into  kinetic 
energy  (heat,  electricity,  etc.),  when  the  molecules  suffer  decompo- 
sition into  their  respective  atoms,  as  happens  in  the  course  of  chemical 
reactions.  The  fact  that  A  and  B  react  to  form  C  and  D  is  expressed 
by  the  equation 

A+B=C+D 

but  this  represents  the  change  in  the  energy  relations  very  indefi- 
nitely and  very  incompletely.  It  does  not  show,  for  example,  whether 
the  energy  set  free  by  the  decomposition  of  A  and  B  is  consumed 
wholly  or  in  part  by  the  formation  of  C  and  D.  It  is  possible  that  the 

JFor  recent  development  of  this  subject  see  Weinberg,  Ber.,  62,  1501  (1919); 
K.  Fajans,  Ibid.,  63,  643  (1920);  also  Steiger,  Ber.,  63,  666  (1920). 


26  THEORIES  OF  ORGANIC  CHEMISTRY 

energy  of  A  +B  may  be  equal  to,  or  else  greater  or  less  than  that  of 
C-\-D.  If  the  energy  A-\-B  is  greater  than  the  energy  of  C+D,  it 
follows  that  chemical  action  is  accompanied  by  the  evolution  of  heat, 
and  that  the  reaction  belongs  to  the  class  known  as  exothermal,  i.e., 


If,  however,  the  energy  of  A  -\-B  is  less  than  that  of  C-f  D,  energy  must 
be  added  to  the  system  in  order  to  induce  the  reaction, 


which  is  now  said  to  be  endothermal  in  character. 

In  the  case  of  unsaturated  aliphatic  compounds  the  addition  of 
hydrogen  belongs  to  the  exothermal  type  of  chemical  reaction.  Thus, 
for  example: 

C2H4  +  H2   =  C2H6  +  31.9  Cal. 

Ethylene  Ethane 

C3H6  +  H2   =   C3H8  +  32.5  Cal. 

Propylene  Propane 

C4H4O4  +  H2   =   C4H6O4  -f  32.5  Cal. 

Fumaric  acid  Succinic  acid 

These  reactions  serve  to  show  that  approximately  the  same  amount 
of  heat  is  liberated  by  the  reduction  of  a  variety  of  unsaturated  com- 
pounds. 

Stohmann  1  now  proceeded  to  determine  the  value  for  the  heat 
liberated  in  the  progressive  reduction  of  terephthalic  acid,  and  obtained 
the  thermal  values  expressed  below  : 

C8H6O4  +  H2   =   C8H804  -  2.8  Cal. 

Terephthalic  Ai&-Dihydrotere- 

acid  phthalic  acid 

C8H804  +  H2   =   C8Hi0O4  +  28.9  Cal. 

A15-Dihydrotere-  Al-Tetrahydrotere- 

phthalic  acid  phthalic  acid 

C8Hi004  +  H2   =   C8Hi2O4  +  23.5  Cal. 

Ai-Tetrahydrotere-  Hexahydrotere- 

phthalic  acid  phthalic  acid 

These  figures  show  that  in  the  first  stage  of  the  process  of  the  reduction 
of  terephthalic  acid,  results  are  obtained  which  are  not  in  harmony 
with  the  general  rule  which  has  been  formulated  in  regard  to  the  addition 
of  hydrogen  to  doubly  bound  carbon  atoms,  since  energy  is  absorbed 
and  not  given  off  in  the  reaction.  The  second  and  third  steps  in  the 
process,  on  the  other  hand,  proceed  quite  regularly.  Further,  the 
thermal  relationships  observed  in  the  case  of  this  particular  series 
have  been  found  to  hold  quite  generally  for  the  reduction  products  of 
1  Jour,  prakt.  Chemie,  41,  13-14,  538  (1890);  45,  475  (1892);  48,  447  (1893). 


LATER  DEVELOPMENTS  IN  STRUCTURAL  CHEMISTRY          27 

benzene  and  its  derivatives.     The  following  values  have  been  obtained  in 
the  reduction  of  benzene  itself: 


C6H6     + 

Benzene 

H2 

-'   C6H8     + 

Dihydrobenzene 

0.8  Cal. 

C6H8     + 

Dihydrobenzene 

H2 

=        CeHiQ       + 
Tetrahydrobenzene 

25  Cal. 

CeHiQ       + 
Tetrahydrobenzene 

H2 

=     C6-Hi2     -f- 

Hexahydrobenzene 

27.8  Cal. 

Hexahydrobenzene 

H2 

Hexane 

11  Cal. 

From  these  results  it  is  established  that  in  the  case  of  substances 
containing  benzene  nuclei  the  first  step  in  the  process  of  reduction  is 
essentially  different  from  the  second  and  the  third  and  that  the  latter, 
in  their  observed  heats  of  reduction,  correspond  to  reductions  which 
involve  the  addition  of  hydrogen  to  doubly  bound  carbon  atoms  in 
aliphatic  compounds.1 

It  thus  appears  that  a  certain  amount  of  resistance  must  be  over- 
come before  the  change  from  benzene  to  dihydrobenzene  can  take 
place,  and  that  energy,  which  would  ordinarily  be  set  free  by  the 
addition  of  hydrogen,  is  used  up  in  this  process.  After  this  initial 
reaction,  further  additions  of  hydrogen  take  place  regularly  with  the 
usual  evolution  of  heat.  In  brief,  the  abrupt  change  in  propertie0 
attendant  upon  the  formation  or  destruction  of  the  benzene  nucleus 
has  been  confirmed  by  physical  as  well  as  by  purely  chemical  investi- 
gation.2 Moreover,  it  follows  that  the  presence  of  three  pairs  of  doubly 
bound  carbon  atoms  in  the  benzene  nucleus  may  no  longer  be  assumed, 
and  that,  therefore,  Kekule's  formula  does  not  correctly  express  the 
constitution  of  benzene  and  its  derivatives.  , 

In  order  to  account  for  these  facts  in  a  more  satisfactory  manner 
Baeyer  discarded  Kekule's  formula  and  substituted  in  its  place  the 
so-called  "  centric  formula  "  which  was  originally  suggested  by  H. 
Armstrong.3 


According  to  this  the  fourth  valency  of  each  carbon  atom  is  repre- 
sented as  merely  directed  toward  the  center  of  the  ring  (as  shown  by 

1  Jour,  prakt.  Chemie,  43,  21  (1891). 

2  Annalen  der  Chemie,  407,  145  (1915). 

3  Jour.  Chem.  Soc.,  51,  264  (1887). 


28  THEORIES  OF  ORGANIC  CHEMISTRY 

the  short  lines)  thus  indicating  that  by  their  mutual  action  the  power 
of  each  is  rendered  latent,  and  that  a  condition  of  equilibrium  is  thereby 
established.  Such  a  centric  method  of  linkage  is  unknown  in  the 
fatty  series  and  may  be  regarded  as  a  prerogative  of  aromatic  com- 
pounds. When  benzene  is  reduced  to  dihydrobenzene  this  condition 
of  equilibrium  is  destroyed  with  the  saturation  of  two  bonds,  and  the 
remaining  four  of  the  six  centric  bonds  rearrange  themselves  to  form 
two  normal  pairs  of  unsaturated  double  linkages: 
H  H 

H|\/|H       H2 


H 

Such  a  conception  accounts  readily  for  the  marked  difference  in  prop- 
erties between  benzene  and  its  reduction  products. 

The  investigations  of  Baeyer  placed  the  chemistry  of  benzene 
and  its  derivatives  in  quite  a  new  light.  They  led  to  the  general 
conclusion  that  the  essential  characteristics  of  aromatic  compounds 
depend  upon  the  peculiar  symmetrical  arrangement  of  the  fourth 
valency  of  each  of  the  six  carbon  atoms  present  in  the  nucleus,  and 
not  upon  ring  formation  nor  yet  upon  the  groupings  of  carbon  link- 
ages.1 This  peculiar  condition  was  referred  to  later  as  the  "  latent  " 
condition  of  these  six  valencies. 

This  conception  of  structure  was  soon  applied  to  other  types  of 
cyclic  compounds.  In  1890  Bamberger2  discovered  that  naphthalene 
is  much  more  readily  attacked  by  reducing  agents  than  are  other 
benzene  derivatives,  and  that  it  adds  first  two  and  then  four  atoms  of 
hydrogen.  With  the  addition  of  four  atoms  a  limit  seems  to  be  reached, 
and  the  further  addition  of  six  hydrogen  atoms  is  observed  to  meet 
with  a  resistance  similar  to  that  observed  in  the  initial  stage  of  the 
reduction  of  benzene.  This  resistance,  along  with  other  chemical 
properties  of  tetrahydro-naphthalene,  marks  the  first  indication  of  a 
true  benzene  nucleus  in  the  naphthalene  molecule.  Such  relationships 
find  no  expression  in  the  formula  of  Erlenmeyer,  Sr.,  and  Graebe, 

H      H 


i    ii 


H      H 

lAnnalen  der  Chemie,  267r  47-48  (1890). 
2  Ibid.,  1  and  following  (1890). 


LATER  DEVELOPMENTS  IN  STRUCTURAL  CHEMISTRY          29 

which  assumes  the  presence  of  two  true  benzene  nuclei  in  naphthalene. 
In  order  better  to  explain  these  facts,  Bamberger  applied  Baeyer's 
idea  of  latent  valencies  centrally  directed  and  in  this  way  evolved  the 
following  constitutional  formula  for  naphthalene: 


This  represents  two  rings  in  each  of  which  are  present  six  latent 
valencies  similar  in  character  to  those  represented  by  the  centric  formula 
of  benzene.  At  the  same  time  it  should  be  noted  that  neither  ring 
represents  a  true  benzene  nucleus.  The  action  of  reducing  agents 
may  be  supposed  to  destroy  the  condition  of  equilibrium  in  this 
system,  and  the  saturation  of  four  of  the  original  ten  latent  valencies 
brings  about  a  rearrangement  which  results  in  the  formation  of  a 
true  benzene  nucleus : 

H  H 

.  H 


H 

The  relationships  expressed  by  these  formulas  show  why  the  benzenoid 
character  of  the  substance  is  accentuated  by  reduction  —  tetra- 
hydronaphthalene  being  regarded  as  a  true  benzene  derivative  in 
which  the  reduced  nucleus  functions  as  an  aliphatic  side  chain.  These 
formulas  also  offer  an  easy  explanation  as  to  why  the  reduction  of 
naphthalene  and  its  derivatives  takes  place  in  stages  which  show  a 
marked  difference  from  each  other.  Bamberger  developed  analogous 
formulas  for  quinoline,  anthracene,  and  phenanthrene. 

The  researches  of  Ciamician  and  Angeli  on  pyrrol  and  its  derivatives 
had  in  the  meantime  become  generally  known.  Although  this  sub- 
stance 

HC— CH 

II      II 
HC     CH 

\/ 

NH 

contains  an  imido  group  it  shows  no  basic  properties,  and  even 
resembles  phenol  in  its  weak  acidity.  When  reduced  to  dihydro- 


30  THEORIES  OF  ORGANIC  CHEMISTRY 

and  tetrahydro-pyrrol,  however,  it  suffers  an  abrupt  change  from 
acidic  to  strongly  basic  properties: 

HC — CH  HC — CH2  H2C — CH2 

ii          ii   i  ii 

/STT       Tir^        r^TI  ^       "U    C*       /^TT 

v^Jtl       — >       tt.\j       V^li2       — '       Jtl2V>'       v^±l<> 

\/  \/ 

N  N  N 

H  H  H 

Weak  acid  Strong  bases 

I  II  III 

The  formulas  which  are  represented  above  do  not  explain  the  abrupt 
and  abnormal  change  in  properties  observed  in  the  transition  from  I 
to  II  as  compared  with  the  gradual  and  normal  change  in  properties 
in  the  transition  from  II  to  III. 

In  order  to  explain  these  relationships  better,  Bamberger  developed 
the  following  formula  for  pyrrol : 


HCr\~7iCH 


HC 


CH 


N 
H 


in  which  he  assumes  the  presence  of  pentavalent  nitrogen  and  also 
a  system  of  six  latent  valencies  similar  in  character  to  those  present 
in  benzene.  The  addition  of  hydrogen  may  be  supposed  to  destroy 
the  equilibrium  of  this  system,  and  this  is  accompanied  by  a  radical 
rearrangement  of  the  molecule: 


CH  HC 


HC 


NH  NH 


CH 


CH 


During  this  transformation  nitrogen  passes  from  the  pentavalent 
to  the  trivalent  condition,  while  at  the  same  time  an  unsaturated 
double  bond  is  established  between  two  of  the  four  carbon  atoms. 
The  abrupt  change  in  the  properties  of  the  substance  during  the  proc- 
ess of  reduction  is  in  this  way  readily  accounted  for. 

Analogous  formulas  were  soon  evolved  for  indol,  thiophene, 
furane  and  other  similar  compounds.  It  was  not  possible,  however, 
to  attach  any  lasting  significance  to  the  new  formulas,  and  they  were 


LATER  DEVELOPMENTS  IN  STRUCTURAL  CHEMISTRY    31 

finally  abandoned,  following  criticism  by  Ciamician  and  Zanetti,1 
Markwald,2  and  others.  The  phenomena  themselves  remained  unex- 
plained, and  continued  to  demand  other  and  more  adequate  interpre- 
tations to  replace  those  which  had  been  rejected. 

To  summarize  briefly,  it  may  be  said  that  the  various  investigations 
which  have  just  been  reviewed  led  to  the  conclusion  that,  while  ring 
compounds  of  the  benzene  type  are  undoubtedly  unsaturated,  this 
characteristic  is  not  so  pronounced  or  so  definite  as  in  the  case  of 
unsaturated  compounds  of  the  paraffine  series.  The  gradation  in  prop- 
erties of  unsaturated  aromatic  and  aliphatic  bodies  undoubtedly 
bears  some  relation  to  the  presence  of  conjugate  double  linkages  in 
the  molecule,  but  there  seems  to  be  no  way  of  expressing  these  rela- 
tionships in  terms  of  the  present  structural  formulas. 

As  a  result  of  work  along  these  lines  a  very  keen  interest  was 
developed  in  regard  to  the  relation  of  unsaturation  to  various  physical 
properties.  Researches  were  undertaken  with  a  view  to  determining 
the  influence  of  double  bonds  upon  molecular  volume,  electrolytic 
conductivity,  and,  even  more  particularly,  upon  molecular  refraction 
and  dispersion.  Developments  in  these  fields  of  research  will  be 
referred  to  again  later  in  this  text.  Still  more  recently,  investigation 
has  concerned  itself  with  the  effect  of  unsaturation  upon  other 
physico-chemical  properties,  such  as  dielectric  constant,  absorption 
spectrum,  etc.  In  1897  F.  Henrich  pointed  out  that  the  so-called 
negative  character  of  certain  groups  of  atoms  was  really  conditioned 
by  their  unsaturation.  He  called  attention  to  the  fact  that  tautomeric 
as  well  as  other  types  of  rearrangements  seem  to  depend  upon  the 
activity  of  unsaturated  atoms  or  groups,  and  that  recent  speculation 
in  regard  to  the  relation  between  color  (fluorescence,  etc.)  and  chemical 
constitution  is  based  upon  a  consideration  of  the  properties  of  doubly 
bound  atoms. 

In  general,  the  study  of  unsaturated  compounds  has  opened  up  an 
almost  infinite  number  of  new  problems,  and  has  done  much  toward 
keeping  theoretical  speculation  free  from  purely  formal  and  inelastic 
conceptions,  such  as  in  the  past  have  so  often  hindered  the  forward 
movement  of  science.  Progress  in  chemistry  at  the  present  tune  is 
largely  due  to  an  appreciation  of  the  fact  that  affinity  does  not  operate 
exclusively  as  sharply  defined  units  (valencies)  but  is  capable  of  almost 
infinite  variation.  It  is  supposed  that  in  the  different  types  of  simple 
combination  residues  of  affinity  are  frequently  left  free,  and  that 
these  may  be  relatively  great  or  small,  depending  upon  a  variety  of 

iBer.,  24,  2122  (1891);  26,  1711  (1893). 

2Annalen  der  Chemie,  274,  331  (1893);  279,  1  (1894). 


32  THEORIES  OF  ORGANIC  CHEMISTRY 

conditions,  but  must  in  all  cases  play  a  quite  definite  role  in  deter- 
mining the  properties  and  the  reactivity  of  the  resulting  compound. 
Certain  theories,  which  have  been  of  importance  in  the  historical 
development  of  the  conception  of  residual  affinity  must  now  be 
considered. 


CHAPTER  IV 
JOHANNES    TREBLE'S    THEORY    OF   PARTIAL   VALENCIES 

THE  idea  that  chemical  action  is  due  to  attractive  forces  (valencies) 
operating  between  atoms  came  to  be  more  and  more  generally  accepted 
after  the  middle  of  the  last  century.  This  led  to  the  deduction  that 
where  free  valencies  were  known  to  exist  chemical  activity  was  to  be 
expected,  and  conversely,  that  chemical  activity  indicated  the  presence 
of  free  valencies.  The  importance  of  the  latter  was  overlooked  by 
organic  chemists  partly  because  of  the  character  of  the  chemical 
formulas  which  were  then  in  use.  While  Kekule  had  originally  assumed 
that  free  valencies  were  present  in  unsaturated  compounds,  the  fact 
that  these  were  always  observed  to  occur  in  pairs  and  that  all  attempts 
to  prepare  methylene  =CH2  resulted  in  the  formation  of  ethylene, 
CH2=CH2,  led  to  the  assumption  that  the  valencies,  which  were  set 
free  in  the  formation  of  unsaturated  compounds,  mutually  saturated 
each  other,  thus  giving  rise  to  double  and  triple  bonds  between  atoms. 
The  formulas  which  were  employed  to  express  these  ideas,  could, 
however,  be  construed  to  mean  something  quite  different  from  that 
for  which  they  were  intended.  In  the  case  of  CH3 — CH3,  CH2=CH2, 
and  CH=CH,  for  example,  the  mere  symbols  suggest  a  greater  stability 
for  ethylene  and  acetylene  than  for  methane,  and  this  is,  of  course, 
directly  contradictory  to  the  facts.  The  first  rational  explanation 
of  the  gradually  decreasing  stability  observed  in  compounds  con- 
taining double  and  triple  bonds  was  offered  by  A.  v.  Baeyer1  in  his 
so-called  " tension  theory"  and  was  based  upon  van't  Hoff's  hypoth- 
esis in  regard  to  the  distribution  in  space  of  the  four  valencies  of 
carbon.  Other  attempts  to  interpret  the  phenomena  of  unsaturation 
were  advanced  by  von  Wunderlich,2  Victor  Meyer  and  P.  Jacobson3 
and  others,  and  were  in  general  agreement  with  that  of  Baeyer. 

In  1899  Thiele4  pointed  out  that  while  it  was  possible  to  explain 
the  behavior  of  substances  possessing  a  single  pair  of  double  or  triple 

'Ber.,  18,  2277  (1885). 

2  Konfiguration  organischer  Molekiile,  Wurzburg,  1886. 
8  Lehrbuch  der  Organischen  Chemie,  Vol.  I. 
4  Annalen  der  Chemie,  306,  87  (1899). 
33 


34  THEORIES  OF  ORGANIC  CHEMISTRY 

bonds  by  means  of  the  theories  which  have  just  been  referred  to,  it 
was  impossible  to  interpret  in  the  same  manner  the  properties  of 
substances  possessing  two  pairs  of  adjacent  double  bonds.  Thus,  for 
example,  R.  Fittig  l  has  shown,  as  the  result  of  a  series  of  investi- 
gations in  regard  to  the  constitution  of  piperic  acid,  that  this  substance, 

CH2(O2)C6H3CH  :  CHCH  :  CHCOOH 

always  gives  as  its  normal  reduction  product  a-hydropiperic  acid 

:  CHCH2COOH 


and  that,  therefore,  hydrogen  always  adds  primarily  in  the  1-4,  never 
in  the  1-2  or  3-4  positions.  The  transformation  to  /3-hydropiperic 
acid, 

CHCOOH 


is  brought  about  only  as  the  result  of  the  action  of  sodium  hydroxide. 

Similar  observations  were  made  by  Baeyer  2  in  connection  with  the 
reduction  of  terephthalic  acid.  Thus  in  the  case  of  A1-3-dihydro- 
terephthalic  acid,  hydrogen  adds  in  the  1-4  and  not  in  the  1-2  or  3-4 
positions,  with  formation  of  a  new  double  bond  between  the  2—3 
positions  of  the  ring. 


COOH 


COOH 


Realizing  the  importance  of  this  discovery  Baeyer,  in  co-operation  with 
Rupe,3  extended  his  research  into  the  aliphatic  series  and  studied 
the  behavior  of  the  atomic  grouping 

— CH=CH  •  CH=CH- 
in  muconic  acid, 

HOOC-CH  :  CH-CH  :  CHCOOH 

1234 

The  results  of  this  investigation  also  show  that  addition  both  of 
hydrogen  and  of  halogen  takes  place  in  the  1-4  and  not  in  the  1-2  or 

i  Annalen  der  Chemie,  162,  47;  172,  158;  216,  171;  227,  46  (1888). 

2  Annalen  der  Chemie,  251,  271  (1889). 

3  Annalen  der  Chemie,  256,  1  (1890). 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES         35 

3-4  positions.  Thiele  contributed  still  other  data  to  those  furnished 
by  Fittig,  Baeyer  and  Rupe,  and  in  every  case  the  evidence  seemed 
to  point  to  the  fact  that  addition  to  unsaturated  systems  possessing 
conjugate  double  bonds  usually  takes  place  in  the  1^4  positions. 

Thiele's  Theory  of  Partial  Valencies  attempts  to  explain  these 
important  facts  on  the  basis  of  previous  assumptions  in  regard  to  the 
chemical  nature  of  the  carbon  atom.1  If  a  substance  containing 
doubly  bound  carbon  atoms,  as  for  example  ethylene,  is  brought  in 
contact  with  one  of  the  halogens,  a  very  vigorous  addition  reaction 
takes  place  even  at  ordinary  temperatures,  with  the  result  that  two 
atoms  of  halogen  combine  respectively  with  the  two  doubly  bound 
carbon  atoms.  These  carbon  atoms  must  be  regarded  as  very  reactive, 
and,  according  to  Thiele,  the  reason  for  this  is  to  be  found  in  the  fact 
that  they  possess  free  valencies.  Such  free  valencies  are  of  a  particular 
type,  however,  and  must  not  be  confused  with  the  ordinary  conception 
of  free  unit  valencies.  In  the  case  of  ethylene,  for  example,  the  two 
unsaturated  carbon  atoms  may  be  imagined  as  joined  together  by 
means  of  four  valencies  which  have  united  to  form  a  double  bond; 
but  in  this  union  all  four  valencies  are  not  mutually  and  completely 
saturated,  since  in  the  case  of  the  second  pair  only  a  fraction  of  each 
unit  of  affinity  is  exercised  in  holding  the  two  carbon  atoms  together, 
while  a  residue  of  affinity  is  left  free.  To  this  residue  of  free  affinity 
Thiele  gives  the  name  "  Partial  Valency,"  and  he  represents  it  graphic- 
ally in  a  chemical  formula  by  means  of  a  dotted  line. 

This  idea  was  developed  still  further  by  Thiele  and  was  applied 
not  only  to  all  doubly  bound  carbon  atoms  but  also  to  other  unsatu- 
rated atoms  in  organic  combinations,  the  symbols  for  which  may  be 
written  in  the  following  way : 

C=C        C=O        C=N        O=N        N=N,    etc. 

The  relative  strength  of  the  partial  valencies  may,  of  course,  be 
assumed  to  be  different  in  different  cases.  Such  formulas  show  why 
unsaturated  groups  of  atoms  are  reactive,  but  they  do  more  in  that 
they  afford  a  basis  for  the  development  of  an  entirely  new  point  of 
view.  To  understand  this  fully  it  will  be  necessary  to  consider  in 
detail  Thiele's  theory  of  partial  valency. 

As  has  already  been  pointed  out,  chemists  at  this  time  had  aban- 
doned the  conception  of  free  valencies  and  had  assumed  the  presence 

1  Annalen  der  Chemie,  306,  87  (1899);  308,  333  (1899). 


36  THEORIES  OF  ORGANIC  CHEMISTRY 

of  multiple  forms  of  union  in  unsaturated  compounds.  In  1892  Nef  l 
again  brought  forward  the  idea  of  free  valencies  and  at  the  same  time 
Armstrong  2  attempted  to  explain  the  cause  of  certain  chemical  reactions 
on  the  assumption  of  so-called  residual  affinities.  Both  theories  vary 
from  that  of  Thiele,  however,  since  the  latter  assumes  that  in  unsatu- 
rated compounds  the  atoms  are  actually  joined  together  by  means 
of  double  or  triple  bonds,  but  possess  in  addition  a  certain  residue 
of  free  affinity.  Thiele' s  conception  finds  support  in  the  thermochemical 
investigations  of  J.  Thomson,3  who  shows  that  the  heat  of  formation 
of  ethylene  is  less  than  should  be  expected  if  two  simple  pairs  of  carbon 
linkages  are  present  in  the  molecule:  "  This  would  signify  that  in  the 
formation  of  an  unsaturated  compound  of  this  type,  not  all  of  the 
energy,  belonging  to  the  four  units  of  valency,  is  exercised  in  holding 
the  two  carbon  atoms  together  or  that,  in  other  words,  they  still 
possess  residues  of  free  affinity  or  partial  valencies." 

In  all  additions  to  ethylene  the  reacting  atoms  or  groups  of  atoms 
are  supposed  to  attach  themselves  by  satisfying  the  residues  of  free 
affinity  present  in  the  molecule  but,  according  to  Thiele,  this  is 
followed  immediately  by  the  ruption  of  one  of  the  double  bonds  between 
the  carbon  atoms,  and  the  final  arrangement  is  one  in  which  the  adding 
atoms  or  groups  completely  saturate  respectively  each  of  two  unit 
valencies  of  the  carbon  atoms.  In  the  case  of  bromine,  for  example, 
the  various  stages  involved  in  the  process  of  addition  to  ethylene 
may  be  represented  in  the  following  manner: 

H2C H2C Br  H2CBr 

f  Br2     ->          ||  ->  | 

H2C Br  H2CBr 

I  II  III 

Reich  4  formulates  this  reaction  as  follows : 


H2C H2C H2C H2C — Br 

-f  Br2=   || 

H2C.         H2C Br2  H2C Br2  H2C— Br 

I 

lAnnalen  der  Chemie,  270,  267  (1892);  280,  291  (1894);  287,  265;  298,  202; 
309,  264.  Also  "The  Fundamental  Conceptions  Underlying  the  Chemistry  of  the 
Element  Carbon,"  see  Chapter  XIV. 

2  Jour.  Chem.  Soc.,  61,  264  (1887). 

sZeitschr.  physikal.  Chemie,  1,  369  (1887). 

4  Such  intermediate  products  have  never  as  yet  been  isolated.  Annalen  der 
Chemie,  306,  105  (1911);  319,  129  (1912).  Also  compare  Reich,  Jour,  prakt.  Chemie 
90,  177 '(1914)  and  Reddelien,  ibid.,  91,  219. 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES         37 

Since  the  different  elements  have  different  attractions  for  each 
other,  it  follows  that  addition  reactions  will  be  greatly  influenced  by 
the  nature  of  the  atoms  involved.  Nitrogen,  for  example,  has  only  a 
very  slight  affinity  for  halogen,  and,  therefore,  N=N  adds  halogen 
^)nly  with  difficulty.  Since  hydrogen  has  a  greater  affinity  for  oxygen 
than  for  carbon,  while  the  reverse  is  true  in  the  case  of  the  group 
— SOsNa,  it  follows  that  the  addition  of  sodium  bisulphite  to  ketone 
and  aldehyde  groupings  takes  place  according  to  the  scheme : 

OH  OH 

II    +    I  -      I 

— C         SO3Na        — CSO3Na 

and  not  according  to  the  equation  below: 

O        S03Na  OSOsNa1 

II    +    I  -      I 

— C        H  — CH 

This  conception  of  free  valency  suggests  the  possibility  that  under 
certain  conditions  compounds  may  exist  in  which  atomic  stability 
within  the  molecule  depends  in  part  upon  the  exercise  of  partial 
valencies.  The  primary  product  (II)  resulting  from  the  addition  of 
bromine  to  ethylene  would  belong  to  this  class  of  bodies,  as  would 
similarly  the  polymerization  products  of  the  ethylene  hydrocarbons: 
C C  C C 

II      +      II  =  II          II 
C C       C C 

Such  complex  molecules  would  be  more  saturated  in  character  than  the 
simple  molecules  from  which  thay  are  derived,  but  would  readily  break 
down  into  their  components.  Metastyrol  and  other  polymerization 
products  showing  saturated  properties  may  possibly  be  regarded  as 
compounds  of  this  type.2 

When  two  pairs  of  doubly  bound  carbon  atoms  occupy  adjacent 
positions  in  the  molecule  they  are  said  to  form  a  "  conjugate  system." 
The  simplest  type  of  such  a  system  may  be  represented  by 

E=E— E=E 

where  E  equals  any  element.  If  the  particular  element  happens  to 
be  carbon,  for  example,  it  follows  according  to  Thiele's  theory  that  each 
atom  present  in  the  chain  possesses  a  partial  valency : 

CH2=CH— CH=CH2 
I  I          I         I 

i  i  i  i 

1  Annalen  der  Chemie,  306,  92  (1899). 

2  Compare  Willstatter,  Ber.,  41,  1464  (1908). 


38  THEORIES  OF  ORGANIC  CHEMISTRY 

In  this  case  the  reduction  of  butylene l  (butadiene)  should  result 
in  the  simultaneous  addition  of  four  hydrogen  atoms  to  each  of  the 
four  carbon  atoms.  Actual  observation  has  shown,  however,  that 
only  two  hydrogen  atoms  add  to  the  hydrocarbon  and  that  they  attach 
themselves  not  to  two  adjoining  carbon  atoms  such  as  1-2  or  3-4, 
but  to  the  end  carbon  atoms  in  the  positions  1  and  4  forming  sym- 
dimethylethylene. 

CH2  :  CH-CH  :  CH2  +  H2  -»  CH3CH  :  CHCH3 

The  addition  of  bromine  takes  place  in  the  same  way,  and  a  sym- 
metrical unsaturated  dibromide  is  formed,  BrCH^CH  :  CH-CH^Br. 

This  remarkable  behavior  has  been  observed  in  the  case  of  innumer- 
able compounds  possessing  analogous  atomic  groupings  and  may  be 
explained  by  supposing  that  in  systems  of  this  type  the  partial  valen- 
cies on  the  atoms  2  and  3  mutually  saturate  each  other,  combining 
to  give  a  new  kind  of  double  bond  between  the  respective  atoms. 
Thiele  calls  this  form  of  combination  an  "  inactive  double  bond." 
The  system  of  atoms  as  a  whole  receives  the  name  of  a  conjugated 
system  and  may  be  represented  graphically  in  the  following  way: 

1234 

CH=CH— CH=CH 


If  the  above  scheme  correctly  expresses  the  relationships  of  the 
atoms  in  compounds  which  contain  two  adjacent  pairs  of  doubly 
bound  carbon  atoms,  it  is  easy  to  see  why  addition  takes  place 
primarily  in  the  1-4  positions,  for  the  adding  atoms  or  groups  will 
naturally  attach  themselves  so  as  to  neutralize  the  free  residual 
affinities  of  the  end  carbon  atoms.  Since  the  adding  atoms  will  them- 
selves require  more  affinity  for  saturation  than  is  afforded  by  the 
partial  valencies,  addition  will  be  immediately  followed  by  a  rupture 
of  the  double  bonds  joining  the  pairs  of  atoms  1-2  and  3^.  The 
free  affinities  on  2  and  3  will  then  combine  to  form  a  double  bond 
which  will  replace  the  inactive  single  bond  between  the  central 
atoms.  In  this  way  the  formation  of  symmetrical  addition  products 
may  be  readily  accounted  for  in  all  cases. 

Substances  which  possess  conjugated  double  linkages  are  more  highly 
saturated  than  substances  which  possess  two  separate  pairs  of  double 
bonds,  and  should,  therefore,  show  lower  heats  of  combustion.  In 
general  this  may  be  said  to  be  actually  the  case. 

der  Chemie,  308,  333  (1899). 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES        39 

Conjugated  systems  may  be  made  up  of  heterogeneous  pairs  of 
atoms,  as,  for  example, 


0=C— C=0,    CH=CH— C=O,  etc. 

and  the  question  naturally  arises  as  to  whether  the  same  general  rules 
for  addition  apply  in  all  cases.  This  question  may  be  answered  by 
considering  the  chemistry  of  benzil,  for  example,  to  which  either  one 
or  the  other  of  the  following  structural  formulas  may  be  assigned : 

CeHs  CeHs 

or       i     2 

;=o 


If  hydrogen  adds  in  the   1-4   position,   dihydroxystilbene   should   be 
formed 


HO—  C=C—  OH 

but  such  is  not  the  case  since  benzil  on  reduction  gives  a  ketone-alcohol 
combination,  i.e., 

C6H5 


O=C  --  CH—  OH 

At  first  sight  this  result  would  seem  to  indicate  3-4  addition,  but  such 
is  not  necessarily  the  case  since  it  is  possible  to  assume  that  the  unsatu- 
rated  alcohol,  dihydroxystilbene,  is  formed  as  a  primary  product  in 
the  reduction  and  then  immediately  rearranges  to  give  the  stable  com- 
bination benzoin  : 

C6H5-C=0  C6H5—  C—  OH            C6H5-CO 

I          +H2  ->                ||  -*                | 

C6H5'C=0  C6H5—  C—  OH            CeHs-CHOH 

I  II                               III 

Benzil  Isobenzoin  or  Stilbendiole  Benzoin 

In  fact,  it  has  been  observed  frequently  that  attempts  to  prepare  the 
unsaturated  atomic  grouping  C(OH)=C  result  in  the  formation  of  a 
ketone  linking  O=C—  CH. 

In  his  interesting  research  on  isobenzil,  H.  Klinger1  has  shown 
that  acid  chlorides  are  reduced  according  to  the  following  equation: 

RCOH  RCOCOR 

4RCOC1  +  4H=     ||        +  2RCOC1=      ||  +  4HC1 

RCOH  RCOCOR 

lEer.,  24,  1268,  1271  (1890);  31,  1217  (1898). 


40  THEORIES  OF  ORGANIC  CHEMISTRY 

In  an  effort  to  prove  that  dihydroxystilbene  is  actually  formed  as 
an  intermediate  product  in  the  reduction  of  benzil,  Thiele  attempted 
to  stop  the  reaction  at  this  stage,  and  he  succeeded  in  doing  this  by  the 
ingenious  device  of  carrying  out  the  reduction  in  the  presence  of  a 
protective  solvent,  namely,  acetic  anhydride  and  concentrated  sul- 
phuric acid.  In  this  way  the  hydroxyl  groups  were  acetylated  as 
rapidly  as  they  were  formed  and  before  sufficient  time  had  elapsed 
to  allow  for  the  rearrangement  of  the  dihydroxystilbene  into  benzoin. 
As  products  of  the  reduction  Thiele  obtained  two  diacetates  which 
were  found  to  be  stereoisomers  and  which  were  different  from  the 
diacetate  obtained  by  the  acetylization  of  benzoin.  They  were 
regarded  by  Thiele  as  the  diacetyl  derivatives  of  dihydroxystilbene, 
and  as,  in  general,  offering  valuable  confirmation  of  his  theory.1 

In  atomic  groupings  such  as  CH==CH  •  CH==CH  and  O=C-C=O, 
the  adjacent  pairs  of  doubly  bound  carbon  atoms  are  identical  in 
character,  and  the  partial  valencies  on  the  atoms  2  and  3  are  therefore 
of  equal  strength,  and  should  exactly  neutralize  each  other  to  form 
an  inactive  double  bond.  The  case  is  somewhat  different  when  the 
conjugated  system  is  made  up  of  unequal  pairs  of  doubly  bound  atoms 
as,  for  example,  in  the  grouping  CH==CH-CH==0.  Here  it  is 

1  234 

necessary  to  suppose  that  the  partial  valencies  of  CH=CH  are  of  dif- 
ferent relative  strength  from  those  of  C=0,  and  that  therefore  an 
inactive  double  bond  between  2  and  3  is  impossible.  In  an  exchange 
of  affinity  between  these  carbon  atoms  either  one  or  the  other  must  be 
left  with  a  residue  of  free  affinity,  or  in  other  words,  with  a  partial 
valency  of  a  lower  order. 

Such  a  mixed  system  of  conjugate  double  linkages  may  or  may  not 
react  as  a  unit.2  It  is  present  in  unsaturated  aldehydes  and  ketories, 
and  its  behavior  on  reduction  may  be  considered  in  the  case  of  benzyl- 
ideneacetone.  According  to  earlier  views  ready  reduction  of  the  car- 
bonyl  group  in  this  substance  was  to  be  expected: 


C6H5CH=CHC=0  +  H2  ->  C6H5CH=CHCHOH 

As  a  matter  of  fact  this  reaction  does  not  take  place,  the  reduction 
product   being    CeHsCH^CEbCOCHs.     The    formation    of    this    sub- 


1  For  other  interpretations  compare  Werner,   Chem.  Zeitung,  1,  4;  also  Erlen- 
meyer,  Jr.,  Jour,  prakt.  Chemie,  66,  351  (1902). 

2  For  a  detailed  discussion  in  regard  to  the  "  Distribution  of  affinity  in  organic 
compounds  "  see  Borsche,  Annalen  der  Chemie,  375,  147  (1910). 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES         41 


stance  may  be  explained,  however,  in  the  terms  of  Thiele's  theory, 
by  supposing  that  the  primary  reaction  is  one  of  1-4  addition : 


CH3 
I 
C6H5-CH=CH—  C=0 


H2 


C6H5-CH=CH— C=O 


C6H5  •  CH2— CH=C— OH 


and  that  this  is  followed  by  the  immediate  rearrangement  of  the 
unstable  enol  combination  into  the  corresponding  tautomeric  ketone, 
viz.,  benzylacetone  CeHsCH^CH^COCHa.  The  latter,  being  the 
more  stable  modification,  forms  the  main  product  of  the  reaction. 
In  this  way  it  is  easy  to  understand  why  the  carbonyl  group,  which 
usually  reduces  so  readily,  fails  in  this  case  to  give  — CHOH. 

Harries l  has  shown  that  the  reduction  of  unsaturated  ketones 
may  readily  take  a  course  which  involves  the  interaction  of  two  mole- 
cules. Benzylideneacetone,  for  example,  may  react  in  the  following 
way: 


V^JLJ- 

CH    + 

1 

CH    +   H2  = 

V^O-J. 

CH2 

V^-LJL 

CH2 

1 

CO 

CO 

CO 

CO 

CH3 

CH3 

CH3 

CH3 

In  such  cases  condensation  always  takes  place  in  the  /?-  and  never 
in  the  a-position  with  respect  to  the  carbonyl.  While  previous  to  this 
time  there  had  seemed  to  be  no  good  reason  for  this  behavior  it,  too, 
may  be  readily  explained  in  terms  of  Thiele's  theory.  For  since  hydro- 
gen has  a  greater  affinity  for  oxygen  than  for  carbon,  one  atom  of 
hydrogen  might  naturally  add  first  to  oxygen  in  the  4-position  giving 


CH3 

1  2  3\        4 

C6H5-CH— CH=C— OH 


CH3 
or     C6H.5CH-CH2— C=O 


1  Annalen  der  Chemie  296,  295  (1897). 


42 


THEORIES  OF  ORGANIC  CHEMISTRY 


and  two  such  residues  might  then  combine  by  the  saturation  of  partial 
valencies  in  the  1 -positions: 


A 


n 


CH 


+ 


C-CH3 

AH 


CH      =  CH      - 

II  II 

C-CH3  C-CH3 

OH  OH 


1 

CeHs 

1 

OGHs 

1 

CH 

1 

CH  

1 

—  CH 

1 

CH 

CH2 

CH2 

C-CH3 

CO 

<4o 

1 

I 

I 

OH 

CH3 

CH3 

The  reduction  of  unsaturated  acids  is  also  explained  in  an  unusually 
plausible  manner.  Ease  in  reduction  had  been  shown  by  Baeyer  l 
to  depend  upon  the  proximity  of  the  ethylene  to  the  carbonyl  groups, 
and  this  may  be  accounted  for  in  the  following  way: 


and  again, 


HOC=0_ 

HOCOH 

HOC=O 

o 

II 

1 

CH 

CH 

CH2 

ii 

CH 

CH 

AH 

D        4 

-  H2  ->           || 

-*                           || 

CH 

CH 

CH 

AH 

AH 

CH2 

HO-C=O..... 

HOCOH 

HOC=O 

Muconic  acid 

Dihydromuconic  acid 

I 

j 

HO—  C=O  

HO—  C—  OH 

HO—  C=O 

CH 

CH 

CH2 

r  4 

"                  CH 

1 
CH2 

HO—  C=O  

Fumaric  acid 

HO—  C—  OH 

HO—  C=O 

Succinic  acid 

II 

Intermediate  products  corresponding  to  I  and  II  have  not  as  yet  been 
obtained,  but  this  may  be  readily  explained  as  due  to  instantaneous 
tautomeric  rearrangements. 

The  crotonic  acids  are  known  to  be  much  less  readily  reducible  than 
fumaric   and   maleic   acids.     This   difference   in   behavior   would   be 

1  Loc.  cit. 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES         43 

difficult  to  explain  on  the  assumption  that  in  these  reduction  reactions 
hydrogen  adds  to  the  unsaturated  ethylene  linkages,  but  may  be  readily 
interpreted  by  means  of  Thiele's  formulas  for  these  substances,  i.e., 

HOC=0 H0-C=0 

CH  CH 

and 
CH  CH 

HOC=0 CH3 

if  the  fact  is  borne  in  mind  that  hydrogen  has  a  much  greater  affinity 
for  oxygen  than  for  carbon. 

The  addition  of  halogen  to  systems  of  conjugated  double  bonds 
takes  place  in  much  the  same  way  as  has  been  described  in  the  case 
of  hydrogen.  A  study  of  the  addition  of  the  halogen  acids  to  a-,  /3-,  un- 
saturated acids  has  shown  that  the  halogen  atom  regularly  assumes 
a  /3-position  with  reference  to  the  carboxyl  group  of  the  acid,  and 
the  phenomenon  has  been  represented  by  means  of  the  following 
equation : 

X  X 

CH     Br     CHBr 
CH   +  H  =  in2 
COOH         COOH 

It  was,  however,  impossible  to  explain  this  behavior  in  any  rational 
way.  The  facts  were  simply  accepted  as  an  empirical  law  and  all 
attempts  to  interpret  them  were  dismissed  dogmatically  by  saying: 
"  halogen  is  repelled  by  carboxyl."  The  negative  character  of  the 
two  might  be  supposed  to  account  for  this,  were  it  not  that  the  NH2 
group,  which  is  commonly  regarded  as  positive  in  character  and  should 
therefore  be  attracted  by  carboxyl,  also  adds  to  unsaturated  a-,  /3- 
acids  in  the  /3-position.  Thus  the  positive  or  negative  character  of  the 
adding  atoms  or  groups  of  atoms  does  not  seem  to  influence  the  reaction. 
Thiele's  theory,  on  the  other  hand,  offers  a  very  plausible  elucidation 
of  the  phenomena.  In  terms  of  this  theory,  free  residual  affinity 
exists  in  two  positions  in  the  acid  molecule, 

OH 
X-CH=CH— C=O 


44  THEORIES  OF  ORGANIC  CHEMISTRY 

and  in  all  additions  involving  hydrogen  it  may  be  assumed  that 
preference  will  be  given  to  the  4-position.  Hydrobromic  acid  will 
therefore  react  according  to  the  following  scheme:  . 

XXX 

CH Br  CHBr  CHBr 

Rearrangement 
CH       +  ->  CH  CH2 

HO— C  HO— C  HO— C 

4 H  in  A 

and  the  same  general  diagram  may  be  used  to  explain  additions  involv- 
ing H-C1,  H-I,  H-OH,  H-NH2,  acids,  etc. 

1-4  addition  to  conjugated  systems  is  not,  however,  a  rule  which 
has  no  exceptions.1  Thiele,2  himself,  has  pointed  out  that  double 
bonds,  present  in  such  systems,  may  at  times  react  quite  independ- 
ently of  one  another.  Such  independent  action  may  be  expected 
when  the  adding  atoms  or  groups  possess  a  strong  affinity  for  a  par- 
ticular pair  of  the  atoms  which  are  held  together  by  double  bonds. 
In  the  case  of  the  complex  C=C — C=0,  for  example,  it  may  be 
anticipated  that  halogen  will  favor  ethylene,  and  that  HCN  on  the 
other  hand  will  add  preferably  to  the  carbonyl  group.  Hydrogen 
alone  adds  almost  exclusively  in  the  1-4  positions.3  The  halogens 
react  very  irregularly,  as  is  shown  from  a  consideration  of  the  following 
facts:  butadiene,  CH2=CH-CH=CH2,  adds  bromine  in  the  1-4 
positions,  giving  CH2BrCH=CHCH2Br  as  the  main  product  of  the 
reaction,  although  small  quantities  of  the  isomer  CH2BrCHBrCH=CH2 
are  also  formed;  phenylbutadiene,  C6H5CH=CHCH=CH2,  on  the 
other  hand,  adds  bromine  almost  exclusively  in  the  3-4  positions 
while  diphenylbutadiene,  C6H5CH=CHCH=CHC6H5,  gives  a  96  per 
cent  yield  of  the  3-4  dibrom  addition  product  and  it  is  even  ques- 
tionable whether  1-4  addition  takes  place  at  all  in  this  case. 

lErlenmeyer,  Annalen  der  Chemie,  316,  43  (1901);  Vorlander,  Annalen  der 
Chemie,  320,  73  (1902);  345,  206  (1906);  Michael,  Jour,  prakt.  Chemie,  60,  467 
(1899);  68,  503,  512  (1903);  75,119;  Thiele  and  Hackel,  Annalen  der  Chemie, 
325,  6  (1902);  Hinrichsen,  Zeitschr.  physikal.  Chemie,  39,  308  (1902);  Ber.,  37, 
1121  (1904);  Annalen  der  Chemie,  336,  168  (1904);  Flurscheim,  Jour,  prakt. 
Chemie,  71,  503  (1905);  Bauer,  Jour,  prakt.  Chemie,  72,  206  (1905);  Bamberger, 
Ber.,  40,  2239  (1907);  Kohler,  Chem.  Centralbl.,  1908,  I,  226;  Meyer  and  Jacob- 
son's  Lehrbuch  der  organischen  Chemie,  Vol.  I,  795  (2d  edition). 

2  Annalen  der  Chemie,  306,  106  (1899). 

3  Ber.  42,  2872  (1909). 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES        45 

F.  Straus  1  sums  up  the  situation  in  regard  to  the  addition  of 
bromine  to  conjugate  systems  of  ethylene  linkages  when  he  says, 
11  1-4  addition  may  take  place,  but  it  is  not  the  rule.  No  single 
instance  has  as  yet  been  observed,  at  least  in  connection  with  open 
chain  compounds,  where  1-4  addition  takes  place  exclusively." 

Although,  as  Wieland2  has  pointed  out,  nitrogen  dioxide  usually 
resembles  halogen  in  addition  reactions,  in  the  case  of  diphenylbutadiene 
it  reacts  quite  differently,  adding  in  the  1-4  positions  to  give  1^1- 
dinitrodiphenylbutylene, 

C6H5CH(NO2)CH=CH— CH(N02)C6H5 

Hinrichsen  3  attempts  to  explain  this  difference  in  behavior  by  supposing 
that  the  mutual  repulsion  of  nitro  groups  is  considerably  greater  than 
that  of  bromine  atoms.  Thus  while  the  bromine  molecule  dissociates 
only  at  high  temperatures,  the  dissociation  of  N2O4  into  2NO2  takes 
place  readily  and  at  relatively  low  temperatures.  The  relationships 
involved  in  additions  to  conjugated  systems  are  undoubtedly  very 
complicated  and,  in  the  words  of  Hinrichsen,  are  "  conditioned  not  only 
by  kinetic  and  stereochemical  influences,  but  quite  as  much  by  the  purely 
qualitative  affinities  operating  between  the  adding  atoms  themselves, 
or  between  them,  on  the  one  hand,  and  the  atoms  or  groups  of  atoms 
present  in  the  unsaturated  molecule."  According  to  this  investigator, 
two  types  of  reaction  may  be  differentiated : 

I.  Where  the  components  of  the  adding  molecules  are  electno- 
chemically  different  in  character.  The  components  in  this  case  exercise 
an  attraction  for  each  other  and  will  attach  themselves  to  positions 
as  close  together  as  possible.  This  condition  favors  1-2  addition 
and  is  illustrated  by  the  following  equation : 4 

C6H5CH=CH— CH=C(COOC2H5)2 

+  HCN  ->  C6H5CH=CH-CH-CH(COOC2H5)2 


Independent  additions  are  naturally  more  frequent  in  the  case  of  hetero- 
geneous systems.  Thus,  in  the  action  of  HCN  upon  cinnamic  aldehyde, 
the  carbonyl  group  alone  is  affected, 

C6H5CH=CHCHO+HCN  ->  C6H5CH=CHCHOHCN  5 


1  Ber.,  42,  2872  (1909). 

2Annalen  der  Chemie,  360,  306  (1908;. 

3Chem.  Zeitung  (1909),  1098. 

4  Compare  also  Ber.,  44,  2974  (1911). 

6  Annalen  dcr  Chemie,  306. 


46  THEORIES  OF  ORGANIC  CHEMISTRY 

and  this  is  also  true  in  the  addition  of  methyl  magnesium  iodide.1 
Cinnamoyl  formic  acid  reduces  to  give  primarily  phenyl  a-hydroxyiso- 
crotonic  acid  :  2 

C6H5CH=CHCO  •  COOH+H2  ->  C6H5CH=CHCHOH  •  COOH 

On  the  other  hand,  Harries  3  and  his  students  have  shown  that  in  the  re- 
duction of  unsaturated  ketones  of  the  general  formula  R2C=CH  —  COR, 
the  ethylene  linkage  is  the  one  which  is  primarily  attacked  while  the 
carbonyl  is  affected  only  secondarily.  The  same  is  true  in  additions  of 
hydroxylamine. 

According  to  Vorlander4  the  ethylene  linkage  alone  is  involved  in 
reactions  between  sodium  malonic  ester  and  derivatives  of  cinnamic 
acid: 

X  NaX 

C6H5-CH=CH—  C=0  +  NaCH(COOC2H5)2  =  C6H5  •  CH—  CHC=O 

CH(COOC2H5)2 

Even  in  the  case  of  two  conjugated  ethylene  linkages,  1-2  or  3-4 
addition  frequently  takes  place.  Thus,  according  to  Hinrichsen,5 
bromine  adds  to  cinnamylidene  derivatives  according  to  the  equation: 

/CN(C6H5,  or  COOC2H5) 

C6H5  -  CH=CH—  CH=C<  +  Br2 

X€OOC2H5 

/CN(C6H5,  or  COOC2H5) 
=  C6H5-CH—  CH—  CH=C< 
Br      Br 


Phenyl,   or  carbethoxy  COOC2Hs,  may  be  substituted  for  cyanogen 
without  affecting  the  course  of  this  reaction. 

The  following  substances  add  to  double  unions  in  a  manner  analogous 
to  that  described  in  the  case  of  hydrogen  cyanide,  viz.,  —  p-toluene 
sulphinic  acid,  C7H7SO2H;  acid  potassium  sulphite,  HKSOs;  ammonia, 
NHs;  the  sodium  salt  of  malonic  ester,  NaCH(COOR)2;  sodium 
ethylate,  NaOC2H5j  benzyl  mercaptan,  CeHsCH^H;  and  hydroxyl- 
amine NH2OH. 

II.  Where  the  components  of  the  adding  molecules  are  electro- 
chemically  similar  in  character  either  one  of  two  things  may  happen: 

ifier.,  36,  2529  (1903). 

2Ber.,  35,  2649  (1902). 

"Ber.,  28,  150  (1895);  29,  375,  380;  30,  230  (1897);  Posner,  36,  4305  (1903),  etc. 

'Annalen  der  Chemie,  320,  60  (1902);  Ber.,  36,  172,  2339  (1903). 

5Annalen  der  Chemie,  336,  323  (1904). 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES        47 

(a)  The  mutual  repulsion  of  the  two  parts  of  the  adding  molecule  may 
be  reinforced  by  the  character  of  the  components  of  the  unsaturated 
molecule.  This  is  illustrated  in  the  addition  of  N2O4  to  diphenyl- 
butadiene,  to  which  reference  has  previously  been  made.  Another 
example  is  the  addition  of  hydrogen  to  phenylcinnamylacrylic  acid: 


C6H5  •  CH=CH— CH=C<  +  H2 

\COOH 

r*  TT 

/^6Al5 

=  C6H5-CH— CH=CH— C< 

jj  jjXJOOH 

In  other  words,  in  reactions  involving  an  addition  of  hydrogen, 
this  element  tends  to  attach  itself  to  the  carbon  atoms  holding  negative 
groups  (C6H5,  COOH,  etc.). 

(6)  The  two  parts  of  the  adding  molecule  may  be  definitely 
attracted  by  atoms  or  groups  of  atoms  present  in  the  unsaturated 
molecule.  Such  a  condition  may  be  supposed  to  account  for  the 
addition  of  bromine  to  cinnamylidene  malonic  ester,  and  also  hydrogen 
to  dibenzalpropionic  acid.  In  the  case  of  the  malonic  ester  3-4  addi- 
tion is  impossible  since  the  two  negative  carbethoxy  groups  actively 
repel  the  bromine  and  1-2  addition  takes  place  exclusively: 

C6H5CH=CH— CH=C(COOR)2  +Br2 

1234 

=  C6Hs— CHBr-CHBr-CH=C(COOR)2 

The  addition  of  hydrogen  to  dibenzalpropionic  acid  is  expressed  as 
follows : 

C6H5-CH=CH—C=CH— Cells  +  H2 


COOH 


=  C6H5-CH=CH— C CH2— C6H5 

"COOH 


Here  again,  in  reactions  involving  an  addition  of  hydrogen,  this 
element  tends  to  join  with  the  carbon  atoms  holding  negative  groups 
(C6H5,  COOH,  etc.). 

Erlenmeyer,  Jr.,1  even  goes  so  far  as  to  say  that  it  is  impossible 
to  predict  how  hydrogen  and  bromine  will  add  to  conjugate  systems 

i  Jour.  Prakt.  Chemie,  66,  354  (1902). 


48  THEORIES  OF  ORGANIC  CHEMISTRY 


of  the  type  Ri=R2 — R3=R4-     The  reaction  may  take  place  according 
to  him  in  any  one  of  three  ways,  viz. : 

I  II  III 

Ri— R2=R3— R4          Ri— R2— R3=R4         Ri=R2— Ra— R4 
H  H  H     H  H      H 

and  the  result  can  be  determined  only  by  experiment. 

If  the  conditions  favoring  addition  reactions  are  reversed,  it  should 
follow  that  atoms  may  be  split  off  from  these  various  addition  prod- 
ucts with  the  result  that  the  original  system,  Ri=R2 — R3=R4  is 
formed.  Such  a  conclusion  has  been  demonstrated  experimentally 
Thus  when  Thiele  reduced  the  dibromides, 

COOH 
C6H5  •  CH— C=CH— CH— C6H5 

and    C6H5-C— CH=CH— CH.C6H5 
Br      COOH      Br 

Br  Br 

by  the  action  of  zinc  dust  and  acetic  acid,  he  obtained  the  acids, 
C6H5  •  CH=C— CH=CH  •  C6H5  COOH 

COOH  and         C6H5  •  C=CH— CH=CH  •  C0H5 

Indeed  he  was  even  able  to  split  off  hydrobromic  acid  from  the  1-4 
positions.     Thus  the  following  change: 

COOH 
C6H5  •  C-CH=CH  •  CHBr  •  C6H5 


COOH 

-»  C6H5-C=CH— CH=CBr-C6H5  +  HBr 

was  brought  about  when  the  dibromide  was  treated  with  potash.1 

Conjugate  systems  consisting  of  three  pairs  of  doubly-bound  atoms 
may  now  be  considered,  viz., 

etc. 
C=C— C=C— C=O 

A  study  of  atomic  groupings  of  this  type  shows  at  once  that  the 
distribution  of  affinity  is  such  that  partial  valencies  appear  only  on 

1  Annalen  der  Chemie,  306,  109;  316,  46-49;  323,  217;  Ber.,  40,  2239  (1908). 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES        49 

atoms  which  are  located  at  the  ends  of  the  chain.  The  magnitude 
of  the  residual  free  affinity  in  these  positions  may  be  assumed  to  be 
relatively  great,  but  need  not  manifest  itself  in  increased  chemical 
reactivity,  since  it  is  quite  possible  that  the  partial  valencies  on  terminal 
atoms  may  mutually  saturate  each  other  according  to  the  following 
scheme: 

HC 


.CH        C 


Such  an  arrangement  would  obviously  tend  to  decrease  rather  than 
increase  the  chemical  reactivity  of  the  substance. 

It  should  be  noted  at  this  point  that  groups  such  as  —  CH2C1, 

—  CHC12,  —  CCla,  or  in  general   /CC1,  may,  according  to  H.  Finkel- 

stein,1  be  regarded  as  unsaturated  and,  therefore,  capable  of  forming 
addition  products.  Thus  the  reduction  of  a  halide  is  comparable  to 
the  reduction  of  a  ketone  to  its  alcohol. 

R2C=0  +  H2    =  R2C=0  ->  R2CH—  OH 

11  i  i 

R3C—  Cl  +  H2    =  R3C—  Cl  ->  R3CH+HC1 
ft    H 

The  interaction  of  silver  hydroxide  and  magnesium  with  halides  is 
expressed  as  follows: 

R3C—  Cl  +  AgOH  =  R3C—  Cl  ->  R3COH+AgCl 

6HAg 

R3C—  Cl  +     Mg     =  R3C—  Cl  -+  R3C—  MgCl. 

!      i  V 

Mg 

If  such  unsaturated  halide  groups  occupy  positions  in  the  molecule  which 
are  adjacent  to  other  unsaturated  groups,  conjugate  systems  may 
arise.  Speculation  along  this  line  has  led,  in  a  number  of  instances, 
to  very  plausib  e  interpretations  of  difficult  phenomena.  Thus,  for 
example,  acid  chlorides  do  not  react  spontaneously  with  metals  like 
zinc,  while  the  corresponding  halogen  substitution  products  under  the 

1  "  Verhandl.  d.d.  Naturforscherversammlung,"  1911,  II,  176  and  following. 


50  THEORIES  OF  ORGANIC  CHEMISTRY 

same  conditions  readily  give  ketones.  The  cause  of  the  increased 
chemical  activity  in  the  second  case  is  indicated  by  the  following 
formulas  which  show  the  addition  of  zinc  to  a  conjugate  system  of 
this  type: 


: 
1  +  Zn  ->        C—  C—  Cl 

^  ---  -- 

i    ii 

Cl   O 

Y 

R\  /Cl 

_+      >C=C<  ->      >C=C=0+ZnCl2 

W  xOZnCl       R/ 

If  three  or  more  conjugate  double  bonds  are  arranged  not  in  a 
straight  but  in  a  branched  chain,  as  for  example, 

1234  4 

E=E—  E=E  E 

or      i      2      |  |s      5     6 
E=E  E=E—  E—  E=E 

5        6 

they  form  what  Thiele  calls  systems  of  "Crossed  Double  Bonds"; 
and  may  be  regarded  as  made  up  respectively  of  two  separate  systems 
namely  : 

1-2-3-4     and    4-3-5-6 

If  E  represents  atoms  of  the  same  element  it  follows  that  the  partial 
valency  of  3  will  exactly  equal  that  of  1,  2,  4,  5  or  6  respectively,  and 
if  this  is  the  case,  it  obviously  cannot  neutralize  completely  the  free 
affinity  of  both  2  and  5.  The  atoms  2  and  5  must,  therefore,  continue 
to  carry  a  certain  residue  of  free  affinity  (equal  to  one-half  of  the 
partial  valency).  '  The  distribution  of  free  energy,  therefore,  in  such  a 
system  may  be  expressed  by  means  of  the  following  diagram: 

E  ...... 

II 
E=E  —  IL  —  E  —  E 


If  the  atoms  are  not  all  exactly  alike,  the  relative  values  of  the  resid- 
ual affinities  will,  of  course,  be  different  from  that  which  is  represented 
above. 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES        51 

It  should  be  noted  at  once  that  not  only  is  the  relative  free  affinity 
of  the  terminal  atoms  greater  than  in  the  case  of  simple  conjugate 
systems,  but  the  total  quantity  of  free  affinity  present  in  the  molecule 
is  also  greater.  The  latter  fact  is  demonstrated  in  a  most  convincing 
way  by  a  study  of  the  physical  constants  of  this  class  of  substances, 
but  a  detailed  discussion  of  this  important  aspect  of  the  subject  must 
be  reserved  for  another  chapter. 

The  chemical  reactions  of  such  a  system  may  be  considered  in 
connection  with  dibenzalpropionic  acid,1  which  serves  as  a  convenient 
illustration : 

C6H5CH 

II     °H 

1  2  3        ft|        6 

C6H5  •  CH=CH— C— 0=0 


The  two  conjugate  systems  which  are  present  in  the  molecule,  are 
respectively  1-2-3-4-  and  4-3-5-6.  Which  of  these  two  systems 
will  take  part  in  a  given  reaction  will  depend  largely  upon  the  nature 
of  the  reagent  employed.  Thus  bromine  adds  to  the  system  1-2-3—4 
since  its  affinity  for  carbon  is  greater  than  its  affinity  for  oxygen. 
The  main  product  of  the  reaction  in  this  case  is  the  dibromide, 

CeHgCHBr 
I     OH 

C6H5  -  CH— CH=C— C=0 

Br 
showing  1^:  addition;  but  the  isomeric  dibromide, 

C6H5-CH 

OH 

C6H5-CH— CH- 


L  L 


representing  1-2  addition,  is  also  formed  as  a  secondary  product. 
Hydrogen  and  hydrobromic  acid,  on  the  other  hand,  add  to  the  system 
4-3-5-6,  since  the  affinity  of  hydrogen  is  greater  for  oxygen  than  for 
carbon.  In  the  case  of  hydrogen  the  reaction  results  in  the  formation 
of  C6H5CH=CH— CH(COOH)-CH2C6H5. 

1  For  other  illustrations  see  Annalen  der  Chemie,  306,  115. 


52  THEORIES  OF  ORGANIC  CHEMISTRY 

The  application  of  Thiele's  theory  to  the  chemistry  of  naphthalene 
and  related  ring  compounds  is  very  interesting  and  will  be  considered 
briefly.  It  has  already  been  noted  in  connection  with  straight  chain 
conjugate  systems  that  the  partial  valencies  on  the  terminal  atoms  may 
mutually  saturate  each  other  if  brought  into  juxtaposition,  and  strictly 
analogous  conditions  may  arise  in  the  case  of  systems  of  crossed  double 
bonds.  It  has  been  observed,  for  example,  that  while  dibenzalacetone, 

o 

C6H5—  <bH          C  CH—  C6Hs 


readily  adds  ethylacetoacetate  in  the  presence  of  pyridine  as  a  con- 
densing agent,  dicinnamalacetone, 

C6H5CH=CH  •  CH=CHCOCH=CH  •  CH=CHC6H5 

does  not.  The  inactivity  of  the  latter  compound  is  interpreted  by 
Borsche  l  as  due  to  the  intramolecular  saturation  of  the  partial  valencies 
on  the  terminal  carbon  atoms  and  may  be  represented  diagrammatically 
in  the  following  manner: 

Cells   C&H.5 


HC 

S  N  /  \ 

HC        O       CH 

II         I 
C       CH 


C        C 
H        H 


It  has  also  been  observed  that  dianisalacetone, 

O 
—  C 


H3CO  •  C6H4  •  CH=CH—  C—  CH=CH  •  C6H4  •  OCH3 

1234  5 

and  tetramethyl-p-p'-diamidodibenzalacetone  : 

O 
(CH3)2N  •  C6H4  •  CH=CH—  C—  CH=CH  -  C6H4  •  N(CH3)2 

1  2345 

1Annalen  der  Chemie,  375,  152  (1910). 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES         53 

fail  to  react  with  ethyl  acetoacetate,  while  compounds  like  benzalanisal- 
acetone, 

O 

C6H5  -  CH  =  CH— C— CH=CH  •  C6H4OCH3 

1  2345 


p-dimethylamidodibenzalacetone  and  benzalcinnamalacetone 

0 
C6H5  •  CH=CH— C— CH=CHC6H4  •  N(CH3)2 

12345 


O 

C6H5-CH=CH—  C  —  CH=CH-CH=CHC6H5 

12  345 


which  contain  at  least  one  CH^CHCeHg  group,  still  react  with 
this  /3-ketone  ester,  although  much  less  readily  than  dibenzalacetone 
itself.  Borsche  explains  the  interference  offered  by  the  substitution 
of  the  — CeEUOCHs  and  the  — CeH^-N^Hs^  groups  by  supposing 
that  the  partial  valencies,  which  are  present  on  the  oxygen  and  nitro- 
gen atoms  of  these  radicals,  are  saturated  by  those  which  are  present 
on  the  carbon  atoms  1  and  5.  The  free  affinity  on  the  oxygen  of  the 
carbonyl  group  is  simultaneously  decreased  by  the  influence  of  the 
CHsO  and  (CHs^N  groups.  These  relationships  may  be  expressed 
by  means  of  the  following  formulas  which  explain  themselves: 


Y 


C6H5— CH==CH— C— CH=CH— CeHs 

1  2345 

Dibenzalacetone 


C— CH=CH.C6H5 
CH 


H 

Benzalanisalacetone 


54  THEORIES  OF  ORGANIC  CHEMISTRY 


(CH3)2Nx     >v     £  j 

c~  CH=CH  • 


U 


H 

Dimethyl-p-amidodibenzalacetone 


CH     CH  C 


H 


O       CH  CH    O 

Q      II       D  1 

CH    C        CH  C 


H     C 


CH    CH  CH     CH 

Dicinnamalacetone  Benzalcinnamalacetone 

The  relative  reactivity  of  unsaturated  linkages  in  different  sub- 
stances seems  to  depend  upon  the  character  of  the  substituents  which 
are  in  union  with  the  doubly  bound  atoms,  although  as  yet  only  a  few 
quantitative  experiments  have  been  made  to  determine  the  exact 
extent  of  such  influences.  An  investigation  of  this  nature  has,  however, 
recently  been  undertaken  by  H.  Staudinger  1  and  N.  Kon  in  connection 
with  their  interesting  researches  on  ketenes.  Diphenylketene,2 
(C6H5)2C=C=O,  was  treated  at  131°  with  various  aldehydes  and 
ketones  of  the  general  formula,  CeHsCH  :  CHCOR,3  when  it  was 
observed  that  the  carbonyl  group  adds  in  every  case  directly  to  the 
ketene.  This  change  is  immediately  followed  by  the  decomposition 
of  the  addition  product  with  the  evolution  of  carbon  dioxide  and 
the  formation  of  an  ethylene  derivative.  The  complete  reaction  is 
expressed  by  the  following  equation  : 

(C6H5)2C=C=O  +  R2C=O4  =  (C6H5)2C—  C=0 

R2C—  O 

(C6H5)2C—  C=0       (C6H5)2-C       C=0 

II  11  +  11 

R2C—  O  R2C       O 

1  Annalen  der  Chemie,  384,  38  (1911);  387,  254  (1912). 

2  In  the  form  of  diphenylketene  quinoline. 

3  Where  R  may  be  CH  =  CHC6H5,  H,  C6H5,  CH3,  OCH«,  N(C6H6)2. 

4  See  Staudinger  "  Die  Ketene,"  Stuttgart,  Enke,  1912. 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES         55 

It  is  possible  to  study  the  relative  reactivity  of  the  carbonyl  group 
in  different  compounds  by  observing  the  relative  speed  of  this  reaction. 
Although  it  is  impossible  to  exclude  side  reactions  entirely  and  so  to 
obtain  invariable  constants,  it  is,  nevertheless,  possible  to  gauge  the 
reactivity  of  the  carbonyl  group  roughly  from  the  quantity  of  alde- 
hyde or  ketone  which  is  decomposed  during  the  first  hour.  Results 
seem  to  show  that  steric  influences  do  not  affect  this  reaction,  and  that 
the  degree  of  unsaturation  of  the  carbonyl  group  depends  upon  differ- 
ences in  the  relative  strength  of  the  partial  valencies  rather  than  upon 
differences  in  the  electrochemical  character  of  the  atoms. 

It  was  observed  that  the  carbonyl  group  is  most  inert  when  present 
in  acid  derivatives,  especially  esters  and  chlorides.  In  general,  com- 
pounds which  contain  carbonyl  in  close  proximity  to  CHs,  CH^R, 
or  CHR2  were  found  to  be  very  unreactive.  The  highest  degree  of 
unsaturation  was  observed  in  the  case  of  substances  which  contain 
carbonyl  in  union  with  unsaturated  groups  as,  for  example, 

C6H5CH=CHCOC6H5     and     (C6H5CH=:CH)2CO,    etc. 


although  aromatic   ketones,  such    as    benzophenone  (CeHs^CO,  and 
aldehydes  of  the  type  RCHO,  are  also  reactive. 

In  the  case  of  unsaturated  ketones  the  reactivity  is  not  limited 
to  the  carbonyl  group,  but  extends  to  the  entire  conjugated  system. 
To  illustrate,  C6H5CH=CH2  and  C6H5CH=O  react  only  slightly 
with  diphenylketene,  while  C6H5CH=CHCOC6H5  reacts  with  extraor- 
dinary ease,  a  difference  which  may  be  readily  understood  by  refer- 
ence to  the  partial  valency  formulas  of  these  substances,  viz., 


X 

=CH— C=O 


C6H5— CH=CH2;     C6H5— CH=O;     C6H5-CH 


Systems  of  crossed  double  bonds  were  observed  to  be  even  more 
reactive  than  simple  conjugate  systems.  Dibenzalacetone  represents 
such  a  system,  the  general  partial  valency  formula  of  which  is 


C 


56  THEORIES  OF  ORGANIC  CHEMISTRY 

To  recapitulate,  substances  possessing  conjugate  systems  of  double 
bonds  are  more  reactive  than  substances  with  simple  bonds,  while  most 
reactive  of  all  are  substances  containing  systems  of  crossed  double 
bonds.  This  statement  is  further  illustrated  by  the  fact  that  butadiene 
is  more  reactive  than  ethylene,  while  fulvene  is  the  most  reactive  of 
these  three  combinations: 

i         2 

CH=CH\  3         4 
CH2=CH2         CH2=CH  •  CH=CH2  >C=  CH2 

Ethylene  Butadiene  CH=CI:r 

6  5 

Fulvene 

The  same  relative  difference  in  the  degree  of  unsaturation  has  been 
noted  in  the  case  of  simple  ketones,  diketones,  and  triketones  where 
the  carbonyl  groups  occupy  adjacent  positions.  The  fact  that  an 
increase  in  the  unsaturated  character  of  substances  is  frequently 
associated  with  the  appearance  of  color  will  be  considered  later  in  the 
chapter  on  "  Color  and  Constitution." 

Staudinger,1  by  careful  investigation  of  the  reactivity  of  carbonyl 
in  aldehyde  combinations,  found  that  diphenylketene  and  oxalyl- 
chloride  interact  more  easily  with  p-methoxy-  and  p-dimethylamino- 
benzaldehyde  than  with  benzaldehyde.  In  other  words,  substitution 
in  the  benzene  nucleus  increases  the  unsaturation  of  carbonyl, 
and  consequently  strengthens  the  partial  valencies  of  this  group,  a 
principle  which  was  confirmed  by  Staudinger  in  his  work  on  autoxida- 
tion  of  aldehydes.  According  to  C.  Engler2  the  autoxidation  of  an 
aldehyde  may  be  expressed  as  follows: 

H 

H 


I 
C 


C6H5C=0  +  0=0  =   .C6H5C— O  -»  C6H5C< 

N)OH 
O— O 

Since  methoxy-  and  dimethylaminobenzaldehyde  are  more  reactive 
towards  ketene  than  benzaldehyde,  exhibiting  greater  partial  valency, 
it  might  be  predicted  that  these  two  aldehydes  would  be  more  sus- 
ceptible to  oxidation  than  benzaldehyde.  Staudinger  found,  however, 
that  the  reverse  was  true  and  that  the  aminoaldehyde  was  less  reactive 
than  anisaldehyde  and  the  latter  more  stable  than  benzaldehyde. 
To  explain  these  results  he  adopted  an  assumption  made  by  Baeyer 

iBer.,  46,  3520  (1913). 

2  "  Kritische  Studien  iiber  die  Vorgange  der  Autoxydation,"  pp.  89,  Braunschweig, 
1904. 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES        57 

and   Villiger,   that   oxygen   adds   to   aldehydes  unsymmetrically,   and 
interpreted  the  mechanism  of  this  reaction   (autoxidation)   as  follows: 
Equation  of  Baeyer  and  Villiger: 

.0  ,0 

+  0=0  =  C6H5-Cr 
H        i       i  X)-OH 

Staudinger's  equations: 

Q      .  Q  A 

r<  TI  r»^     _L  \n     r>      n  TT    c^v  rearrangement  n  ^  ^/f 

V^6Al5^\  r     .-'vJ=(J  — >  V_^6^l5 '  U\  ^G-tlsLxf 

XH     •-"  X)=O  X)— OH 

H 

According  to  Staudinger,  this  explanation  is  strictly  in  accord  with 
Thiele's  theory  of  partial  valency.  In  the  two  aldehydes,  CeHsCHO 
and  (CHs^N-CeH^HO,  the  hydrogen  of  the  latter  is  much  more 
firmly  bound  than  in  benzaldehyde  and  shows  less  tendency  to  add 
to  oxygen;  consequently  this  aldehyde  is  less  susceptible  to  oxidation 
influences  than  benzaldehyde.  In  other  words,  unsaturation  of 
carbonyl  groups  connotes  a  firm  union  between  carbon  and  hydrogen 
of  the  — CHO  radical.  Of  the  three  isomeric  aldehydes,  CHsO  •  CeH4  • 
CHO,  the  o-derivative  was  found  to  be  the  most  resistant,  and  the 
m-compound  intermediate  between  o  and  p  in  their  reactivity  towards 
oxygen.  In  other  words,  the  o-compound  contains  the  most  unsatu- 
rated  carbonyl  group. 

Staudinger  1  expresses  the  formation  of  benzoin  from  benzaldehyde 
according  to  the  principle  of  autoxidation  as  follows: 


H 


According  to  this  interpretation,  only  aldehydes,  which  contain 
relatively  unsaturated  carbony!  groups  and  also  relatively  mobile 
hydrogens,  polymerize  with  formation  of  benzoins.  Benzaldehyde, 
anisaldehyde  and  p-chlorbenzaldehyde  behave  in  accord  with  this 
principle.  Dimethylaminobenzaldehyde,  on  the  other  hand,  does 
not  polymerize  to  a  benzoin,  although  it  contains  a  strongly  unsatu- 
rated carbonyl  group.  If,  however,  this  is  mixed  with  another  alde- 
hyde, which  contains  a  sufficiently  mobile  hydrogen  atom,  as  benzalde- 
hyde, anisaldehyde  or  p-chlorbenzaldehyde,  mixed  benzoin  combina- 

^er.,  46,  3535  (1913). 


58  THEORIES  OF  ORGANIC  CHEMISTRY 

tions  are  formed.  These  somewhat  meager  illustrations  help  to  show 
how  fruitful  Thiele's  theory  has  been  in  the  elucidation  of  problems 
in  the  chemistry  of  aliphatic  combinations.  It  also  serves  an  even  more 
important  function  in  connection  with  the  chemistry  of  benzene  and  its 
derivatives. 

The  more  important  aspects  of  Baeyer's  theory  in  regard  to  ben- 
zene have  already  been  referred  to.  Kekule's  formula  for  benzene, 
with  its  three  pairs  of  double  linkages,  failed  to  account  for  the 
saturated  character  of  the  substance,  and  this  failure  was  the  more 
striking  because  hydro-derivatives,  possessing  one  and  two  pairs  of 
double  bonds  respectively,  showed  typically  unsaturated  properties. 
In  order  to  explain  this  difference  in  behavior  Baeyer,  in  conjunc- 
tion with  Armstrong,  advocated  the  so-called  "  centric  formula  "  for 
benzene,  in  which  six  valencies  were  represented  as  mutually  saturating 
each  other,  but  he  was  unable  to  offer  a  rational  explanation  as  to  how 
the  saturation  of  the  six  valencies  was  effected.  Such  an  explanation 
is,  however,  possible  in  terms  of  Thiele's  theory,  according  to  which 
Kekule's  formula  for  benzene  becomes : 


the  six  partial  valencies  being  represented  as  united  to  form  three 
"  inactive  double  bonds." 

That  a  substance  possessing  such  a  system  of  conjugate  double 
bonds  should  be  saturated  in  character  is  obvious.  In  the  words  of 
Thiele:  "  the  saturation  of  the  partial  valencies  renders  the  original 
three  pairs  of  unsaturated  double  bonds  inactive,  so  that  it  becomes 
impossible  to  distinguish  any  difference  between  them  and  the  three 
secondary  double  linkages."  Benzene  may  then  be  represented  by 
the  formula: 

H 

HC^ 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES        59 

Such  an  expression  obviates  all  need  for  "  Kekule's  oscillation 
hypothesis"  since  it  does  not  presuppose  a  difference  between  the 
or£/io-positions  1-2  and  1-6. 

Thiele's  theory  of  partial  valencies  not  only  explains  the  satu- 
rated character  of  benzene,  and  the  identity  of  the  two  or^o-positions 
in  its  molecule,  but  also  allows  for  a  satisfactory  interpretation  of  reduc- 
tion phenomena.  -The  transformation  of  the  saturated  benzene 
nucleus  into  its  unsaturated  hydro-derivatives  may  be  readily  under- 
stood by  referring  to  the  partial  valency  formulas  of  the  latter,  viz. : 


These  formulas  elucidate  the  thermal  relationships  already  alluded  to 
in  this  text  and  also  explain  the  observed  similarity  between  the  ortho- 
and  para-positions  as  well  as  the  isolation  of  the  raeta-position.1 

As   has   been    stated    1-4    addition   takes   place   in   the    system 
C=C — C=C  because  free    residual   affinity   exists   at   these  points. 

1234 

Addition  to  the  benzene  ring,  on  the  other  hand,  involves  a  disruption 
of  the  entire  system  and  may  result  in  either  1-2  or  1-4,  but  never 
1-3  addition.  The  conditions  which  determine  the  direction  which 
addition  reactions  will  take  in  any  particular  case  have  not  as  yet 
been  determined  with  accuracy.  The  simultaneous  appearance  of 
ortho-  and  para-substitution  products  can  easily  be  understood  in  the 
light  of  this  theory  if,  as  Armstrong2  assumes,  the  process  of  substi- 
tution consists  primarily  in  addition  reactions  which  are  accompanied 
by  intermolecular  rearrangements  and  which  ultimately  result  in  the 

1  Ber.,  17,  2719  (1884);  Armstrong,  Jour.  Chem.  Soc.,  51,  258,583  (1887);  Morley, 
Ibid.,  51,  579;    Crum,  Brown  and  Gibson,  Ibid.,  61,  367  (1892);    Fliirscheim,  Jour, 
prakt.  Chemie,  66,  321  (1902);  also  see  71  and  76. 

2  Jour..  Chem.  Soc.,  61,  258  (1887). 


60  THEORIES  OF  ORGANIC  CHEMISTRY 

splitting  off  of  simple  molecules.     The  application  of  Thiele's  theory 
to  a  few  individual  aromatic  compounds  may  now  be  considered. 

In  spite  of  the  fact  that  phenol  contains  a  saturated  benzene 
ring,  as  its  formula  shows, 

OH 


it  is  an  exceptionally  reactive  substance.  This  property  vanishes, 
however,  if  the  hydrogen  of  the  hydroxyl  group  is  replaced  by  sub- 
stituents  such  as  CHs,  C2Hs,  CHsCO,  etc.  Now  the  group  — C=C  •  OH 
which  is  present  in  phenol,  readily  rearranges,  in  the  case  of  compounds 
of  the  aliphatic  series,  to  give  the  system  — CH — C=0.  A  similar 
rearrangement  in  the  case  of  phenol  would  result  in  the  formation 
of  a  substance  possessing  either-  one  or  the  other  of  the  following 
formulas : 

0 0 


Both  formulas  indicate  the  presence  of  free  residual  valencies  in  the 
molecule,  and  it  is  perfectly  reasonable  to  suppose  that  the  chemical 
activity  of  free  phenol  depends  upon  a  condition  of  tautomerism 
involving  dynamic  equilibrium  between  the  enol  and  the  more  active 
keto  forms. 

Aromatic  acids  contain  carboxyl  groups  in  direct  union  with 
doubly-linked  carbon  atoms,  and  this  condition  gives  rise  to  a  system 
of  crossed  double  bonds, 

OH 


with  a  partial  valence  on  the  oxygen  atom  of  the  carbonyl  and  another, 
somewhat  smaller  partial  valence  on  carbon  in  the  nucleus.     Addi- 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES        61 


tion  to  such  a  system  may  be  illustrated  by  the  action  of  hydrogen 
on  ortho-  and  para-phthalic  acids: 

OH 
0— (X_ 


OH 

Phthalic  acid 


0=0- 

AH 

Paraphthalic  acid 


In  both  instances  the  hydrogen  may  be  supposed  to  add  to  the  partial 
valencies  on  the  oxygen,  giving, 


and 


HO-/C/ 
HO/ 


II 


but  this  is  followed  immediately  by  rearrangements  which  result  in 
the  formation  of  the  final  products,  viz.: 


COOH 


COOH 


and 


COOH 


In  the  case  of  the  corresponding  aldehydes  the  course  of  the  reaction 
is  the  same,  but  reduction  takes  place  much  more  readily. 

Experience  has  shown  that  certain  hydrogen  atoms  of  the  benzene 
ring  are  very  mobile  in  quinone,1  but  this  fact  finds  no  satisfactory 
explanation  in  the  formula  commonly  assigned  to  this  substance. 
Reference  to  the  partial  valency  formula,  however,  shows  at  once  that 
four  small  residual  affinities  are  present  on  the  carbon  atoms  2,  3,  5 
and  6,  and  two  others,  relatively  greater,  on  the  oxygen  of  the  car- 
bonyl  group: 

iRer.,  31,  978  (1898). 


62 


THEORIES  OF  ORGANIC  CHEMISTRY 


It  is  obvious  that  additions  in  the  1^4  positions  are  impossible.  The 
course  taken  in  addition  reactions  depends  in  great  measure  upon  the 
chemical  nature  of  the  adding  atoms  or  groups  of  atoms.  Thus,  for 
example,  hydrogen  adds  primarily  to  oxygen,  as  is  shown  by  the  for- 
mation of  hydroquinone,  while  halogen,  on  the  other  hand,  shows  an 
affinity  for  carbon  and  adds  in  the  5-6  position,  forming  dihalides 
corresponding  to  the  formulas  below: 


O 


OH 


H2 


Hal. 


Hal. 
H 


Since  the  formation  of  such  substances  is  accompanied  by  a  decrease 
in  the  free  energy  in  the  molecule,  due  to  loss  of  residual  valencies 
in  positions  2  and  3,  the  addition  of  a  second  molecule  of  halogen 
should  take  place  less  readily  than  in  the  case  of  the  first,  and  this  is, 
in  fact,  true.1 

In  the  addition  of  hydrogen  chloride  to  quinone,  hydrogen  adds  to 
oxygen  and  chlorine  to  carbon,     Thus 


0 


H 


1  Nef .  Jour,  prakt.  Chemie,  42,  182. 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES         63 
aniline  adds  in  a  similar  manner  to  form 

OH 


OH 


as  a  primary  product. 

If  the  oxygen  of  quinone  is  replaced  wholly  or  in  part  by  nitrogen 
or  by  carbon,  the  following  types  result: 


Such  atomic  systems  are  present  in  a  great  variety  of  colored  com- 
pounds, and  show  properties  analogous  to  those  observed  in  the  case 
of  quinone.  Thus,  for  example,  sulphurous  acid  reacts  with  quinone- 
diimide  to  form  a  sulphonic  acid  derivative  of  p-phenylene  diamine, 


NH 


HNH 


NHs 


NH 


S03H 


NH 


QJEi 


ITh.   Posner l   uses  formulas  which  differ  slightly  from  those  of 
hiele.     For    example,    when    thiophenol    reacts    with    quinone,    an 
addition  product  consisting  of  two  molecules  of  thiophenol  and  one 
olecule  of  quinone  is  formed,  to  which  Posner  assigns  the  following 
utomeric  formulas: 

1  Annalen  der  Chemie,  336,  106  (1904);  Jour,  prakt.  Chemie,  83,  471  (1911). 


64 


THEORIES  OF  ORGANIC  CHEMISTRY 


OH 


0«H«S  I 


'6  L±5 


Keto  form 


Posner's  theory  in  regard  to  the  distribution  of  affinity  allows 
for  tautomerism  of  still  another  type.  He  supposes  that  quinone, 
and  all  true  derivatives  of  quinone,  are  able  to  exist  in  two  tautomeric 
forms: 


A  comparison  of  these  formulas  with  that  of  Thiele 

0. 


shows  a  slight  difference  in  one  respect.  Thiele  assumes  that  the  free 
residual  affinity  on  the  carbon  atoms  1  and  4  is  distributed  equally 
on  two  sides,  thus  saturating  the  atoms  2-6  and  3-5  respectively, 
while  Posner  imagines  that  this  affinity  acts  as  a  single  force  to  saturate 
in  the  one  case  2  and  5  (I)  and  in  the  other,  6  and  3  (II). 

The  addition  of  halogen  hydrides,  aniline  and  other  substances 
to  quinone  and  derivatives  of  quinone  is  supposed  by  Posner  to  take 
place  in  a  manner  strictly  analogous  to  that  which  has  just  been 
described  in  the  case  of  thiophenol. 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES        65 


-H 


OH 

Enol  form 


X 


Keto  form 


All  these  addition  products  behave  in  a  tautomeric  manner  and  are 
able  to  pass  readily  into  true  derivatives  of  benzene.  Addition,  accord- 
ing to  Posner,  represents  the  first  phase  of  the  change.  The  trans- 
formation into  true  benzene  compounds  is  the  second  phase  and 
involves  a  loss  of  hydrogen,  giving  dihalogenated  hydroquinones, 
dianilinonoquinone,  etc. 

Tautomerism  of  the  enol-keto  type  may  be  supposed  to  account 
for  the  existence  of  colorless  and  colored  compounds  as  well  as  for 
variations  in  the  intensity  of  color  in  the  case  of  substances  possessing 
quinoidal  structure.  Since  the  keto  is  the  more  unsaturated  of  the 
two  tautomeric  modifications,  this  arrangement  may  be  assumed  to 
be  the  one  which  is  present  in  a  strongly  colored  substance  belonging 
to  this  class. 

An  elucidation  of  the  chemistry  of  aromatic  nitro-compounds 
was  effected  by  Meisenheimer  1  by  the  use  of  partial  valency  formulas. 
Up  to  this  time  no  rational  explanation  had  been  offered  for  the  fact 
that  trinitrobenzene  and  similar  substances  dissolve  in  alcohol  in  the 
presence  of  a  base,  to  give  salts,  which,  when  isolated,  are  always  found 
to  contain  residues  of  the  given  alcohol.  Meisenheimer  interprets 
this  phenomenon  by  supposing  that  the  reaction  is  one  of  simple  addi- 
tion. He  assumes  that  the  reactivity  of  the  carbon  atom  in  the  para- 
position  with  respect  to  N(>2  is  greatly  augmented  by  the  introduction  of 

1  Annalen  der  Chemie,  323,  219  and  241  (1902). 


66  THEORIES  OF  ORGANIC  CHEMISTRY 

other  nitro-groups,  and  that  K  and  OCH3,  for  example,  add  respectively 
to  the  free  affinity  which  is  present  on  carbon  and  oxygen,  forming 

OCH3 

V 

NO2 


KON=0 


An  analogous  configuration  may  be  assigned  to  the  salt  formed  by  the 
action  of  potassium  methoxide  upon  trinitroanisol,  etc.  The  cases 
of  nitro-naphthalene  and  anthracene  will  be  reserved  for  somewhat 
more  detailed  discussion  later. 

Potassium  cyanide  also  reacts  with  the  more  highly  nitrated  de- 
rivatives of  benzene  and  naphthalene  in  a  manner  analogous  to  that 
which  has  just  been  considered  in  the  case  of  potassium  methoxide. 
Red  salts  are  formed  to  which,  in  the  case  of  trinitrobenzene  for  example, 
the  following  formula  has  been  assigned  : 

H       CN 


KON=O 


The  most  generally  accepted  formula  for  naphthalene  is  that  which 
has  been  proposed  by  E.  Erlenmeyer,  Sr.,  and  which  assumes  the 
presence  of  two  condensed  benzene  nuclei: 

H        H 
C        C 

HC       C        CH 

I         II          I 
HC       C        CH 

v\/ 

C        C 
H        H 

According  to  this  formula  naphthalene  might  be  expected  to  resemble 
benzene  very  closely  in  its  chemical  properties.  Bamberger  has  shown, 
however,  that  naphthalene  is  much  more  reactive  than  benzene,  and 
that  in  the  presence  of  reducing  agents,  for  example,  it  readily  adds  first 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES        67 

two  and  then  four  hydrogen  atoms.  Further  additions  of  hydrogen 
do  not  take  place  so  readily,  and  this  is  explained  by  supposing  that 
tetrahydronaphthalene  contains  a  true  benzene  nucleus,  thus  offering 
increased  resistance  to  the  action  of  reducing  agents.  The  rather 
remarkable  course  of  this  reduction  was  originally  interpreted  by 
Bamberger  in  terms  of  his  theory  of  potential  valencies,  but  this 
theory  proved  inadequate  to  explain  the  further  observation  of  Bam- 
berger that  additions  of  hydrogen  and  other  atoms  always  take  place 
in  the  a-positions.  A  more  satisfactory  and  rational  interpretation 
of  the  phenomenon  is  offered  by  Thiele's  theory  of  partial  valencies. 
This  may  be  developed  in  brief  as  follows: 

According   to   the   Erlenmeyer   formula   for   naphthalene,    partial 
valencies  are  assumed  to  exist  on  all  of  the  ten  carbon  atoms, 


If.  however,  such  free  valencies  are  imagined  as  uniting  to  form  con- 
jugate systems  it  becomes  apparent  that  the  free  affinity  on  the  carbon 
atoms  2,  3  and  6,  7  is  fully  equalized.  The  partial  valencies  of  9  and 
10  do  not,  however,  suffice  to  fully  saturate  those  on  both  1  and  8 
and  4  and  5  respectively.  Thus,  while  the  total  affinity  on  9  and  10 
is  completely  neutralized,  carbon  atoms  1  and  4  and  also  5  and  8  possess 
residues  of  affinity.  In  other  words,  partial  valencies  may  be  imagined 
as  present  on  all  a-carbon  atoms  as  is  indicated  below : 


These  "  half  "  partial  valencies  offer  initial  points  of  attack  to  adding 
atoms  or  groups  and  the  phenomenon  of  1-4  addition  is  therefore  readily 
explained.     In   reduction,   for   example,    the   course   of   the   reaction 
may  be  represented  by  means  of  the  following  scheme: 
H 


H 


68 


THEORIES  OF  ORGANIC  CHEMISTRY 


This  formulation  shows  not  only  why  primary  addition  takes  place  in 
the  a-positions,  but  also  why  four  and  only  four  atoms  add  with  rela- 
tive ease.  The  formation  of  a  true  benzene  nucleus  in  dihydronaph- 
thalene,  as  shown  above,  readily  explains  the  resistance  of  tetrahy- 
dronaphthalene  to  the  further  action  of  reducing  agents.1 
According  to  the  usual  formula  for  anthracene, 


H 
C 


H 
C 


H 
C 


HC 


HC 


C 
H 


C 


C 


CH 

2 


3 

CH 


H        H 


this  substance  might  be  expected  to  be  very  unreactive,  since,  in  addition 
to  two  benzene  nuclei,  the  molecule  contains  only  carbon  atoms  in 
simple  forms  of  combination.  As  a  matter  of  fact,  however,  the  car- 
bon atoms  in  the  positions  9  and  10  are  readily  attacked  by  a  variety 
of  reagents,  and  in  particular,  by  the  lower  oxides  of  nitrogen.2 

Thiele's  theory  affords  a  rational  explanation  of  the  chemical 
activity  of  the  carbon  atoms,  by  assuming  the  presence  of  free  affinity 
at  these  points.  His  formula  for  anthracene  does  away  with  the 
necessity  of  a  single  bond  between  the  carbon  atoms  9  and  10  as  shown 
above,  and  represents  the  molecule  as  composed  of  carbon  atoms 
united  by  means  of  conjugate  systems  of  double  bonds:3 


It  is  of  interest  to  call  attention  here  to  an  o-quinoidal  formula  for 
anthracene  which  has  been  proposed  by  Armstrong,  and  which 

/VS/V 


1  Meisenheimer,  Annalen  der  Chemie,  323,  218  (1902). 

2  Annalen  der  Chemie,  323,  205  (1902);  330,  133  (1904). 


3  Annalen  der  Chemie,  306,  141  (1899) ;  also  compare  K.  H.  Meyer's  recent  work 
in  Annalen  der  Chemie,  379,  37  (1910);  396,  133,  152,  166  (1912);  and  Annalen  der 
Chemie,  420,  113,  126,  134  (1920). 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES         69 


has  recently  received  support  through  the  spectrochemical  researches 
of  Auwers.1 

By  means  of  the  Thiele  formula  Meisenheimer  has  been  able  to 
elucidate  the  action  of  the  lower  oxides  of  nitrogen  upon  anthracene. 
In  1880  Liebermann  and  Lindemann2  treated  anthracene  with  the 
oxides  of  nitrogen  in  the  hope  of  getting  nitro-derivatives.  Two 
different  products  were  obtained,  depending  upon  the  conditions  of 
the  experiment,  namely,  what  may  be  called  an  anthracene  nitrate 
of  formula  CuHio-HNOs,  and  anthracene  nitrous  oxide,  Ci4HioN204. 
Meisenheimer  has  since  shown  that  both  of  these  compounds  are  true 
derivatives  of  anthracene  and  correspond  structurally  to  the  formulas  3 
I  and  II  respectively: 


NO 


H 


Both  substances,  when  treated  with  sodium  hydroxide,  give  the  same 
product,  which  Liebermann  and  Lindemann  called  nitrosoanthron, 
but  which  the  later  researches  of  Meisenheimer  and  Dimroth4  have 
shown  to  be  nitroanthracene : 

H 


This  substance  has  also  been  obtained  by  Perkin,5  who  prepared  it  by 
nitrating  anthracene  in  alcohol  and  decomposing  the  resulting  product 
with  alkali.  Both  Liebermann  and  Perkin  observed  that  in  addition 
to  nitroanthracene,  which  is  insoluble  in  alcohol,  a  soluble  product 
was  formed  during  the  course  of  these  reactions.  This  was  called 
nitrosohydranthron  and  given  the  empirical  formula  Ci4HnNO2  by 
the  first  investigator,  while  the  latter  called  it  pseudomtrosoanthron 
and  gave  it  the  formula 


^er.,  63,  941  (1920). 

2Ber.,  13,  1584  (1880). 

3  Ber.,  33,  3547  (1900). 

4Ber.,  34,  219  (1901). 

6  Jour  Chem.  Soc.,  59,  634  (1891). 


70  THEORIES  OF  ORGANIC  CHEMISTRY 

Meisenheimer l  has  since  shown  that  nitrosohydranthron  and 
psuedonitrosoanthron  are  identical,  that  they  correspond  to  Perkin's 
formula  and  that  they  may  be  regarded  as  a  monoxime  of  anthra- 
quinone: 

O 


NOH 

He  was  able  to  show  further  that  the  substance  could  be  prepared  in 
any  desired  quantity  by  treating  nitroanthracene  with  potassium 
methylate  in  methyl  alcohol  solution. 

The  transformation  of  nitroanthracene  into  the  monoxime  of 
anthraquinone  may  be  regarded  as  a  change  which  involves  intra- 
molecular oxidation  and  which  results  in  the  formation  of  a  nitroso- 
phenol,  which  then  rearranges  to  the  oxime. 

O 
H 


N02  NOH 


It  is  very  improbable,  however,  that  this  represents  the  true  mech- 
anism of  the  reaction,  since  when  nitroanthracene  is  treated  with 
potassium  alcoholate,  secondary  products  containing  rnethoxy-groups 
are  formed.  By  changing  the  conditions  of  the  experiment  it  is  even 
possible  to  obtain  such  substances  as  the  principal  products  of  the 
reactions.  Thus,  for  example,  if  nitroanthracene  is  shaken  with  a 
solution  of  potassium  in  methyl  alcohol  it  is  transformed  into  a  potas- 
sium salt  having  the  constitution  represented  by  the  formula: 


N02K 
1  Annalen  der  Chemie;  323,  204  (1902). 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES 


71 


This  salt  is,  in  fact,  an  addition  product,  and  may  be  regarded  as  the 
metallic  derivative  of  nit romethoxy anthracene, 

OCH3 


NO2 

It  can  indeed  be  obtained  from  it.  Prolonged  treatment  of  nitro- 
anthracene  with  potassium  methoxide  leads  to  the  addition  of  a 
second  molecule  of  potassium  methoxide  and  results  in  the  formation 
of  a  substance  having  the  formula: 

H3C(X     /OCH3 


02K 


These  reactions  may  be  readily  explained  on  the  basis  of  Thiele's 
theory  of  partial  valencies.  The  reaction  between  anthracene  and 
nitric  acid  is  imagined  as  taking  place  according  to  the  following 
scheme : 


Either  substance  (I  or  II)  when  treated  with  methyl  alcohol  will  give 
dihydronitroanthranol  methyl  ether, 

)CH8 


72 


THEORIES  OF  ORGANIC  CHEMISTRY 


by  the  substitution  of  a  methoxy-group  for  an  OH  or  an  — ONO  group 
respectively.  By  loss  of  H^O,  HNCb,  or  CHaOH  respectively,  each 
of  these  three  substances  may  in  turn  be  transformed  into  nitroan- 
thracene : 


N=O. 


The  formation  of  the  oxime  of  anthraquinone  and  of  nitroanthrondi- 
methylacetal  from  nitroanthracene  may  be  considered  as  taking  place 
according  to  the  following  scheme: 

OCH3         Hv    /OCH3 


_KOH  = 


N-OK 


OCH3     CH30X    /OCRs1 


K  N N 

OK  OH 

^or  further  details  consult  Annalen  der  Chemie,  355,  249  (1907). 


JOHANNES  THIELE'S  THEORY  OF  PARTIAL  VALENCIES         73 
The  commonly  accepted  formula  for  phenanthrene, 


supposes  that  three  complete  benzene  nuclei  are  present  in  a  molecule 
of  the  hydrocarbon.  This  structure  does  not  account,  however,  for 
the  marked  reactivity  of  the  two  central  carbon  atoms.  Tkiele's 
formula  for  phenanthrene,  on  the  other  hand,  shows  that  the  rffmty 
of  these  two  carbon  atoms  is  only  partially  saturated,  and  that  each 
therefore  possesses  a  residue  of  free  chemical  affinity.  On  this  basis 
their  reactivity  is  readily  understood.1 

Other  ring  systems  may  obviously  be  explained  in  the  terms  of 
Thiele's  hypothesis  but  the  present  discussion  must  be  limited  to 
those  which  have  just  been  considered.  The  relations  existing  between 
the  physical  constants  of  the  chemical  coir  pounds  represented  by  the 
various  systems  of  combination  of  atoms  will  be  considered  in  some 
detail  in  later  chapters. 

From  the  very  beginning  Thiele's  theory  has  been  the  subject  of 
violent  controversy  and,  as  a  result,  has  been  modified  in  several 
important  respects.  The  basic  principle  of  the  hypothesis  still  hclds 
and  its  application  has  served  to  account  for  many  phenomena  which 
would  have  been  inexplicable  without  it.  It  has  been  of  still  greater 
service  in  opening  new  channels  for  the  further  development  of  the 
theory  of  organic  chemistry. 

Following  the  appearance  of  Thiele's  theory,  E.  Knoevenagel 2 
and  E.  Erlenmeyer,  Jr.,3  attempted  to  explain  all  of  the  phenomena 
with  which  it  dealt  by  means  of  stereochemical  conceptions.  These 
ideas  were  considered  in  some  detail  in  an  earlier  edition  of  the  present 
text  but  will  be  omitted  from  this  text  because  no  use  has  been  ir.ui'e 
of  them  in  the  intervening  time  and  they  have  undergone  no  further 
development.  All  stereochemical  assumptions  demand  so  many 
special  hypotheses  which  cannot  be  proved  that  it  is  questionable 
whether  they  help  to  elucidate  a  given  problem  or  confuse  it. 

When  compared  with  other  theories  which  have  been  advanced 
to  explain  the  phenomena  of  unsaturation,  Thiele's  assumption  of 
partial  valencies — or,  in  other  words,  fixed  positions  of  free  affinity 
on  the  atom — may  be  said  to  represent  the  simplest  working  hypoth- 

1  See:  Ber.,  33  and  following  for  the  work  of  J.  Schmidt  and  A.  Werner. 
2Annalen  der  Chemie,  311,  203  (1900);   Ber.,  36,  2806  (1903). 
3  Annalen  der  Chemie,  316,  50. 


74  THEORIES  OF  ORGANIC  CHEMISTRY 

esis  known  at  the  present  time  if  we  accept  the  idea  of  residual 
affinity  or  partial  valency.  It  is  especially  valuable  when  used 
in  conjunction  with  certain  definite  conceptions  in  regard  to  the 
specific  relationships  which  exist  between  the  different  atoms,  but 
this  is  a  matter  which  will  be  referred  to  again  later.  It  may  be  said 
at  this  point,  however,  that  the  form  in  which  Thiele's  theory  is  most 
frequently  used  at  the  present  time,  is  one  which  represents  an  exten- 
sion of  Thiele's  original  conception  by  the  injection  of  certain  other 
fundamental  ideas.  The  latter  embody  what  is  known  as  Werner's 
theory  and  must  now  be  considered  in  some  detail. 


CHAPTER  V 
THE  THEORY   OF  ALFRED   WERNER 

RESEARCHES  in  Organic  Chemistry  led,  in  the  second  half  of  the 
last  century,  to  the  supposition  that  the  valency  of  carbon  was  always 
four,  and  could  be  represented  by  four  separate  forces  acting  in 
definite  directions  in  space.  This  hypothesis  is  open  to  many  objec- 
tions and  in  1881,  it  was  vehemently  attacked  by  A.  Glaus.1  To 
quote  his  words,  "  facts  support  the  assumption  that  the  carbon 
atom  functions  ordinarily  as  a  tetravalent  element,  but  to  suppose 
that  the  chemical  attraction  inherent  in  its  atom  is  divided  into  parts 
each  of  which  acts  independently  of  every  other  is  as  unnatural  as  it 
is  unfounded."  It  seemed  much  more  rational  to  Claus  to  suppose 
that  the  total  affinity  of  carbon  is  unified  and  acts  as  one  force,  sepa- 
rating into  parts  only  when  carbon  enters  into  combination  with 
other  atoms,  in  which  case  the  partition  that  takes  place  is  deter- 
mined to  a  large  extent  by  the  relative  affinity  of  the  reacting  atoms. 
Thus,  for  example,  in  such  compounds  as  CO2,  C$2,  etc.,  the  chemical 
energy  of  carbon  may  be  regarded  as  divided  into  two  equal  parts, 
because  the  two  atoms  with  which  it  is  combined  are  alike;  while 
in  the  compound  COS  the  two  affinities  of  carbon,  which  are  exercised 
in  holding  oxygen  and  sulphur  respectively,  are  unequal  because  the 
chemical  energies  of  these  two  elements  are  different. 

The  objections  raised  by  Claus  and  by  others  2  were  unheeded 
at  the  time  because  the  attention  of  chemists  was  centered  in  the 
controversy  as  to  whether  the  carbon  atom  was  to  be  regarded  as  a 
sphere  with  valencies  directed  toward  the  corners  of  a  tetrahedron, 
or  as  itself  a  tetrahedron  with  valencies  situated  in  the  corners  or 
on  the  sides.3  And  since  this  controversy  was  productive  of  valuable 
experimental  results,  ten  years  elapsed  before  there  was  any  further 
theoretical  amplification  of  this  fundamental  idea  of  Claus.  In  1891 
A.  Werner  finally  succeeded  in  formulating  a  theory4  which  has 

'Ber.,  14,432  (1881). 

2  Lessen,  Annalen  der  Chemie,  204,  327  (1886);  Ber.,  .20,  3306  (1887). 

3  Wunderlich,  "  Konfiguration  organischer  Molekiile,"  Wurzburg,  1886. 

4  Vierteljahrsschr.  d.  naturf.  Ges.  in  Zurich,  36,  129  (1891);    "  Neuere  Anschau- 

75 


76  THEORIES  OF  ORGANIC  CHEMISTRY 

received  very  wide  and  increasing  recognition  and  by  means  of  which 
it  has  been  possible  to  interpret  the  chemical  properties  of  compounds 
containing  asymmetrical  cobalt,  rubidium  and  other  atoms.  Although 
this  theory  has  developed  in  connection  with  phenomena  in  the  field 
of  Tnorganic  Chemistry,  it  is  also  applicable  to  Organic  Chemistry 
and  is  beginning  to  be  used  more  and  more  in  the  interpretation  of 
organic  problems. 

According  to  Werner,  an  atom  may  be  regarded  as  spherical,  homo- 
geneous and  of  definite  size.  The  idea  of  four  individual  valencies 
operating  in  definite  directions  in  space  is  replaced  by  the  view  that 
the  property  of  attraction  emanates  from  the  center  of  the  atom  and 
is  evenly  distributed  over  its  surface.  A  priori,  there  is  no  division 
of  this  attractive  force  into  separate  parts.  In  the  case  of  the  carbon 
atom,  Werner  believes  that  evidence  goes  to  show  that  it  can  attract 
and  hold,  at  most,  four  other  atoms,  [f  four  atoms  combine  with  a 
carbon  atom  they  will  distribute  themselves  so  as  to  produce  the 
greatest  possible  neutralization  of  their  reciprocal  affinities,  and  the 
surface  attraction  of  the  carbon  will  divide  itself  among  the  various 
atoms  according  to  their  nature.  Each  fractional  part  of  the  total 
affinity  of  the  carbon  atom,  may,  therefore,  be  regarded  as  distributed 
over  a  definite  sector  of  the  surface  of  the  sphere  representing  the 
mass  of  the  atom,  and  constitutes  the  so-called  "  Binderlache  "  (com- 
bining surface)  of  the  atom.  The  intensity  of  this  surface  attraction 
varies  in  the  different  elements.  The  most  stable  arrangement  of 
atoms  will  be  that  in  which  the  largest  part  of  the  surfa.ce  of  the  central 
sphere  is  covered,  without  an  overlapping  of  the  segments,  and,  on 
the  other  hand,  chemical  union  between  two  atoms  will  be  weakened 
if  the  combining  surfaces  of  the  atoms  are  only  partially  covered  by 
their  reciprocal  attractions. 

If  all  four  atoms  attached  to  carbon  are  similar,  as  in  methane- 
each  will  require  an  equal  amount  of  surface  attraction,  and  they  will, 
therefore,  arrange  themselves  around  the  central  carbon  atom  in  the 
form  of  a  regular  tetrahedron.  This  represents  the  most  stable 
arrangement  for  carbon,  since  any  displacement  of  an  atom  from  its 
position  at  the  corner  of  the  tetrahedron  brings  about  a  condition 
where  the  lines  of  force  emanating  from  the  central  atom  are  only 
partially  neutralized  by  the  reciprocal  attraction  emanating  from  the 
combining  atom,  and  thus  the  union  between  the  two  atoms  is 
weakened. 

If  any  of  the  four  atoms  in  union  with  carbon  are  different,  as  in 

• 

ungen  auf  dem  Gebiete  der  anorganischen  Chemie,"  Braunschweig,  Fr.  Vieweg  &  Sohn, 
3d  Ed.  1913. 


THF  THEORY  OF  ALFRED  WERNER  77 

CR'2  R?  and  CR1  R11  R?1,  the  distribution  of  affinity  will  be 
irregular;  and  if  all  four  atoms  are  different  as  in  CR1  R11  Rm  RIV 
an  asymmetric  tetrahedral  grouping  of  the  atoms  will  result,  and  two 
enantiomorphic  configurations  will  be  possible.  The  theory  is  thus 
able  to  account  for  the  existence  of  space  isomers.  The  new  conceptions 
developed  by  Werner  to  explain  the  mutual  rearrangement  of  optical 
or  geometrical  isomerides  can  not,  however,  be  discussed  at  this  time. 

The  nature  of  the  union  between  the  carbon  atoms  in  compounds 
containing  single,  double  and  triple  bonds  must  now  be  considered  in 
detail  in  the  light  of  Werner's  theory,  and  an  effort  must  be  made  to 
see  whether  the  theory  is  able  adequately  to  explain  the  facts  observed 
in  regard  to  the  relative  strength  of  the  various  types  of  union. 

In  the  case  of  so-called  single  union  between  two  carbon  atoms  — 
as,  for  example,  in  the  system: 


a/  \a 

—the  full  force  of  affinity  will  be  exerted  only  at  the  points  of  contact 
of  the  two  carbon  atoms,  and  at  every  other  point  on  the  hemispheres 
the  strength  of  affinity  will  be  represented  by  the  component  of  the 
total  affinity  which  is  parallel  to  the  axis  joining  the  centers  of  the 
two  spheres.  In  Fig.  1  the  force  of  affinity  at  the  point  where  the  dotted 
line  meets  the  circumference  of  the  sphere  may  be  resolved  into  two 
forces,  Ci  and  C2,  of  which  Ci  alone  will  be  active  in  binding  the  two 
carbon  atoms. 


FIG.  1. 

The  value  of  this  force  gradually  decreases  as  the  distance  between  the 
surfaces  of  the  atoms  increases,  thus  leaving  an  amount  of  free  affinity 
which  E.  Bloch  l  has  estimated  "  at  less  than  one-half  and  more  than 
one-third  of  the  bound  energy  which  is  exercised  in  actually  holding 
the  atoms  together"2  (i.e.,  represented  in  the  mutual  saturation  of 
their  valencies). 

1  "A.  Werner's  Theorie  der  Kohlenstoff atoms,"  p.  14,  Leipzig,  1903. 

»A.  Werner,  Ber.,  39,  1278  (1906). 


78  THEORIES  OF  ORGANIC  CHEMISTRY 

In  the  case  of  so-called  double  bonds,  as  for  example,  in  the  system 
(a)2C=C(a)2,  it  is  assumed  that  the  two  carbon  atoms  are  bound 
together  by  that  part  of  the  total  affinity  of  each  carbon  atom  which 
is  left  after  deducting  from  the  total  the  amount  of  affinity  necessary  to 
hold  the  substituents.  This  residue  is  greater  than  the  amount  of 
free  affinity  neutralized  in  the  simple  union  between  two  carbon  atoms. 
Werner1  subdivides  it  into  two  parts,  of  which  one  is  situated  outside 
the  combining  zones  of  a  and  a  (represented  by  x,  and  xf,  in  Fig.  2), 
and  the  second,  in  the  segment  between  a  and  a  (represented  by  a  and 
a',  in  Fig.  2). 


The  forces  radiating  from  x  and  x',  act  so  uniformly  that  the  two 
atoms  are  able  to  revolve  about  a  common  axis.  The  forces  radi- 
ating from  a  and  a',  however,  are  not  so  symmetrically  distributed 
and  under  certain  conditions  must  interfere  with  the  free  rotation 
of  the  atoms.  The  latter  fact  is  without  stereochemical  significance. 
According  to  Bloch,  in  the  case  of  a  double  bond  the  amount  of  free 
affinity  is  nearly  equivalent  to  that  which  is  bound  and  is  relatively 
three  or  four  times  as  great  as  the  free  affinity  estimated  in  the  case  of 
single  bonds. 

By  a  similar  disposition  of  two  spheres,  Werner  represents  trebly 
bound  carbon  as  in  the  acetylene  series,  aC=Ca,  but  since  only  one 
other  atom  is  attached  to  each  carbon  sphere  the  amount  of  affinity 
left  for  binding  the  two  atoms  together  is  greater  than  in  the  case 
of  the  singly  or  doubly-linked  carbon  systems.  Bloch  found  that  the 
amount  of  free  affinity  is  twice  as  great  as  that  which  is  bound,  and  is 
relatively  twice  as  great  as  the  free  affinity  estimated  for  the  double 
bond  and  six  times  that  estimated  for  the  single  linkage.  According 
1  Vierteljahrsschr.  d.  naturf.  Ges.  in  Zurich,  36,  145. 


THE  THEORY  OF  ALFRED  WERNER  79 

to  this  conception,  the  amount  of  affinity  neutralized  in  the  manifes- 
tation of  double  and  triple  bonds  is  greater  than  in  the  case  of  single 
bonds,  and  this  appears  to  be  in  harmony  with  the  fact  that  acetylene 
is  stable  at  high  temperatures.  To  account  for  the  relatively  greater 
reactivity  of  the  so-called  unsaturated  compounds,  Werner  makes 
a  distinction  between  stability  and  reactivity.  Thus  reactivity  is 
determined  by  the  amount  of  free  affinity  present  on  the  atom  (see 
component  €2,  Fig.  1),  and  is  greater  for  derivatives  of  ethylene  and 
acetylene  than  for  derivatives  of  ethane. 1  It  may  be  imagined  as  radi- 
ating beyond  the  contour  of  the  molecule,  and  in  this  way  operating 
to  attract  other  atoms  to  the  molecule  and  so  to  bring  about  chemical 
action.  From  these  considerations  it  is  obvious  that  the  triple  bond 
produces  the  most  reactive,  the  single  bond,  the  least  reactive  molecule. 
Werner  points  out,  however,  that  the  amount  of  free  affinity  is  not  a 
constant  either  for  singly  or  doubly  bound  carbon  but  that  it  varies 
according  to  the  nature  and  constitution  of  the  particular  compound. 

Three  or  more  carbon  atoms  may  enter  into  combination  to  form 
either  a  chain  or  a  ring.  This  may  be  formulated  in  the  terms  of 
Werner's  theory  as  follows:  "  If  the  atoms  uniting  with  carbon  attract 
each  other  only  slightly  or  not  at  all,  the  valencies  may  be  regarded 
as  operating  through  the  corners  of  a  regular  tetrahedron.  If,  however, 
the  valencies  are  deflected  from  these  positions,  the  stability  of  the 
union  with  carbon  is  lessened,  and  this  is  the  case  in  many  ring  com- 
pounds. The  tension  in  ring  compounds  observed  by  von  Baeyer  may 
be  explained  in  terms  of  Werner's  theory  as  due  to  the  tendency  on  the 
part  of  the  atoms  to  return  to  positions  of  the  greatest  possible 
neutralization  of  their  affinities,  and  von  Baeyer's  law  governing  tension 
in  such  cases  harmonizes  with  Werner's  conceptions  in  all  essentials. 

The  increased  reactivity  of  hydrogen  atoms  in  union  with  doubly 
and  trebly  bound  carbon  may  be  regarded  as  analogous  to  the  mobility 

of  the  a  hydrogen  present  in  acids  of  the  general  formula  / CH  •  COOH. 

The  latter  has  been  interpreted  by  Werner2  as  due  to  the  fact  that 
the  oxygen  atom  in  the  carbonyl  group  requires  a  relatively  large 
part  of  the  combining  surface  of  the  carbon  atom  so  that  only  a 
relatively  small  part  of  the  affinity  of  the  carbon  is  available  for  holding 
the  adjoining  carbon  atom.  The  result  is  to  increase  the  total  amount 
of  free  affinity  on  this  second  carbon  atom.  Since  free  affinity  is  to  be 
regarded  as  radiating  beyond  the  contour  of  the  molecule,  it  is  obvious 
that  the  substance  will  exercise  an  increased  attraction  for  other  atoms. 

'Ber.,  39,  1278  (1906). 

2Theorie  der  Kohlenstoffatoms.  p.  166 


80  THEORIES  OF  ORGANIC  CHEMISTRY 

The  groupings  C=N  and  C=C  exert  an  influence  analogous  to  that 
of  C=O.  Bloch  1  believes  that  stereochemical  explanations  should 
not  be  sought  in  such  cases,  and  interprets  the  reactivity  of  the  a 
hydrogen  as  due  to  the  acidifying  influence  of  the  double  bonds. 

In  the  case  of  conjugated  double  bonds,  as,  for  example,  in  the 
system  C=C — C=C,  there  would  be  less  free  affinity  on  the  two 
middle  than  on  the  end  carbon  atoms,  so  that  the  phenomena  may 
be  interpreted  in  much  the  same  way  in  terms  of  Werner's  theory  as 
in  terms  of  the  theory  of  partial  valencies  as  developed  by  Thiele.2 

Werner  conceives  that  in  benzene  the  six  carbon  atoms  are  joined 
together  in  the  form  of  a  ring,  the  constitution  of  which  may  be 
represented  by  Fig.  3. 


FIG.  3. 

Because  the  six  carbon  atoms  are  combined  in  the  same  ring,  each 
comes  within  the  sphere  of  attraction  of  every  other,  and,  at  the  same 
time,  any  movement  outside  of  this  sphere  of  attraction  is  restricted. 
And  since  in  benzene  every  carbon  atom  is  exactly  equivalent  to  every 
other  in  its  combining  power,  the  static  condition  of  the  molecule  will 
be  that  in  which  each  and  every  atom  is  bound  to  its  respective 
position  by  the  exercise  of  the  greatest  possible  fraction  of  its  total 
affinity,  although  the  value  for  this  fraction  may  vary  for  the  indi- 
vidual atoms. 

Werner  pictures  the  reciprocal  exchange  of  chemical  affinity  between 
the  carbon  atoms  of  the  benzene  ring  in  the  following  way:  "  The 
force  of  attraction  emanating  from  a  carbon  atom  is  comparable  to  the 
emission  of  light.  Thus  atom  1  may  be  imagined  as  luminous  and  as 
radiating  light  upon  the  other  five.  It  follows  that  of  these,  the  carbon 
atoms  in  the  or^o-positions  (atoms  2  and  6)  will  receive  the  same, 
and,  relatively  to  other  atoms,  the  greatest  illumination.  The  carbon 
lTheorie  der  Kohlenstoff atoms,  pp.  25,  26.  2  Ibid.,  p.  20. 


THE  THEORY  OF  ALFRED  WERNER  81 

atoms  in  the  meta-position  (atoms  3  and  5)  are,  on  the  other  hand, 
partly  shaded  by  the  or^o-carbon  atoms,  and  so  receive  very  slight 
illumination.  Moreover,  the  light  which  is  received  by  these  atoms 
is  less  intense  because  of  the  greater  distance  through  which  it  must 
pass.  Finally,  the  carbon  atom  in  the  para-position  (number  4) 
receives  a  relatively  great  amount  of  direct  light,  but  this  light  loses 
somewhat  in  intensity  by  reason  of  the  distance  through  which  it  must 
travel." 

This  conception  finds  expression  in  the  following  hexagonal  formula 
for  benzene : 


To  quote  Werner's  words,  "  the  six  carbon  atoms  of  the  benzene  ring 
may  be  assumed  to  lie  in  the  same  plane.  The  hydrogen  atoms  are 
also  distributed  symmetrically  in  this  plane  but  are  so  arranged  as  to 
lie  outside  the  ring.  Carbon  atoms  in  the  or^o-positions  are  mutually 
saturated  by  the  exercise  of  a  somewhat  greater  fraction  of  their  total 
affinity  than  is  supposed  to  be  represented  in  the  ethylene  type  of 
combination.  A  small  fraction  of  affinity,  equal  roughly  to  a  partial 
valency,  may  be  assumed  to  act  between  carbon  atoms  in  the  para- 
positions,  and  it  may  even  happen  that  still  smaller  fractions  enter  into 
the  mutual  saturations  of  carbon  atoms  in  the  meta-positions."1  It 
is  obvious  that  these  valencies  correspond  to  neither  single,  double, 
nor  diagonal  bonds. 

Such  a  formula  may  be  said  to  resemble  the  Armstrong-Baeyer 
centric  formula  more  nearly  than  any  other  which  has  been  advanced 
to  explain  the  chemical  and  physical  properties  of  benzene,  but  it 
differs  from  it  in  certain  respects.  It  seems  indeed  to  embody  an 
entirely  new  conception  of  the  atomic  relationships  in  benzene  and  its 
derivatives.  Werner's  conceptions  have  been  somewhat  amplified 
by  E.  Bloch  and  have  been  presented  in  such  a  form  as  to  give  a  very 
comprehensive  interpretation  of  the  chemistry  of  aromatic  compounds. 
As  applied  in  the  most  diverse  fields  of  organic  chemistry  by  A. 
Werner,  P.  Pfeiffer,  and  E.  Bloch,  these  conceptions  have  served  to 
represent  the  most  complicated  relationships  in  such  a  way  as  to  free 
them  from  the  limitations  frequently  imposed  by  the  use  of  structural 
formulas. 

Werner  has  been  unusually  successful  in  the  application  of  his  theory 
the  field  of  inorganic  chemistry  and  has  been  able  to  break  down 
1  "  Handworterbuch  der  Xaturwissenschaften,"  1915,  p.  178. 


82  THEORIES  OF  ORGANIC  CHEMISTRY 

the  wall  of  separation  between  so-called  molecular  and  atomic  com- 
pounds. The  mechanism  by  which  whole  molecules  of  apparently 
saturated  compounds  are  held  together  is  inexplicable  in  terms  of  the 
usual  conceptions  of  structural  chemistry  but  is,  on  the  other  hand, 
readily  comprehensible  in  terms  of  Werner's  theory,  since  this  assumes 
that  the  atoms  which  are  bound  together  in  the  various  forms  of 
saturated  combinations  still  possess  residual  valencies  and  are  there- 
fore capable  of  further  reactions. 

As  has  been  noted,  Werner  regards  the  valency  of  each  atom  as 
distributing  itself  in  such  a  way  as  to  satisfy  not  only  the  affinity  of 
the  contiguous  atoms  but  also  certain  necessities  in  the  special  arrange- 
ments of  these  atoms  in  the  molecule.  First  of  all,  he  distinguishes 
between  compounds  of  the  first  order  and  of  the  second  order  by 
supposing  that,  while  the  former  represent  compounds  in  which  the 
principal  valencies  of  the  respective  atoms  are  saturated,  this  does  not 
necessarily  exhaust  the  affinity  at  the  disposal  of  the  atoms  in  question. 
If  further  combinations  take  place,  compounds  of  the  second  order 
will  be  formed.  In  developing  this  conception  Werner  has  introduced 
the  terms  principal  and  auxiliary  valencies  to  denote  the  above  two 
kinds  of  combination.  Principal  valencies  correspond  to  ordinary 
valencies  and  bind  together  such  atoms  or  groups  as  are  capable  of 
existing  as  independent  ions,  or  are  equivalent  to  such  ions,  as  for 
example 

Cl,        Na,        NO2,        CH3,    etc. 

Auxiliary  valencies,  on  the  other  hand,  represent  an  entirely  new  con- 
ception and  may  be  expressed  by  means  of  dotted  lines.  They  are 
met  with  in  the  case  of  such  radicals  as  are  capable  of  functioning  as 
whole  molecules  and  which  cannot  exist  or  function  as  independent 
ions,  as  for  example 

H20      ;     H3N      ;     HC1     ;     02       ;     (CH3)3B      ; 

v      etc. 


These  radicals  may  be  regarded  as  entering  into  chemical  combinations 
only  through  the  exercise  of  auxiliary  or  residual  affinities. 

The  two  kinds  of  valencies  may  be  distinguished  by  their  energy 
content,  the  principal  valencies  representing,  in  general,  greater 
affinity  than  the  auxiliary.  The  difference  is,  however,  one  of  degree 
and  depends  upon  the  relative  amount  of  saturation  of  the  other 
valencies.  Both  types  must  be  regarded  as  components  of  the  total 
affinity  of  the  atom,  and  there  is  no  really  sharp  line  of  demarcation 
between  them.  They  merge,  and,  under  certain  conditions,  pass  one 


THE  THEORY  OF  ALFRED  WERNER  83 

into  the  other,  for  it  is  obvious  that  the  degree  of  saturation  of  one 
kind  of  valence  must  tend  either  to  strengthen  or  weaken  the  other. 

The  number  of  principal  valencies  which  a  given  atom  is  capable 
of  exercising  is  not  a  fixed  quantity,  but  depends  upon  the  nature  of 
the  atoms  with  which  it  is  in  combination.  This  follows  from  the 
fact  that  the  relative  strength  of  combination  between  atoms  varies 
with  the  nature  of  the  atoms,  and  any  change  in  the  amount  of  affinity 
exercised  by  the  valencies  must  be  accompanied  by  a  change  in  their 
number,  if  the  total  affinity  of  the  atom  in  question  is  to  be  saturated. 
Thus  iron,  for  example,  has  a  maximum  of  three  principal  valencies 
for  chlorine,  while  this  number  may  be  increased  for  oxygen.  The 
total  affinity  exercised  by  the  iron  in  each  case  may  be  regarded  as  the 
resultant  of  the  strengths  of  attraction  of  all  the  atoms  entering  into 
chemical  combination  with  it.  Allowing  for  variation  in  the  number 
of  valencies,  there  is,  nevertheless,  a  maximum  number  of  principal 
valencies  for  each  element. 

Although  only  a  relatively  small  part  of  the  total  affinity  of  the 
atom  is  needed  in  the  exercise  of  auxiliary  valencies,  there  exists, 
nevertheless,  a  maximal  number  for  each  element.  In  order  to  answer 
the  question  as  to  what  factors  govern  the  limiting  value  of  this  num- 
ber in  any  given  case,  it  is  necessary  to  consider  the  conditions  under 
which  the  saturation  of  such  valencies  takes  place.  Structural  formulas 
are  based  upon  the  assumption  that  the  radicals  which  unite  to  form 
a  given  chemical  compound  are  adjacent  to  each  other  and  in  direct 
union.  It  is  not  necessary  to  suppose  that  such  groups  are  immovable, 
but  it  must  be  assumed  that  no  atom  or  group  occupies  wholly  or  in 
part  the  space  which  belongs  to  some  other  atom  as  its  particular 
sphere  of  rotation.  Now,  if  the  atoms  in  a  molecule  are  joined  to  a 
central  atom  by  means  of  principal  and  auxiliary  valencies,  it  follows 
from  what  has  just  been  said  that  only  a  limited  number  will  find  places 
in  the  space  immediately  surrounding  this  central  atom.  The  space 
contiguous  to  a  central  atom  is  spoken  of  as  the  "  first  sphere,"  and 
the  atoms  occupying  positions  within  the  first  sphere  may  be  regarded 
as  in  direct  union  with  the  central  atom.  The  "  coordination  number  " 
of  the  atom  is  the  term  used  by  Werner  to  express  the  number  of 
atoms  which,  in  a  given  case,  can  hold  positions  within  the  first  or 
undissociable  zone  of  the  central  atom.  Such  atoms  or  groups  may  be 
regarded  as  bound  to  the  central  atom  either  by  principal  or  auxiliary 
valencies;  they  may  also  be  bound  to  other  atoms  which  occupy 
positions  in  the  second  or  dissociable  zone  with  respect  to  the  central 
atom. 

While  the  values  of  the  coordination  number  of  the  different  elements 


84  THEORIES  OF  ORGANIC  CHEMISTRY 

may  vary  and,  in  case  of  a  given  element,  may  even  be  different  under 
different  conditions,  only  the  maximal  value  need  be  considered  at 
present.  By  this  is  meant  the  maximum  number  of  atoms  which  are 
capable  of  combination  in  the  first  sphere.  It  is  relatively  independent 
of  the  nature  of  these  atoms  and  is  governed  largely  by  the  amount 
of  space  constituting  the  first  zone.  Those  compounds  in  which  the 
maximal  value  has  been  reached  are  said  to  be  coordinately  saturated. 
The  maximal  value  of  the  coordination  number  of  a  given  atom  may 
be  obtained  from  a  study  of  the  constitution  of  all  of  its  known  com- 
pounds, and  in  the  case  of  a  very  great  number  of  elements  is  equal 
to  six.  In  the  case  of  carbon,  however,  it  is  equal  to  four;  and  it  has 
been  found  that  the  elements  which  occupy  positions  next  to  carbon 
in  the  Periodic  System,  namely,  boron  and  nitrogen,  also  have  coor- 
dination numbers  equal  to  four.  The  coordination  number  and  the 
number  representing  the  principal  valencies  are  identical  in  the  case 
of  carbon,  but  in  the  case  of  a  very  great  number  of  other  elements 
this  is  not  true.  Usually  the  coordination  number  (equal  to  6)  and 
the  maximum  number  of  auxiliary  valencies  are  identical.  Finally 
it  should  be  noted  that  in  many  of  the  compounds  of  a  given  element 
its  maximum  coordination  number  is  not  reached,  such  compounds 
being  spoken  of  as  coordinately  unsaturated. 

The  existence  of  partial  valencies  has  been  demonstrated  in  the 
case  of  substances  which  contain  singly  bound  atoms  as  well  as  in  the 
case  of  those  having  unsaturated  groupings.  This  fact  was  first 
recognized  by  R.  Anschutz, 1  but  its  general  application  was  established 
by  Werner.  Suppose,  for  example,  that  the  atom  Me  is  attached 
to  several  atoms,  among  which  is  the  atom  X,  by  means  of  single  unit 
valencies.  If  the  character  of  its  union  with  the  other  atoms  is  such  as 
to  engage  a  very  large  fraction  of  its  total  affinity,  it  follows  that  it 
will  have  relatively  less  affinity  available  for  saturating  the  valency 
offered  by  X.  The  valency  of  X  will  not,  therefore,  be  fully  neutralized 
by  its  union  with  Me  and  the  fraction  of  affinity  which  remains  free 
may,  under  certain  conditions,  function  as  a  partial  valency  in  the 
formation  of  molecular  compounds  such  as  MeX  ...  A. 

In  order  to  test  the  experimental  accuracy  of  this  deduction  from 
his  theory,  Werner  2  first  investigated  inorganic  compounds  of  the  gen- 
eral formula  MeXn,  where  n  represents  the  maximal  number  of  valen- 
cies of  the  atom  Me,  and  where  the  compound  may,  therefore,  be 
regarded  as  apparently  fully  saturated.  He  was  able  to  show  that  the 
halides  SOU,  SeCU,  PBr5,  PC15  and  others  react  readily  with  chlorides, 

iZeitschr.  Electrochemie,  32,  (1904);  and  Annalen  der  Chemie,  346,  397  (1906). 
2Ber.,  39,  1278  (1905). 


THE  THEORY  OF  ALFRED  WERNER  85 

such  as  AuCl3,  FeCl3,  A1C13,  SbCl3,  SnCU,  etc.,  to  form  addition 
products  of  the  formula  SCl4-AuCl3,  SCl4-FeCl3,  (SCUVSnCU,  etc. 
From  this  Werner  concluded  that  one  of  the  halogen  atoms  in  each 
member  of  the  first  series  of  compounds,  as  for  example  C1SC13, 
actually  possesses  the  power  of  exercising  a  partial  valency. 

Organic  halides  have  in  numerous  instances  been  observed  to 
behave  in  the  same  way.  Triphenylmethyl  chloride,  for  example, 
reacts  with  chlorides,  such  as  ZnCk,  A1C13,  SnCU,  etc.,  to  form  addition 
products  of  the  type 

(C6H5)3CC1  .  .  .  SnCU 

In  the  case  of  this  particular  substance  the  chlorine  atom  is  so  active 
that  it  enters  into  combinations  with  water,  alcohol  and  other  reagents. 
Werner  was  able  to  demonstrate  that  this  reactivity  is  not  conditioned 
by  the  negative  character  of  the  three  phenyl  groups,  since  tribenzoyl- 
methylbromide  (CeHsCO^CBr,  contains  an  inactive  halogen ,  atom, 
but  that  it  depends  upon  a  weakening  of  the  fourth  valency  of  the 
methane  carbon  atom  in  the  tripheny  line  thy  1  radical.  Such  a  weak- 
ening of  valency  may  be  accounted  for  by  supposing  that  almost  all 
of  the  affinity  of  the  methane  carbon  atom  is  required  for  the  satura- 
tion of  the  three  phenyl  groups  and  that,  therefore,  very  little  is  avail- 
able for  holding  a  fourth  atom  or  group.  This  interpretation  is  further 
supported  by  the  fact  that  the  aliphatic  hydrogen  in  triphenylmethane 
is  much  more  reactive  than  a  hydrogen  in  methane  and  the  molecule 
is  capable  of  entering  into  combination  with  other  molecules  to  form 
addition  products. 1 

Similar  observations  have  been  made  in  the  case  of  many  other 
apparently  saturated  compounds,  so  that  it  may  be  said  in  general  that 
the  affinity  represented  by  single  bonds  is  frequently  subject  to  very 
considerable  variations.  This  conception  may  be  applied  to  the 
solution  of  diverse  problems  of  general  theoretical  importance  as 
Siegfried  Skraup  2  has  recently  pointed  out. 

Ammonium  compounds  in  organic  chemistry  have  been  studied 
in  great  detail  by  Werner,  and  the  interpretation  of  their  constitution 
from  the  standpoint  of  his  theory  may  be  considered  at  this  point. 
Ammonium  chloride,  according  to  Werner,  has  neither  the  formula 
NH3-HC1  (molecular  formula)  nor  the  formula  NH4C1  (valence  for- 
mula), but  must  be  regarded  as  built  up  in  the  following  way:  both 
ammonia  and  hydrochloric  acid  possess  auxiliary  valencies,  the  former 
on  the  nitrogen,  the  latter  upon  the  hydrogen  atom;  and  the  union  of 

^er.,,  39,  1284  (1905). 

2  Annalen  der  Chemie,  419,  1  (1919),  also  see  Dissertation,  Wurzburg,  1919. 


86  THEORIES  OF  ORGANIC  CHEMISTRY 

their  molecules  to  form  ammonium  chloride  is  accompanied  by  the 
neutralization  of  these  valencies.  Thus: 

H3N  ...  +  ...  HC1=H3N  .  .  .  HC1 

A  study  of  the  product  shows  that  the  properties  of  the  chlorine 
present  in  it  differ  in  no  important  way  from  the  properties  of  the 
chlorine  ion  in  hydrochloric  acid;  'and  from  this  it  follows  that  the 
union  between  chlorine  and  hydrogen  must  be  similar  in  the  two  cases. 
Hydrogen,  on  the  other  hand,  is  seen  to  have  lost  its  ionic  properties 
as  the  result  of  the  change.  Such  a  compound  of  the  higher  order  is 
referred  to  by  Werner  as  an  addition  product  ("Anlagerungsver- 
bindung  ")  and  belongs  to  a  class  of  ammonium  compounds  which  must 
be  sharply  differentiated  from  a  second  class,  namely  the  alkyl 
ammonium  salts. 

The  hydriodide  of  methylamine,  which  results  from  the  interaction 
of  methyliodide  and  ammonia,  may  be  taken  as  a  typical  illustration 
of  an  alkyl-ammonium  salt.  The  carbon  atom  in  methyliodide  must 
be  regarded,  according  to  Werner's  theory,  as  coordinately  saturated, 
and  from  this  it  follows  that  combination  with  ammonia  takes  place 
as  a  result  of  the  displacement  of  one  of  the  atoms  occupying  a  coor- 
dinate position.  The  change  in  the  properties  of  iodine  as  a  result 
of  this  reaction  leads  to  the  assumption  that  it  has  been  detached  in 
some  way  from  its  original  position  with  reference  to  carbon.  For 
example,  the  iodine  might  shift  its  position  from  carbon  to  pentavalent 
nitrogen  as  is  expressed  in  the  following  equation ; 

H3C-I+N=H3C-NI 
H3  H3 

According  to  Werner,  such  an  assumption  is  both  capricious  and  false 
and  should  be  discarded.  He  explains  the  reaction  as  follows:  in  this 
transformation  the  iodine  atom  continues  to  remain  in  union  with  the 
central  carbon  atom  even  after  its  removal  from  a  coordinate  position 
with  reference  to  the  latter.  This  conception  supposes  that  the  satu- 
ration of  the  affinity  of  iodine  takes  place  outside  the  first  sphere  of  the 
carbon,  since  the  intrusion  of  nitrogen  has  filled  all  four  coordinate 
positions  within  the  first  sphere.  According  to  this  interpretation 
the  reaction  is  to  be  formulated  as  follows: 

H3CI  +  NH3  =  I(H3C  .  .  .  NH3) 

The  elements  or  groups  directly  attached  to  the  central  atom  whether 
by  principal  or  auxiliary  valencies,  and  represented  as  in  the  first 


THE  THEORY  OF  ALFRED  WERNER  87 

zone,  are  included  within  brackets.  The  position  of  the  halogen 
outside  the  brackets  is  intended  to  indicate  a  difference  in  its  attachment 
to  the  central  atom,  and  to  show  a  tendency  to  ionization  in  solution. 
This  latter  property  is  possessed  in  common  by  all  atoms  or  groups 
occupying  positions  in  the  second  or  "  dissociable  zone." 

All  salt-like  derivatives  of  organic  bases  (ammonium,  phosphonium, 
arsonium  and  thionium  salts)  may  be  formulated  in  an  analogous 
manner,  as  for  example : 

X(H3C  .  .  .  NR3);      X(H3C  .  .  .  PR3); 
X(H3C  .  .  .  AsR3);       X(H3C  .  .  .  SR3) 

Werner  calls  compounds  of  this  type  intra-addition  products  ("  Ein- 
lagerungsverbindungen"),  since  their  configuration  presupposes  a  change 
in  the  position  of  a  nitrogen,  phosphorus,  or  other  atom  from  the  outer 
to  the  inner  sphere  of  the  central  atom.  Werner  assumes,  further, 
that  under  certain  conditions  it  is  possible  for  compounds  of  this  type 
to  rearrange  to  form  normal  addition  products. 

In  other  words,  a  tautomeric  relation  may  be  imagined  to  exist 
between  these  two  classes  of  compounds  as  is  expressed  below : 

I(H3C  .  .  .  NH2)  <=»H3C— NH2 


HI 

Carbonium  form  Hydronium  form 

and  analogously: 

HO(H3C  .  .  .  NH2)  ? 


HOH 

It  is  possible  that  in  many  instances  isomers  of  this  type  are  present 
in  solutions.  In  the  case  of  tetra-alkyl  ammonium  compounds,  how- 
ever, isomerism  is  obviously  out  of  question  and  these  substances  are, 
therefore,  assumed  to  possess  the  carbonium  structure. 

The  salts  of  quaternary  unsaturated  bases  represent  two  important 
classes  of  compounds,  which  may  be  more  clearly  understood  when 
their  structure  is  interpreted  in  the  terms  of  Werner's  theory.  The 
salts  of  the  quaternary  unsaturated  bases  include  all  compounds  which 
are  analogous  in  constitution  to  the  alkyl  quinolonium  salts  and  to  the 
salts  of  the  basic  dyes  derived  from  triphenylmethane.  Substances 
of  this  type  are  not  capable  of  being  transformed  into  stable  ammonium 
bases.  When  such  combinations  are  hydrolyzed  into  the  corresponding 
bases  they  suffer  an  intramolecular  rearrangement  which  involves 
a  shifting  of  the  hydroxyl  group  from  nitrogen  to  carbon  and  the 


88 


THEORIES  OF  ORGANIC  CHEMISTRY 


formation  of  products  which  very  closely  resemble  the  hydronium 
compounds.  Werner  explains  this  reaction  as  follows:  "  the  carbon 
atom  adjoining  the  nitrogen  in  these  compounds  has  one  free  coor- 
dinate position.  The  salt  reacts  to  give  primarily  a  base  of  the  car- 
bonium  type  which  then  suffers  rearrangement.  In  this  process  the 
hydroxyl  group  shifts  from  its  original  position  to  the  free  coordinate 
position  on  the  carbon  and  thus  enters  into  direct  union  with  this 
element.  This  is  accompanied  by  a  simultaneous  readjustment  of 
affinity  relations  in  the  molecule.  The  reaction  may  be  represented 
thus: 


I(CH3  .  . 


CH 


HO(H3C  .  .  .  N^          >CI 

C 
>H      H 


H3C-N 


HO 


The  same  general  scheme  holds  for  analogous  rearrangements  which 
take  place  in  the  case  of  the  basic  derivatives  of  triphenylme  thane. 
In  regard  to  diazonium  salts,  Werner  makes  this  statement:1  "the 
analogy  which  exists  between  the  theory  of  the  formation  of  diazonium 
salts  and  the  theory  of  the  formation  of  methyl  ammonium  iodide 
from  methyliodide  and  ammonia  is  suggested  by  the  following 
expression  : 

C6H5I+N     ->     (C6H5  .  . 


i 


A  study  of  the  above  formulas  reveals  the  interesting  fact  that, 
although  the  carbon  atom  in  union  with  iodine  in  phenyliodide  is 
represented  as  in  combination  with  only  three  atoms  and  therefore 
must  be  regarded  as  coordinately  unsaturated,  it  actually  behaves 
like  a  coordinately  saturated  atom.  This  behavior  must  be  due  to 
the  constitution  of  the  benzene  ring.  It  follows  from  a  consideration 
of  space  relationships  that  the  saturation  of  a  fourth  coordinate 
1  Annalen  der  Chemie,  322,  290  (1902). 


THE  THEORY  OF  ALFRED  WERNER  89 

position  in  the  case  of  any  single  carbon  atom  would  weaken  the 
affinity  by  which  the  ring  is  held  together  and  in  this  way  destroy  the 
aromatic  character  of  the  ring  system.  The  behavior  of  carbon  atoms 
in  aromatic  rings  differs  markedly  from  the  behavior  of  aliphatic  carbon, 
and  is  characterized  in  particular  by  the  fact  that  the  atoms  thus 
bound  together  in  a  ring  are  unable  to  enter  into  indirect  forms  of 
combination  with  atoms  or  radicals  outside  the  ring.  Thus  the  nature 
of  the  union  of  the  carbon  atoms  in  aromatic  compounds  sets  a  definite 
stamp  upon  the  character  of  the  coordination  formulas  of  diazonium 
salts,  and  also  of  salts  of  aniline  and  of  iodonium  salts.  This  may  be 
summed  up  by  saying  that  the  negative  radical,  which  is  joined  to  a 
carbon  atom  in  the  ring  by  the  saturation  of  one  valency,  may  be 
displaced  from  its  position  in  the  first  sphere  to  a  position  in  the 
second  sphere  with  reference  to  that  carbon,  by  the  interposition  of 
other  atoms  or  groups  such  as  N,  I,  NHs,  etc.,  but  not  by  the  inter- 
position of  hydrogen.  The  compounds  resulting  from  reactions  in 
the  cases  mentioned  may  be  formulated  as  follows  : 


,     (C6H5  .  .  .  1)1,     (C6H5  .  .  .  NH3)I 

III  I 

N 


While  in  the  case  of  the  first  two  compounds  these  formulas  correspond 
to  those  which  are  usually  assigned  to  these  substances,  their  signifi- 
cance from  the  point  of  view  of  valence  chemistry  is  quite  different, 
for  they  serve  to  express  the  strongly  marked  individual  character 
of  this  class  of  bodies  as  compared  with  the  behavior  of  ordinary 
ammonium,  phosphonium,  sulphonium,  etc.,  derivatives.  For  example, 
the  great  instability  of  the  salts  of  the  aromatic  amines,  and  the 
similarity  of  behavior  between  iodonium  and  thallium  salts  finds  expres- 
sion in  these  formulas. 


CHAPTER  VI 
RECENT  THEORIES  IN  REGARD  TO  VALENCY 

THE  development  of  Werner's  theory  in  regard  to  the  mechanism 
of  chemical  combinations  has  opened  anew  the  whole  problem  as  to 
the  nature  of  valency.  While  this  question  is  one  which  cannot  readily 
be  settled,  it  may  be  said  in  general  that  there  is  a  tendency  at  the 
present  time  to  interpret  valency  in  terms  of  electrochemical  concep- 
tions. The  whole  matter  is,  however,  in  a  state  of  flux  and  no  explana- 
tion which  is  of  universal  application  has  as  yet  been  found.  The 
various  theories  of  valency  which  have  been  advanced  from  time  to 
time  have  had,  nevertheless,  a  more  or  less  pronounced  influence 
upon  the  theory  of  structural  organic  chemistry,  so  that  their  origin 
and  development  must  now  be  considered. 

Hugo  Kauffmann  has  recently  advanced  a  theory  in  regard  to 
the  nature  of  atomic  relationships  which  attempts  to  express  Thiele's 
and  Werner's  ideas  concerning  valency  in  terms  of  current  electro- 
chemical conceptions.  According  to  Kauffmann,  a  single  valency 
is  to  be  regarded  not  as  a  separate  unified  force  but  as  made  up  of  a 
bundle  of  force  lines.  If  the  lines  of  force  which  constitute  such  a 
bundle  are  distributed  among  a  number  of  different  atoms,  this  fact 
is  represented  by  as  many  lines  as  there  are  valency  parts.  Valency 
is  then  said  to  be  dispersed  ("  zersplittert  ").  If,  on  the  other  hand, 
all  of  the  lines  of  force  which  roughly  correspond  to  a  unit  valency 
are  used  to  hold  but  one  other  atom,  this  fact  is  represented  by  a 
single  line  or  bond.  This  is  equivalent  in  all  essentials  to  the  sign 
which  is  commonly  employed  to  represent  the  current  conception  of 
unit  valencies,  except  that  it  cannot  be  regarded  as  of  uniform  strength, 
since  Kauffmann's  theory  supposes  that  the  strength  of  the  single 
bonds  which  operate  between  chemical  atoms  varies  greatly  under 
different  conditions.  In  any  given  instance,  the  strength  of  valency 
may  be  measured  by  the  relative  number  of  force  lines  which  consti- 
tute a  bundle. 

A  system  of  valency  lines  is  assumed  to  radiate  into  space  in  much 
the  same  way  as  in  the  case  of  electric  lines  of  force,  and  the  space 
which  they  fill  is,  therefore,  referred  to  as  a  valence  field.  Such  a 

90 


RECENT  THEORIES  IN  REGARD  TO  VALENCY       91 

valence  field  may  become  an  electric  field  if  the  course  of  the  valence 
lines  is  interrupted  by  the  interposition  of  electrons.1  The  valence  fields 
which  are  thus  assumed  to  be  present  on  every  molecule  are  represented 
in  the  same  way  as  other  force  fields,  namely,  by  means  of  arching 
lines  which  extend  out  into  the  surrounding  space.  It  follows  that 
the  relative  amount  of  space  which  is  occupied  by  a  given  valence 
field  will  correspond  roughly  to  the  relative  outside  diameter  of  the 
bundle  of  valence  lines  which  constitute  it.  This  conception  is  in 
agreement  with  the  observed  fact  that  molecules  which  contain 
double  bonds  have  relatively  greater  molecular  volumes  than  those 
which  contain  single  bonds.  The  arch,  which  is  formed  by  the  junction 
of  valence  lines  radiating  from  definite  positions  on  the  surfaces  of 
each  of  the  two  different  atoms,  must  be  regarded  as  corresponding 
to  a  condition  of  tension  similar  to  that  in  a  bent  spring.  This  tension 
will  obviously  depend  upon  the  relative  curve  of  the  arch,  or,  in  other 
words,  it  will  vary  in  proportion  to  the  extent  of  the  valence  field. 
Kauffmann  assumes  that  such  points  of  strain  in  a  molecule  are 
simultaneously  points  of  increased  chemical  reactivity.  The  relatively 
great  chemical  activity  of  atoms  which  are  in  union  by  means  of  double 
bonds  is,  therefore,  explained  as  due  to  tension  in  the  valence  field 
which  results  from  this  union.  The  tension  is  relieved  by  breaking 
the  arch  in  those  outermost  regions  where  the  strain  is  greatest.  The 
affinity  which  is  set  free  in  this  way  then  acts  to  hold  new  atoms  or 
groups. 

According  to  this  conception,  partial  valencies  represent  varying 
fractions  of  valency  and  are  to  be  distinguished  from  principal  valen- 
cies only  by  the  fact  that  they  are  appreciably  weaker.  Changes  in 
the  valency  of  a  given  element  are  interpreted  as  due  merely  to  changes 
in  the  distribution  of  the  total  affinity  of  its  atom. 

lonization  is  supposed  to  be  due  to  the  same  general  cause,  resulting 
in  a  particular  condition  of  valence  dispersion.  The  following  formulas 
have,  for  example,  been  advanced  to  explain  the  ionization  of  ammonium 
chloride  and  nitric  acid: 


Kauffmann's  theory    has  found    its   most    important  application 
in  connection  with  the  development  of  conceptions  which  are  embodied 
in  the  Auxochrome  theory  and  it  will,  therefore,  be   referred  to  again 
1  "  Die  Valenzlehre,"  p.  531. 


92  THEORIES  OF  ORGANIC  CHEMISTRY 

in  a  later  chapter  which  deals  with  the  relation  between  color  and 
constitution  in  organic  compounds. 

The  hypothesis  which  O.  Hinsberg  has  developed  in  regard  to 
valency  centers  in  the  atoms  should  be  mentioned  in  this  connection 
but  can  only  be  referred  to  in  passing,1  since  lack  of  space  in  the 
present  volume  prevents  its  detailed  consideration. 

Finally,  the  theory  of  J.  Stark  must  be  considered.  This  theory 
differs  from  those  which  have  just  been  discussed  in  that  it  seeks  to 
connect  absorption  phenomena  in  organic  chemistry  with  certain 
fundamental  physical  assumptions  which  underlie  the  electromagnetic 
theory  of  light.  Stark  points  out  that  while  chemists  have  been 
content  with  the  assumption  that  material  bodies  are  built  up  from 
indivisible  particles  called  atoms,  physicists,  on  the  other  hand,  have 
long  held  the  view  that  the  atoms  are  themselves  composite  bodies. 
This  conclusion  was  first  arrived  at  as  a  result  of  observations  obtained 
in  the  spectral  analysis  of  the  emanations  of  radioactive  bodies. 
Further  study  of  radioactive  substances  led  to  the  formulation  of  an 
hypothesis  in  regard  to  the  direct  disintegration  of  the  atom,  which 
received  immediate  verification  and  which  resulted,  among  other 
things,  in  the  formulation  of  the  following  general  assumptions: 

1.  The   chemical  elements  disintegrate   to  give   minute   particles, 
which,  whatever  their  source,  are  alike  in  that  each  possesses  a  mass 
equal  to  1/1700  that  of  a  hydrogen  atom,  and  carries  a  negative  electric 
charge  equal  to  4.7X10"10  electrostatic  units.     The  minute  electrified 
particles  are  called  electrons,  and  when  formed  from  the  various  elements 
are  not  only  identical  in  mass  but  carry  identical  electrical  charges. 

2.  The  residue,  which  is  left  after  one  or  more  electrons  have  been 
split  off  from  a  chemical  atom,  is  found  to  possess  an  equivalent  positive 
charge  (positive  ion).     The  mass  of  such  a  positively  charged  particle 
belongs  to  the  same  mathematical  order  of  magnitude  as  does  that 
of  an  atom.     So  far  it  has  not  been  possible  to  separate  the  positive 
charge  on  this  particle  without  destroying  the  integrity  of  the  whole 
individual.     It  follows  that  the  positive  ions  formed  from  different 
chemical  atoms  are  different.     In  other  words,  while  the  most  varied 
chemical  individuals  all  give  rise  to  one  and  the  same  kind  of  negative 
electrons,  they  do  not  give  identical  positive  quanta. 

The  assumption  that  the  chemical  atom  is  composed  of  a  positively 
charged  nucleus  and  of  an  electron  in  union  with  it  does  not  preclude 
the  possibility  that  the  positive  residues  are  themselves  built  up 
of  electrons,  but  as  yet  there  is  no  positive  assurance  as  to  whether 

1  Jour,  prakt.  Chemie,  93,  302;  94,  179  (1916);  95,  121;  96,  166  (1917);  98, 
145  (1918);  99,232(1919);  Ber.,  52,26  (1919). 


RECENT  THEORIES  IN  REGARD  TO  VALENCY       93 

this  is,  or  is  not,  the  case.  According  to  Stark  1  the  positive  charge 
of  an  ion  is  not  distributed  uniformly  through  the  particle  but  is  located 
at  that  particular  point  on  the  atom  from  which  the  electron  was 
discharged.  The  positive  and  negative  quanta  that  thus  constitute 
a  chemical  atom  must  be  imagined  as  in  combination  with  each  other 
not  only  on  the  upper  surface  of  the  atom  but  also  in  the  interior  of  the 
atom,  and  it  must  be  assumed,  further,  that  the  separation  of  any 
single  electric  quantum  from  this  state  of  combination  results  in  the 
formation  of  positive  and  negative  ions. 

Stark  assumes  that  the  electric  quanta  have  a  certain  freedom  of 
motion  inside  the  space  defined  by  the  atom,  and  that  these  movements 
correspond  to  currents  (electric  fields),  which  in  turn  give  rise  to 
corresponding  magnetic  fields.  But  while  electric  and  magnetic  forces 
must  thus  be  imagined  as  operating  on  the  upper  surfaces  of  the  atoms, 
the  latter  must  be  regarded  as  insignificant  as  compared  with  the 
former,  so  far  as  may  be  judged  from  observations  up  to  the  present 
time.  Though  the  part  played  by  the  magnetic  forces  in  the  attrac- 
tion and  binding  together  of  atoms  is  slight,  it  is,  nevertheless,  not 
impossible  that  these  forces  act  in  conjunction  with  electric  forces. 
Stark  calculates,  further,  that  forces  of  gravity  are  so  small  as  to  be 
negligible  when  compared  with  other  forces  operating  between  atoms. 

A  close  study  by  chemists  and  physicists  of  the  association  of  atoms 
indicates  that  the  chemical  atoms  do  not  interpenetrate  in  their  com- 
binations, but  are  bound  together  mainly  by  their  outer  surfaces.  It 
is  supposed  that  the  surface  of  a  chemical  atom  may  be  adequately 
represented  by  a  three-dimensional  arrangement  of  positive  and  nega- 
tive electrical  charges.  If  the  great  difference  in  size  between  the 
fundamental  units  of  positive  and  negative  electricity  is  considered, 
the  surface  of  the  atom  must  be  pictured  as  made  up  of  extended  zones 
of  positive  electricity,  and  between  or  even  above  these  the  compara- 
tively small  point-like  negative  electrons.2  Since  lines  of  .force  radiate 
from  the  electrons  to  the  positive  quanta  on  the  atom,  the  upper 
surfaces  of  the  latter  may  be  regarded  as  constituting  an  electric  field. 

Different  kinds  of  atoms  differ  not  only  in  the  number  of  valence 
electrons  but  also  in  the  distance  of  these  electrons  from  the  upper 


.  der  Radioaktivitat  und  Elektronik  5,  124  (1908);  6,  12  (1909);  9.  15 
(1912);  Physikal.  Zeitschr.,  9.  85;  H.  Ley,  "  Farbe  und  Konstitution,"  68  and  103 
(1911).  also  J.  Stark  "  Die  Atomionen  chemischer  Elemente  und  ihre  Kanalstrahl- 
enspektra,"  Berlin  1913;  "Elektrische  Spektralanalyse  chemischen  Atome,"  Leipzig, 
1914;  "  Die  Elektrizitat  in  chemischen  Atom,"  Leipzig,  1915. 

2  Compare  Rutherford  and  Bohr,  Phil.  Mag.,  21,  669  (1912):  25,  10  (1913);  and 
volumes  26  to  30;  also  L.  Zehnder,  "  Ueber  den  Atombau,"  Verhandl.  der  Deutechen 
Phys.  Ges.,  1916,  324. 


94  THEORIES  OF  ORGANIC  CHEMISTRY 

surface  of  the  atom,  the  extension  of  the  lines  of  force  radiating  from 
them,  the  extension  of  the  positively  charged  zones  on  the  atom,  etc. 
It  also  follows  that  one  and  the  same  atom  may  possess  electrons  located 
at  different  distances  from  the  center  of  the  atom  and  corresponding 
positive  zones  of  varying  extensions. 

If  an  electron  on  the  upper  surface  of  a  chemical  atom  is  at  rest 
it  follows  that  the  resultant  of  all  the  forces  acting  upon  it  must  be 
zero  and,  further,  if  it  is  displaced  from  this  position  work  must  be 
done.  For  example,  if  an  electron  is  imagined  as  removed  from  its 
position  of  rest  by  the  action  of  a  force  at  right  angles  to  the  axis  of 
symmetry  of  its  field  of  force,  it  will  simultaneously  tend  to  be  pulled 
back  into  its  original  position  by  the  development  of  an  equal  force 
acting  in  the  opposite  direction,  although  it  is  also  conceivable  that 
an  electron  might  move  over  the  whole  upper  surface  of  its  atom 
without  the  development  of  any  resistance.  If  the  electron  is  removed 
to  a  greater  and  greater  distance  from  its  atom,  the  resistance  to  be 
overcome  gradually  decreases  until  a  limit  is  reached  when  the 
resistance  is  equal  to  zero  and  when  the  electron  separates  completely 
from  its  atom. 

According  to  Stark,  atoms  are  much  more  variable  in  constitution 
than  has  previously  been  supposed,  since  every  displacement  of  an 
electron  from  its  position  of  rest  is  attended  by  fundamental  changes 
in  its  electric  field.  These  effects  extend  below  the  surface  of  the  atom, 
since  changes  in  the  position  and  extension  of  the  positive  zones  must 
produce  simultaneous  internal  changes.  Thus  each  position  of  a  valence 
electron  corresponds  to  a  definite  condition  of  the  atom  as  a  whole, 
or  in  other  words,  a  condition  of  equilibrium  exists  between  the 
electron  and  its  atom.  If,  however,  an  electron  is  displaced  from  its 
position  of  rest,  as,  for  example,  by  the  chemical  action  of  another 
atom,  the  equilibrium  in  the  system  representing  the  relation  of  the 
electron  to  its  atom  is  destroyed. 

Negative  electrons  located  on  the  upper  surfaces  of  the  atoms  may, 
according  to  Stark,  be  regarded  as  valence  electrons,  since  they  serve 
to  bind  one  atom  to  another  and  therefore  perform  the  same  functions 
as  have  in  the  past  been  attributed  to  valency.  In  terms  of  Stark's 
theory  the  valency  of  a  chemical  element  corresponds  to  the  number 
of  electrons  which  are  present  on  the  upper  surfaces  of  its  atoms. 
Combinations  between  atoms  result  from  the  attractions  exercised 
by  the  positive  zones  of  one  atom  for  electrons  located  on  the  upper 
surfaces  of  others.  Moreover,  since  the  structure  of  an  electric  field 
is  usually  represented  by  the  course  and  density  of  the  lines  of  force 
which  originate  at  positive  charges  and  end  at  equally  great  negative 


RECENT  THEORIES  IN  REGARD  TO  VALENCY 


charges,  we  must  imagine  a  system  of  lines  of  force  radiating  in  all 
directions  from  the  point-like  negative  electrons  and  terminating  in  the 
extended  positive  zones.  The  point-like  origin  and  the  diverging  course 
of  the  lines  of  force  are  important  facts  to  be  remembered  in  consider- 
ing the  union  between  one  atom  and  another,  since  disparity  in  the  size 
of  the  positive  and  negative  units  of  electricity  must  be  regarded  as 
the  determining  factors  in  establishing  simple  relationships  between 
atoms. 

Stark  divides  chemical  atoms  into  three  general  classes,  namely 
electro-positive,  electro-negative  and  electro-dual  ("  electrozwiefach  "). 
For  example,  an  atom,  or,  in  other  words,  the  valence  field  of  an  atom, 
may  be  regarded  as  electro-positive  in  character  when  its  electron 
is  situated  at  a  distance  from  the  atom  equal  to  or  greater  than  the 
diameter  of  the  atom.  Fig.  1  represents  diagrammatically  a  sector  of 
a  positive  valence  field  in  a  plane  which  cuts  both  the  atom  and  its 
electron.  The  figure  also  indicates  the  extension  of  the  field  in  which 
the  electron  may  be  regarded  as  at  rest  with  reference  to  its  atom. 


FIG.  1. 


FIG.  2. 


The  picture  presented  by  a  negative  valence  field  is  quite  different, 
as  Fig.  2  shows.  Here  the  valence  electron  is  represented  as  in  close 
proximity  to  the  surface  of  its  atom.  The  indentation  serves  to  show 
that  the  free  movement  of  the  electron  is  limited  and  that  the  field 
in  which  it  may  be  regarded  as  at  rest  with  reference  to  its  atom  is 
much  contracted  and  almost  point-like  as  compared  to  that  shown  in 
Fig.  1.  The  lines  of  force  which  radiate  from  the  electron  to  the 
positive  zone  on  the  surface  of  the  atom  are  represented  as  strongly 
deflected,  and  the  electric  field  which  they  cover  is,  therefore,  much 
more  extensive  than  in  the  case  of  an  electro-positive  atom. 

Electro-positive  and  electro-negative  atoms  differ  in  their  ability 
to  combine  with  the  negative  electrons  of  other  atoms  and  thus  to  enter 
into  various  forms  of  chemical  combination.  Electro-positive  atoms,  for 
example,  are  so  fully  engaged  in  holding  the  valence  electrons  which 


96  THEORIES  OF  ORGANIC  CHEMISTRY 

properly  belong  to  them  that  they  may  be  assumed  to  have  very 
little  free  affinity  with  which  to  attract  and  hold  the  electrons  of  other 
atoms.  Under  these  circumstances  chemical  combination  with  other 
atoms  is  brought  about  by  the  action  of  the  negative  electrons.  In 
other  words,  the  union  of  electro-positive  atoms  with  other  atoms  results 
from  the  saturation  of  lines  of  force  emanating  from  their  electrons 
rather  than  from  the  positive  zones  of  the  atoms  themselves.  Electro- 
negative atoms,  on  the  other  hand,  possess  the  power  to  attract  and 
hold  a  greater  number  of  electrons  than  properly  belong  to  them  and 
they  are,  therefore,  capable  of  entering  into  chemical  relationships 
with  other  atoms  through  the  saturation  of  lines  of  force  emanating 
from  the  positive  zones  of  their  atoms.  It  follows  that  chemical 
combination  between  electro-positive  and  electro-negative  atoms 
results  from  the  saturation  of  lines  of  force  emanating  from  negative 
electrons  attached  to  the  electro-positive  atoms  and  terminating  in 
positive  zones  on  the  surfaces  of  the  electro-negative  atoms. 1 

Electro-dual  atoms  are  assumed  by  Stark  to  be  more  or  less  inter- 
mediate in  structure  between  electro-positive  and  electro-negative 
atoms.  They  resemble  the  latter  in  that  the  free  movement  of  their 
valence  electrons  is  restricted.  In  other  words,  the  field  in  which 
the  electron  is  at  rest  with  reference  to  its  atom  is  contracted  and 
almost  point-like  in  extent.  They  differ  from  electro-negative  atoms 
in  that  their  electrons  are  subject  to  less  strain  as  the  result  of  displace- 
ments from  their  normal  positions  of  rest  to  other  positions  perpen- 
dicular to  the  axes  of  the  atom  at  successive  points  and  equidistant 
from  the  center.  Electro-dual  atoms  differ  from  electro-positive 
atoms  in  that  their  positive  zones  possess  greater  power  to  hold  the 
electrons  of  other  atoms.  This  power  is,  however,  less  than  that  of 
electro-negative  atoms. 

Atoms  may  be  assumed  to  differ  not  only  in  the  structure  of  their 
valence  fields  but  in  the  number  of  these  fields.  They  may,  therefore, 
be  classified  as  mono-,  di-,  or  tri-polar,  according  to  the  number  of 
valence  electrons  which  they  possess.  Carbon  atoms  may  be  regarded 
as  having  four  electrons  of  the  electro-dual  type,  nitrogen  and  phos- 
phorus, three;  and  oxygen  and  sulphur,  two.  Halogens,  on  the  other 
hand,  are  unipolar  and  represent  the  most  pronounced  type  of  electro- 
negative atoms. 

In  all  cases,  chemical  action  is  due  to  the  saturation  of  lines  of 

force  emanating  from  the  valence  electrons  of  one  atom  and  terminating 

in  the  positive  zones  of  another.     In  the  simplest  type  of  combination 

an  electron  is  bound  to  only  one  other  atom  besides  its  own,  but  since 

1  "Die  Elektrizitat  in  Chemischen  Atom,"  p.  71. 


RECENT  THEORIES  IN  REGARD  TO  VALENCY 


97 


the  atoms  of  different  elements  are  characterized  by  the  presence  of 
one,  two,  three  or  more  electrons,  this  condition  may  obviously  be 
duplicated  on  a  given  atom.  Under  these  circumstances  the  atoms 
in  question  are  said  to  be  bound  together  by  means  of  double  or  triple 
bonds.  Such  a  condition  must  be  sharply  distinguished  from  so-called 
multiple  unions  where  the  lines  of  force  from  a  given  electron  terminate 
in  the  positive  zones  of  two  or  more  atoms  besides  its  own. 

In  the  union  of  two  atoms  by  means  of  single  bonds  a  number  of 
typical  arrangements  may  be  postulated.  For  example,  the  combina- 
tion of  two  electro-negative  atoms  may  be  assumed  to  take  place  as 
represented  in  Figs.  3a  and  36,  while  that  of  two  electro-positive  atoms 
is  shown  in  Fig.  4. 


FIG.  3a. 


FIG.  36. 


FIG.  4. 


In  both  instances  the  union  of  the  two  atoms  is  supposed  to  be 
accompanied  by  only  a  very  slight  contraction  of  their  respective 
fields  of  force  and,  therefore,  represents  at  best  a  weak  form  of  chemical 
combination.  The  union  between  two  electro-positive  atoms  may  be 
assumed  to  be  even  weaker  than  in  the  case  of  electro-negative  atoms, 
since  the  former  have  actually  been  observed  to  exist  in  monoatomic 
condition  in  the  gaseous  state.  Reactions  between  electro-positive  and 
electro-negative  atoms  must  be  considered  in  somewhat  greater 


98  THEORIES  OF  ORGANIC  CHEMISTRY 

detail.  Fig.  5  represents  such  a  union  and  shows  that  the  electro- 
negative atom  An  has  attracted  the  valence  electron  Ep  of  the 
electro-positive  atom  Ap  to  a  position  which  is  nearer  to  its  circum- 
ference than  that  of  its  own  electron  En.  The  atom  Ap  and  the 
system  Ep-Ara-En  thus  appear  to  revolve  upon  a  common  axis.  If 
an  outside  force  now  acts  upon  both  atoms  in  such  a  way  as  to  tend 
to  separate  them  from  each  other,  the  effect  will  be  to  charge  the 
relative  distance  of  Ep  from  Ap  more  rapidly  than  that  of  Ep  from 
An.  If  the  disturbing  influence  is  sufficiently  great  to  overcome  the 
resistance  offered  by  the  attraction  operating  between  Ap  and  Ep, 
the  electron  may  become  separated  from  its  own  atom  while  still 
continuing  to  be  bound  to  the  stronger  atom.  Such  a  condition  would 
result  in  the  dissociation  of  the  compound  AP-AB  into  positive  and 
negative  ions,  corresponding  respectively  to  Ap  and  the  system 


FIG.  5. 

Combinations  of  so-called  electro-dual  atoms  with  each  other  or 
with  electro-positive  or  electro-negative  atoms  take  place  with  only 
minor  modifications  of  this  general  scheme.  In  visualizing  these 
relationships  it  is,  however,  necessary  to  remember  that  the  valence 
electrons  of  electro-dual  atoms  occupy  positions  somewhat  further 
away  from  the  surfaces  of  their  atoms  than  in  the  case  of  electro- 
negative atoms.  Moreover,  while  such  an  electron  may  be  assumed 
to  possess  a  definite  position  of  rest  with  reference  to  its  free  atom, 
it  may,  as  a  consequence  of  the  chemical  combination  of  this  atom, 
become  displaced  from  this  position  without  the  immediate  develop- 
ment of  strain.  Carbon  and  hydrogen  represent  examples  of  atoms 
of  this  type  and  may  be  imagined  as  entering  into  chemical  combina- 
tions through  the  exercise  of  single  bonds  according  to  the  following 
scheme  (Fig.  6).  While  a  carbon  atom  may  be  assumed  to  possess  four 
valence  electrons  arranged  above  its  surface  in  positions  which 
correspond  roughly  to  the  four  corners  of  a  tetrahedron,  the  above 


RECENT  THEORIES  IN  REGARD  TO  VALENCY 


99 


diagram  serves  to  represent  only  a  single  sector  of  the  atom  and  shows 
but  one  of  its  four  poles. 

In  the  combination  of  two  carbon  atoms  the  valence  electrons  may 
be  assumed  to  have  suffered  displacement  from  their  original  positions 
of  rest  with  reference  to  their  free  atoms,  with  the  result  that  each 
electron  is  bound  to  both  atoms.  The  union  of  EI  to  Cn  is,  however, 
relatively  weaker  than  the  union  of  EI  to  Ci  so  that  dissolution  of  the 
compound  takes  place  without  ionization.  It  is  to  be  noted  further 
that  the  valence  fields  Ei-Ci  and  Ej-di  resemble  each  other  much 
more  closely  than  the  valence  fields  EP-AP  and  Ep-An  and  that  the 
former  arrangement  may,  therefore,  be  regarded  as  representing 
relatively  the  strongest  type  of  union  between  chemical  atoms.  These 
considerations  taken  in  conjunction  with  the  assumption  of  the  tetra- 
valence  of  carbon  serve  to  account  for  the  number  and  variety  of 
hydrocarbons  1  and  also  for  the  fact  that  such  compounds  do  not 
undergo  electrolytic  dissociation. 


FIG.  6. 

If  an  electro-dual  atom  (A2)  enters  into  chemical  combination 
with  an  electro-negative  atom,  the  resulting  valence  fields  will  closely 
resemble  those  which  have  just  been  described  in  connection  with 
the  union  of  two  electro-dual  atoms.  Since,  however,  E2  may  be 
assumed  to  possess  relatively  great  elasticity  as  regards  the  limits 
of  its  displacement  with  reference  to  its  own  atom,  it  is  free  to  respond 
to  the  attraction  of  An  without  weakening  its  relation  to  its  own  atom. 
In  the  union  of  two  such  atoms  En  is  simultaneously  drawn  to  Az, 
but  since  the  valence  field  En-A2  represents  a  relatively  small  attract- 
ive force  distributed  over  a  relatively  great  area,  it  forms  the  weakest 
part  of  the  union  between  the  two  atoms.  It  may,  therefore,  be 
assumed  to  offer  the  least  resistance  to  the  action  of  an  outside  force 
and,  in  opening,  to  afford  points  of  attraction  for  an  adding  molecule. 
Such  a  conception  serves  to  account  for  the  mechanism  of  the  formation 
of  addition  products  or,  in  general,  of  all  so-called  molecular  compounds. 
1  "  Die  Elektrizitat  in  Chemischen  Atom,"  p.  82. 


100  THEORIES  OF  ORGANIC  CHEMISTRY 

The  combination  of  an  electro-dual  and  an  electro-positive  atom 
is  attended  by  the  expenditure  of  very  little  energy.  Such  unions 
may  be  fairly  stable,  as  in  the  case  of  the  metallic  carbides.  In 
the  case  of  zinc  methyl  on  the  other  hand,  and  in  the  case  of  those 
metallic  derivatives  of  the  hydrocarbons  which  have  been  described 
by  Schlenk,  this  type  of  combination  appears  to  be  distinctly  unstable. 
The  latter  compounds  are  especially  sensitive  to  the  action  of  non- 
metals,  under  the  influence  of  which  the  valence  field  EZ-AP  opens, 
and  thus  affords  an  opportunity  for  the  union  of  Ap  with  the  electro- 
negative atom. 

Two  atoms  may  enter  into  combination  by  means  of  so-called 
double  bonds  if  four  electrons — two  for  each  atom — are  free  to  engage 
in  establishing  the  union.  In  visualizing  such  an  arrangement  it  must 
be  assumed  that  the  two  pairs  of  valence  fields  lie  in  different  planes. 
Figs,  la  and  7b  represent  sections  of  two  doubly-bound  carbon  atoms 
which  are  supposed  to  lie  at  right  angles  to  each  other: 


FIG.  7a.  FIG.  76. 

The  valence  electron  of  one  atom  is  thus  imagined  as  sending  lines 
of  force  to  positive  zones  which  lie  between  two  valence  electrons  of 
the  second  atom.  It  is  obvious  that  atoms  which  are  bound  together 
in  this  way  are  not,  like  singly-bound  atoms,  free  to  rotate  about  a 
common  axis.  If,  for  example,  the  atom  Ci  (represented  in  Fig.  7a), 
were  to  rotate  about  the  axis  of  symmetry  of  its  valence  field  while  Cn 
remained  motionless,  the  effect  would  be  to  bring  the  valence  electrons 
of  Ci  nearer  to  those  of  Cn.  Such  a  movement  would  involve  a 
lengthening  of  the  fields  of  force  between  Ci  and  its  electrons  since 
only  in  this  way  would  the  latter  be  enabled  to  pass  outside  the  force 
fields  of  Cn  and  its  electrons.  Since,  however,  any  force  tending  to 


RECENT  THEORIES  IN  REGARD  TO  VALENCY  101 

separate  an  electron  from  its  position  of  rest  with  reference  to  its  own 
atom  is  always  met  by  an  equally  great  resistance,  a  torsion  in  the 
opposite  direction  should  act  simultaneously  to  bring  Ci  back  to  its 
original  position  with  reference  to  Cn.  The  fact  that  freedom  of 
movement  of  Ci  with  reference  to  Cn  is  obviously  impossible  under 
these  conditions  serves  to  account  for  the  phenomenon  of  stereo- 
isomerism  which  has  been  observed  in  the  case  of  fumaric  and  maleic 
acids  and  other  substances  of  the  general  formula; 

RiCR2  R2CRi 

II         and  || 

R3CR4  R3CR4 

Double  bonds  may  be  assumed  to  open  more  readily  than  single 
bonds  under  the  action  of  an  outside  force  and,  in  so  doing  to  afford 
points  of  free  affinity  within  the  molecule.  This  conception  serves 
to  account  for  the  ease  with  which  substances  containing  doubly 
bound  atoms  react  to  form  addition  products,  polymerization  products 
and  molecular  compounds. 

The  usual  method  of  representing  unsaturated  linkages  by  means 
of  double  bonds  is  retained  by  Pauly  and  Stark,  but,  according  to  the 
terms  of  Stark's  theory,  must  be  regarded  as  indicating  merely  a  normal 
and  stable  mean  condition.  Such  a  condition  is  possible  only  when  the 
two  carbon  atoms  are  symmetrically  arranged  and  when  the  molecule 
is  itself  in  a  condition  of  rest  and  unacted  upon  by  outside  forces. 1 

The  earlier  papers  published  by  Stark  interpreted  the  phenomenon 
of  unsaturation  in  quite  a  different  manner  from  that  which  has  just 
been  described,  and  assumed  that  the  two  unsaturated  atoms  were 
held  together  by  means  of  a  single  bond  and  that  each  was  characterized 
by  the  presence  of  a  free  ("  gelockert  ")  valence  electron.  While  the 
development  of  this  portion  of  Stark's  theory  has  been  omitted  from 
the  present  edition  2  it  must,  nevertheless,  be  conceded  that  this  con- 
ception has  been  the  basis  for  a  satisfactory  interpretation  of  certain 
reactions  which  are  inexplicable  in  terms  of  any  of  the  other  theories. 
Thus,  for  example,  H.  Pauly,3  in  conjunction  with  von  Buttlar,  has 
been  able  to  demonstrate  experimentally  that  phenolic  aldehydes  are 
markedly  less  reactive  than  other  aldehydes.  This  is  shown  in  the 
fact  that  benzoin  condensations,  intramolecular  oxidations  and  reduc- 
tions in  the  presence  of  alkali  (Cannizzaro),  the  formation  of  hydrox- 

1  Ber.,  48,  2017  (1915). 

2  Compare  1913  Edition. 

3Annalen  der  Chemie,  383,  230,  288  (1911);  Ber.,  48,  2010  (1915);  Jour,  prakt. 
Chemie,  98,  106  (1918). 


102  THEORIES  OF  ORGANIC  CHEMISTRY 

amic  acids  (Angeli),  the  reaction  with  ortho-formic  esters  (Claisen), 
acetalization  with  alcohol  and  hydrochloric  acid  (E.  Fischer),  the 
naphthoquinone-carboxylic  acid  reaction  (Doebner)  and  finally  the 
fuchsine-sulphuric  acid  reaction,  all  take  place  less  readily  in  the  case 
of  hydroxy-aldehydes  than  in  the  case  of  unsubstituted  aldehydes,  and 
in  certain  instances  it  may  even  happen  that  the  normal  course  of  the 
reaction  is  definitely  interfered  with  as  a  result  of  the  introduction  of 
the  hydroxyl  group.  It  has,  moreover,  been  established  by  these 
investigators  that  the  phenolic  oxygen  as  a  whole  is  the  determining 
factor  in  influencing  these  reactions. 

Pauly  has  been  unable  to  explain  these  phenomena  in  term*  of  any 
of  the  current  theories  with  the  single  exception  of  the  electro-atomic 
theory  of  Stark.  According  to  Stark's  earlier  method  of  representation, 
the  presence  of  single  bonds  between  atoms  is  indicated  by  means  of 

the  symbol  < >,  and  free  electrons  by  the  symbol  —  °.     The  aldehyde 

group  and  the  hydroxy-aldehyde  group  may,  therefore,  be  represented 
in  terms  of  Stark's  theory  by  means  of  the  diagrams  shown  respectively 
in  Fig.  8  and  Fig.  9.  It  will  be  observed  that  in  Fig.  9  the  double 


FIG.  8.  FIG.  9. 

arrow,  < >,  between  the  carbon  atom  and  the  hydroxyl  group  is  printed 

in  heavy  type.     This  serves  to  indicate  that  the  union  is  relatively 

a  very  stable  one.   The  double  arrow,  < >,  on  the  other  hand  is  printed 

in  light  type  and  thus  indicates  that  this  form  of  combination  between 
carbon  and  oxygen  is  relatively  labile  in  character.  The  facts  which 
are  here  represented  may  be  expressed  in  terms  of  Stark's  theory 
by  saying  that  the  positive  field  of  force  on  the  aldehyde  oxygen  atom  is 

not  fully  occupied  in  holding  the  carbon  atom  (< >)  and  can,  therefore, 

engage  its  free  electron  (— °)  with  a  relatively  great  fraction  of  its 
affinity.  Such  a  condition  corresponds  to  a  decrease  in  the  reactivity 
of  the  carbonyl.  A  further  study  of  Fig.  9  shows  that  the  hydrogen 
atom  of  the  hydroxyl  group  is  in  a  loose  form  of  combination  with 
oxygen.  Such  a  condition  may  result  in  the  ionization  of  the  hydrogen 
under  the  action  of  a  solvent.  In  this  event  the  hydrogen  atom  may 
be  assumed  to  have  lost  its  negative  valence  electron  and  must,  there- 
fore, be  regarded  as  bound  to  the  oxygen  atom  only  by  those  lines  of 


RECENT  THEORIES  IN  REGARD  TO  VALENCY  103 

force  which  operate  between  the  negative  valence  electron  of  the  oxygen 
atom  and  the  positive  zone  of  the  hydrogen  atom.  Such  a  one-sided 

type  of  union  is  indicated  by  the  single  arrow,  — >  in  Fig.  10.  This 
figure  represents  diagrammatically  an  hydroxy-aldehyde  after  ioni- 
zation  of  the  hydrogen  has  occurred; 


FIG.  10. 
Another  type  of  hydroxy  aldehyde  is  shown  in  Fig.  11: 

$      1 


o)A(|> 


b 


c^Wc^fc? 


FIG.  11. 


uu 

. 


Here  the  strong  union  represented  by  the  double  arrow,  <  -  »,  results 
not  only  in  the  ionization  of  the  hydrogen  of  the  hydroxyl  group  but 
also  affects  other  parts  of  the  molecule.  The  most  immediate  effect 
is  upon  the  valence  electron  on  C3  which  is  represented  as  in  close  con- 
tact with  its  atom,  thus  indicating  that  the  negative  charge  of  this 
atom  is  fully  engaged.  The  union  between  C3  and  C2  is  simultaneously 

weakened  and  this  in  turn  acts  to  strengthen  the  linkage  <  -  »  between 
C2  and  C1.  The  fact  that  C1  is  thus  called  upon  to  exercise  a  relatively 
large  part  of  its  free  affinity  in  holding  C2  leaves  this  atom  with  rela- 
tively little  affinity  for  its  union  with  oxygen.  The  latter  is  thus  able 
to  engage  its  free  ("  gelockert  ")  valence  electron  more  energetically 
.d  the  latter  moves  to  a  position  near  the  surface  of  the  atom.  As 
in  the  preceding  case  this  condition  is  reflected  in  a  decrease  in  the 
reactivity  of  the  carbonyl  group.  Both  illustrations  not  only  serve 
to  show  how  a  change  in  the  relative  strength  of  the  union  between 
two  atoms  in  a  molecule  is  inevitably  accompanied  by  simultaneous 
changes  in  other  parts  of  the  molecule,  but  also  help  to  support 
Stark's  general  theory  that  no  linkage  can  be  strengthened  except  at 
the  expense  of  another.  In  the  light  of  this  theory  it  is  easy  to 
understand  how  the  introduction  of  a  given  substituent  into  a  chain 
of  carbon  atoms  or  into  a  ring  compound  may  come  to  have  an  important 


104 


THEORIES  OF  ORGANIC  CHEMISTRY 


influence  upon  the  reactivity  of  other  atoms  or  groups  even  when 
these  are  located  in  a  distant  part  of  the  molecule. l 

When  two  atoms  unite  by  means  of  triple  bonds,  six  valence 
electrons  are  engaged,  three  for  each  atom.  In  this  case  Stark,  in 
his  recent  treatises,  assumes  that  all  three  electrons  belonging  to  a 
given  atom  lie  in  the  same  plane,  and  that  the  two  planes  represented 
by  the  two  sets  of  electrons  are  parallel  to  each  other  but  at  some 
distance  apart.  The  union  of  two  nitrogen  atoms  by  means  of  triple 
bonds,  NE==N,  is  shown  in  Fig.  12a,  and  Fig.  126. 


E 


3  En, 


FIG.  126. 


Here  each  figure  represents  a  section  taken  in  such  a  way  as  to  cut 
the  three  valence  electrons  as  well  as  the  positive  zones  of  each  atom. 
Since  the  two  sections  lie  in  different  planes,  one  above  the  other  and 
at  some  distance  apart,  they  obviously  cannot  be  represented  in  the 
same  drawing  although  the  relative  positions  of  the  electrons  of  the 
second  atom  with  reference  to  the  positive  zones  of  the  first  may  be 
indicated  in  each  case  by  means  of  dotted  projections.  As  in  the  case 
of  doubly -bound  atoms,  free  rotation  of  the  atoms  is  obviously 
impossible.  A  consideration  of  the  fields  of  force  leads  to  the  con- 
clusion that  this  form  of  combination  is  less  stable  than  in  the  case 
of  single  bonds  but  that  it  is  less  sensitive  to  the  action  of  an  outside 
force  than  in  the  case  of  double  bonds. 

Stark  has  attempted  to  deduce  the  properties  of  a  large  number 
of  chemical  compounds  from  a  consideration  of  the  force  fields  in 
atomic  groupings  such  as  N=N,  C=N,  CHs,  NH^OH,  etc.,  and,  as 
a  result,  has  been  able  to  demonstrate  that  many  of  the  variations 
in  the  chemical  relationships  of  the  atoms  which  the  older  theories  of 
valency  have  failed  to  elucidate,  may  be  accounted  for  readily  in  terms 
of  his  theory.  His  system  serves  to  explain  the  most  varied  phenomena 
Compare  Meerwein,  Annalen  der  Chemie,  419,  121  (1919). 


RECENT  THEORIES  IN  REGARD  TO  VALENCY 


105 


in  inorganic  as  well  as  in  organic  chemistry,  such  as  changing  valency, 
intermolecular  combinations,  etc. 

It  may  be  added  that  H.  Pauly  represents  the  various  types  of  union 
between  atoms  by  means  of  symbols  which  possess  the  advantage 
of  being  simpler  than  those  which  have  been  suggested  by  Stark: 


Types  of  Electro-valency 

According  to 
J.  Stark 

According  to 
H.  Pauly 

1.  Ionic  
2    Simple  linkages. 

O->H 
C<=±C 

O-^H 

c  —  c 

3.  Double  linkages  (benzene)  
4    Double  linkages  (olefine)  

esc 
figfl 

C^C 

c=c 

5    Acetylene  linkages 

V^VJ 

flSf! 

c=c 

\J+=>\J 

It  should  be  noted,  however,  that  in  the  case  of  Pauly's  symbols  a  single 
line  is  always  understood  to  represent  the  saturation  of  two  valence 
electrons. 

Among  the  more  recent  interpretations  which  have  been  advanced 
to  explain  the  various  relationships  which  exist  between  the  atoms, 
must  be  mentioned  the  kineto-electro-magnetic  theory  of  J.  Beck- 
enkamp1  and  the  kinetic  theory  of  A.  von  Weinberg.2  Neither  of  these 
theories  can,  however,  be  more  than  referred  to  at  this  tune. 

In  conclusion  it  may  be  said  that  Stark's  theory  in  regard  to  atomic 
relationships  is  of  especial  interest  to  chemists  because  of  the  fact  that 
it  seeks  to  establish  a  relation  between  the  structure  of  different  valence 
fields  and  the  optical  properties  of  the  substances  in  which  they  are 
found.  In  this  connection  it  may  be  said  briefly  that  the  valence 
fields  of  the  atoms  present  in  the  molecules  of  chemical  elements  and 
compounds  are  assumed  to  vibrate  between  definite  positions  of 
equilibrium,  and  that  these  movements  are  electro-magnetic  in 
character  and  are  accompanied  by  the  emission  and  absorption  of 
light.  The  waves  of  light  which  are  emitted  or  absorbed  by  a  par- 
ticular valence  field  are  assumed,  moreover,  to  possess  the  same  fre- 
quency as  the  valence  field  itself,  so  that  by  measuring  the  frequency 

iVerhandl.  d.  Phys.  Med.  Geo.  zu  Wiirzburg,  46,  135  (1918);  "  Leitfaden  der 
Krystallographie,"  by  J.  Beckenkamp,  p.  387  and  following.  Pub.  by  Bornstrager, 
Berlin,  1919. 

2Ber.,  62,  928  and  1501  (1919). 


106  THEORIES  OF  ORGANIC  CHEMISTRY 

or  intensity  of  these  waves  of  light  it  is  possible  to  determine  the 
character  of  the  valence  field  and  thus  to  arrive  at  certain  definite 
conclusions  in  regard  to  the  nature  and  strength  of  union  of  valence 
electrons  in  their  positions  of  equilibrium.  The  constitution  of  the 
molecule  may  in  this  way  be  deduced  from  a  study  of  the  optics  of 
its  respective  valence  fields. 

Stark's  theory,  thus  briefly  outlined,  represents  in  its  fundamental 
aspects  a  resurrection  of  the  electrochemical  theory  of  Bej-zelius. 
Stark' s  particular  contribution  consists  in  bringing  the  more  funda- 
mental electrochemical  conceptions  into  harmony  with  modern 
physical  ideas  in  the  field  of  atomicity,  and  in  this  way  developing  a 
much  broader  interpretation  of  the  mechanism  of  chemical  action 
than  is  possible  in  terms  of  the  prevailing  theory  of  valency.  Stark's 
conceptions  allow  of  a  much  more  intimate  understanding  of  the 
chemical  relationships  of  the  atoms  than  is  otherwise  possible  and 
at  the  same  time,  they  anticipate  much  finer  differences  in  structure 
than  is  afforded  by  the  application  of  any  other  single  theory.  On  the 
other  hand,  it  cannot  be  denied  that  these  conceptions  are  somewhat 
involved  and  do  not  lend  themselves  easily  to  any  brief  summary. 
Stark  himself  acknowledges  1  that — "  many  chemists  may  be  dismayed 
by  the  profusion  of  different  images  which  may  be  derived  from  our 
valence  hypothesis,  may  regard  it  as  intricate,  and  may  even  hesitate 
to  penetrate  into  the  labyrinth  of  types  of  chemical  combination 
which  we  have  described."  Nevertheless,  Stark's  theory  is  as  compre- 
hensive as  any  current  at  the  present  time.  In  that  it  attempts  to 
explain  in  terms  of  modern  physical  conceptions  the  relation  which 
has  been  observed  to  exist  between  the  optical  properties  of  a  substance 
and  the  chemical  constitution  of  its  molecule,  it  serves  to  combine  the 
interest  of  both  physicists  and  chemists  upon  the  solution  of  a 
problem  which  offers  the  most  promising  developments  of  any  now 
under  investigation. 

1  "  Die  Elektrizitat  in  chemischen  Atom,"  p.  94. 


CHAPTER  VII 
THE  ELECTRON   CONCEPTION   OF  VALENCY 

ON  the  basis  of  the  electron  theory,  Falk  and  Nelson  1  have 
developed  a  set  of  conceptions  which  embody  J.  J.  Thomson's  ideas  2 
regarding  the  constitution  of  the  atom.  They  start  with  the  assump- 
tion that  each  individual  atom  represents  a  close  confederation  of 
electrons,  or,  as  J.  J.  Thomson  calls  them,  "  corpuscles,"  and  that 
these  are  in  constant  motion.  If,  by  the  action  of  physical  or  chemical 
forces,  one  or  more  electrons  are  separated  from  the  atom,  the  latter 
receives  a  corresponding  positive  charge.  Since  such  a  positive  charge 
acts  as  an  attractive  force  in  holding  electrons  together  in  the  molecule, 
it  follows  that  the  number  of  electrons  which  may  be  separated  in  this 
way  is  very  small. 

The  atoms  of  the  different  elements  differ  from  each  other  in  the 
ease  with  which  their  electrons  dissociate.  According  to  Thomson, 
certain  atoms  have  also  the  power  to  combine  with  free  electrons  and, 
in  so  doing,  to  receive  a  negative  electric  charge.  Such  atoms  differ 
both  as  to  the  ease  with  which  they  add  electrons  and  in  their  ability 
to  hold  the  electrons  which  have  been  added.  Elements  whose  atoms 
behave  in  this  way  are  called  electro-negative  in  .contradistinction 
to  the  electro-positive  elements  whose  atoms  tend  to  lose  electrons 
and  so  to  receive  a  positive  charge. 

If  the  affinity  of  the  elements  for  each  other  is  electrical  in  character 
it  follows  that  the  ability  of  an  atom  to  enter  into  chemical  combina- 
tions will  depend  upon  its  power  to  hold  an  electric  charge.  In  terms 
of  this  theory,  a  monovalent  atom  is  one  which  carries  a  unit  charge 
of  electricity,  while  a  multivalent  atom  is  one  which  bears  two  or 
more  such  units,  the  number  varying  according  to  the  valency  of  the 
element. 

Every  chemical  combination  between  atoms  is  accompanied  by  an 
exchange  of  electrons — one  atom  losing  an  electron  and  becoming 
thereby  positively  charged,  and  the  other  atom  receiving  an  electron 
and  becoming  negatively  charged.  To  denote  this  change  Thomson 

1  Jour.  Am.  Chem.  Soc.,  32,  1637  (1910). 

2  The  Corpuscular  Theory  of  Matter,  p.  138  (1907). 

107 


108  THEORIES  OF  ORGANIC  CHEMISTRY 

used  the  so-called  "  Faraday  Tubes  of  Force  "  instead  of  the  usual 
+  and  •  -  signs,  while  Falk  and  Nelson  uses  an  arrow,  — >.  The 
direction  of  the  arrow  is  used  to  denote  the  direction  taken  by  the 
electron  in  its  passage  from  one  atom  to  the  other. 

Hydrogen  shows  by  its  properties  that  it  is  an  electro-positive 
element,  or,  in  other  words,  that  it  tends  to  lose  an  electron  in  the 
process  of  chemical  combination  with  other  atoms.  It  does  not  seem 
to  possess  the  opposite  faculty  of  adding  electrons.  Carbon,  on  the 
other  hand,  shows  from  its  properties  that  it  possesses  the  power  both 
to  add  and  to  lose  electrons;  in  other  words  it  is  both  electro-negative 
and  electro-positive  in  character.  This  is  indicated,  in  the  case  of  two 
of  its  compounds,  by  means  of  the  following  formulas: 

H  Cl 

H->C<-H  C1«-C-*C1 

H  Cl 

Methane  Carbon  tetrachloride 

The  formula  for  ethane  becomes: 

H        H 

I         I 
H->C >C<-H 

n    t2 

H        H 

and  differs  from  the  ordinary  structural  formula  assigned  to  ethane 
in  that  the  carbon  atoms  1  and  2  are  not  alike  in  all  respects,  since 
C2  possesses  one  electron  more  than  Ci.  Similar  relationships  are 
even  more  strikingly  evident  in  the  formulas  of  other  compounds 
which  will  be  referred  to  later,  and  serve  to  account  for  the  fact,  so 
frequently  observed,  that  one  of  two  apparently  identical  groups  is 
actually  more  reactive  than  the  other.  The  significance  of  this  is 
brought  out  most  conspicuously  in  the  case  of  the  homologous  series 
of  dibasic  acids.  In  the  case  of  this  series,  a  regular  increase  in  the 
number  of  carbon  atoms  is  not  accompanied  by  a  regular  variation 
in  melting  point,  solubility,  etc.  A  close  study  of  the  properties  of 
these  substances  shows,  further,  that  acids  possessing  an  even  number 
of  carbon  atoms  (as  2,  4,  6,  etc.)  resemble  each  other  much  more 
closely  and  regularly  than  the  acids  immediately  following  each  other 
in  the  series  (as  1,  2,  3,  etc.).  The  same  general  statement  holds  true 
for  acids  having  an  uneven  number  of  carbon  atoms  (as  3,  5,  7,  etc.)- 


THE  ELECTRON  CONCEPTION  OF  VALENCY  109 

These  facts,  which  are  not  accounted  for  in  any  way  by  the  ordinary 
structural  formulas  of  the  compounds,  may  be  explained  by  the 
electronic  formulas  of  Falk  and  Nelson.  Thus  the  acids  I  and  II, 

HOOC<-CH2-^COOH    and     HOOC  <-  CH2  ->  CH2  ->  COOH 
I  II 

show  a  symmetrical  distribution  of  electrons  for  acids  having  an  odd 
number  of  carbon  atoms,  and  an  unsymmetrical  distribution  of  electrons 
for  acids  having  an  even  number  of  carbon  atoms.  The  fact  that 
the  symmetrical  arrangement  alternates  with  the  unsymmetrical 
arrangement  for  the  series  as  a  whole  accounts  for  the  irregularities 
in  properties  of  acids  which  immediately  follow  each  other  in  the  series. 

Since  substituting  groups,  each  in  its  own  characteristic  way, 
affect  the  mobility  of  electrons  in  a  given  atom,  it  is  necessary  to  dis- 
tinguish between  two  classes  of  olefines,  viz.,  those  in  which  the  two 
halves  of  the  molecule  are  similar  (as,  for  example,  R2C=CR2  and 
RR/C=CRR/)  and  those  in  which  the  two  halves  are  dissimilar 
(as,  R2C=CRR',  R2C=CR2',  R2C=CR'R,"  etc.). 

According  to  Falk  and  Nelson,  the  union  of  two  carbon  atoms  by 
means  of  a  double  bond  involves  the  transference  of  two  electrons 
from  one  carbon  atom  to  the  other,  and  this  may  take  place  in  either 
of  two  ways: 

R2C=SCR2,       or       R2C±=>CR2 

RRiCntCRR1,     or     RR'C^CRR1 

Of  the  two  isomers  possible  in  each  case,  one  must  be  more  stable 
than  the  other.  Because  no  isomerism  has  as  yet  been  discovered  in 
the  case  of  substances  belonging  to  the  class  represented  by  R2C=CR2, 
Falk  and  Nelson  assume  that  one  of  the  two  isomers  theoretically 
possible  is  so  unstable  that  it  is  transformed  instantly  into  the  other 
stable  form. 

Compounds  of  the  general  formula  RR/C=CRR'  are  commonly 
found  in  two  isomeric  modifications;  but  according  to  the  present 
theory  three  isomers  are  possible,  as  for  example: 

CH3CH<=±CHCH3;    CH3CH  =£  OHCH3 ;    and    CH3CH^CHCH3 

To  explain  the  discrepancy  between  fact  and  theory  it  may  be  assumed, 
as  in  the  preceding  case,  that  one  of  the  three  possible  isomers  is  so 
unstable  that  it  cannot  be  isolated.  It  may  be  said,  however,  that  the 
present  lack  of  harmony  between  fact  and  theory  in  such  cases  must  be 
regarded  as  an  evidence  of  weakness  in  the  interpretations  of  Falk  and 


110  THEORIES  OF  ORGANIC  CHEMISTRY 

Nelson.  Similar  objections  hold  in  the  case  of  triply  bound  carbon 
where  a  number  of  isomers  are  theoretically  possible  and  where  only 
one  substance  having  the  formula  RC  2  CR,  is  assumed  to  be  stable. 
Nitrogen  has  the  power  to  add  three  electrons  as  the  formula  of 
ammonia  shows.  That  nitrogen  has  also  the  power  to  lose  five  elec- 
trons is  obvious  from  a  consideration  of  its  oxygen  compounds.  Thus 
the  nitrogen  in  ammonia  differs  from  the  nitrogen  in  nitric  acid  by 
eight  electrons.  The  chemical  properties  of  compounds  containing 
nitrogen,  in  which  that  element  is  in  simple  union  with  other  atoms 
or  groups  of  atoms,  depend  upon  whether  such  atoms  or  groups  are 
electro-positive  or  electro-negative  in  relation  to  nitrogen.  For  example, 
Falk  and  Nelson  assume  that  in  hydrazine  the  following  relation 
exists  : 


Thus  the  a-atom  is  seen  to  hold  one,  while  the  /3-atom  holds  three 
negative  charges,  and  expression  is  given  to  the  fact  that  the  two  nitro- 
gen atoms  are  not  equivalent.  The  difference  in  chemical  properties 
between  them  is  shown,  for  example,  in  the  ease  with  which  they  combine 
respectively  with  hydrogen  chloride.  Thus  the  dichloride  is  unstable 
and  readily  loses  a  molecule  of  hydrochloric  acid,  so  that  the  hydra- 
zine functions  more  as  a  mono-  than  as  a  di-acid  base.  Similar  differ- 
ences are  even  more  strongly  marked  in  the  case  of  derivatives  of 
hydrazine. 

Isomeric  diazo-compounds  are  formulated  as  follows: 

R-+N£N;    R-*N:t=N-»X;   R->N  <=±N-+O<-H 
X 

Diazonium  salt  Syn-diazo  compound  Anti-diazo  compound 

If  the  atoms  of  different  elements  are  in  simple  forms  of  combina- 
tion, it  is  usually  possible  to  predict,  from  the  nature  of  the  elements. 
in  question,  the  direction  in  which  the  electron  of  a  given  atom  will 
move.  If  two  atoms  are  doubly  bound,  the  second  electron  may 
move  in  the  same  direction  as  the  first  or  in  the  opposite  direction. 
In  this  way  three  structural  isomers  may  arise,  which,  of  necessity, 
must  differ  as  to  stability.  In  the  case  of  the  carbonyl  group,  for 
example,  the  electrons  may  be  distributed  as  follows: 


I  II  III 

Of  these  forms,  I  must  be  extremely  stable,  III  must  be  so  very  unstable 
that  it  cannot  be  isolated,  while  II  represents  a  mean  between  I  and 


THE  ELECTRON  CONCEPTION  OF  VALENCY       111 

III  in  its  properties.  The  stable  and  labile  forms  of  benzophenone, 
which,  according  to  Schaum,1  differ  as  to  their  chemical  behavior, 
may  be  regarded  as  representing  types  I  and  II  respectively. 

The  well-known  isomerism  of  the  aldoximes  is  regarded  by  Falk 
and  Nelson  as  due  not  to  space  isomerism  but  to  the  following  dis- 
tribution of  electrons: 


RHC=tN->O<-H,     and 

Anti-form  Syn-form 

The  difference  in  chemical  properties  shown  by  the  two  forms,  must, 
in  terms  of  the  present  theory,  be  due  to  the  difference  in  the  union 
of  carbon  and  nitrogen  in  the  two  cases.  A  similar  interpretation  holds 
for  the  ketoximes.  Thus  the  Beckmann  rearrangement  depends 
upon  the  potential  difference  between  the  group  which  substitutes 
for  OH,  and  that  which  contains  the  nitrogen  atoms. 

Since  the  individual  parts  of  a  chemical  compound,  must,  in  general, 
bear  different  charges,  it  follows  that  one  part  of  a  molecule  bearing  a 
positive  charge  will  tend  to  attract  a  part  of  some  other  molecule  which 
carries  an  opposite  charge.  This  kind  of  affinity  must  differ,  both  in 
nature  and  in  intensity,  from  that  which  operates  by  'the  transfer 
of  an  electron  to  produce  simple  types  of  chemical  combination.  Falk 
and  Nelson  assume  that  it  corresponds  in  character  to  partial  or 
residual  valencies.  The  electrical  attraction  exercised  by  an  atom 
or  group  of  atoms  for  others  in  other  molecules  of  the  same  or  of  different 
compounds  may  give  rise  to  combinations  which  are  sufficiently  stable 
under  ordinary  conditions  to  produce  the  characteristic  properties  of  a 
new  substance,  such  as,  for  example,  its  distinctive  color. 


Classification  of  Organic  Reactions 

According  to  the  electronic  conception  of  chemical  change,2  the 
phenomena  of  oxidation,  reduction  and  valency  may  be  briefly 
defined  as  follows:  oxidation  of  an  atom  involves  the  loss  of  electrons, 
or  the  gain  of  positive  charges,  while  reduction  is  the  gain  of  electrons, 
or  the  loss  of  positive  charges.  The  valence  of  an  element  is  the  number 
of  corpuscles  (negative  electrons)  which  an  atom  of  that  element 
loses  or  gains  when  it  enters  into  chemical  combination.  Based  on  the 
above  conceptions,  organic  reactions  (or  inorganic  reactions)  may  be 
classified  as  follows: 

1  Chem.  Zeitung,  47,  417  (1910). 

2  Nelson.  Beans  and  Falk,  Jour.  Am.  Chem.  Soc.,  36,  1810  (1913). 


112  THEORIES  OF  ORGANIC  CHEMISTRY 

1.  Reactions  in  which  the  algebraic  sum  of  the  positive  and  negative 
charges  on  a  definite  atom  of  the  molecule  changes  : 

(a)  The  number  of  electrons  increases. 
(6)  The  number  of  electrons  decreases. 

2.  Reactions  in  which  the  algebraic  sum  of  the  positive  and  nega- 
tive charges  on  the  atom  remains  constant: 

(a)  The  arithmetical  sum  changes. 

(6)  The  arithmetical  sum  remains  constant. 

Reactions  classified  under  la  and  16  are  those  involving  reduction 
and  oxidation  respectively.  The  oxidation  and  reduction  of  the  atoms 
of  carbon,  nitrogen  and  iron  are  represented  in  the  following  reversible 
electronic  equations: 

-+  -+  -+  +  + 

CH4  ->  CHaCl  ->  CH2C12  ->  CHC13  -»  CCU 

-+  -+  -  +  +  + 

CH4  -*  CHsOH  ->~CH20  ->+CH2O2  ->  CO2 

-  +  +  +  +  + 

+  +  +  +  + 

NH3  ->  NH2OH  -*  NH(OH)2  -+  NO(OH)  -»  N02(OH) 


FeCl2  -*  FeCl3 

Oxidation 


Reduction 

Reactions  characteristic  of  Class  2a  are   those  which  Nelson  and 
Falk  term  the  "  onium  "  type.     For  example,  the  addition  of  hydrogen 


chloride,  HC1,  to  ammonia,  NHs,  forms  ammonium  chloride, 
producing  thereby  an  arithmetical  increase  in  electrical  charges  but 
no  algebraic  increase.     Other  representatives  of  this  group  of  "  onium  " 


i±._ 


compounds  are  iodonium,  I  R2  X,  sulphonium,  S  Rs  X,  and  arsonium 


compounds,  As  R  X.  The  reverse  transformations  would  also  appear 
in  this  classification.  Some  interesting  "  onium  "  formations  are  later 
discussed  in  connection  with  the  phenomena  of  esterification,  saponi- 
fication,  etc.  Reactions  characteristic  of  Class  26  are  those  repre- 
sented by  the  term  metathetic,  and  include  all  ionic  transformations 


THE  ELECTRON  CONCEPTION  OF  VALENCY  113 

which  do  not  involve  oxidation  or  reduction.     Tautomeric  rearrange- 
ments, are  also  included  here. 


The  Significance  of  "  Onium  "  Compounds  in  Certain 
Chemical  Reactions  l 

The  addition  of  alkyl  halides  to  amines  with  formation  of  quaternary 
ammonium  salts  is  a  reversible  reaction,  dissociation  being  favored 
by  rise  of  temperature  and  the  action  of  alkali.  These  transforma- 
tions lead  to  two  types  of  equilibria  when  more  than  one  alkyl  group 
is  present,  and  the  change  which  predominates  is  that 

+     (1) 
R\  /R1     <=±    R2R1N     +     RX 


R— 

+ 
R/ 


X      <=±    R3N 


one  which  proceeds  with  the  greatest  velocity,  or  in  which  one  or  more 
of  the  products  formed  is  removed  from  the  sphere  of  reaction.  The 
addition  product  or  quaternary  salt  is  an  "  onium  "  compound  of 
nitrogen. 

Dimethyl  ether  and  hydrogen  chloride  combine  at  a  low  tempera- 
ture to  form  an  addition  product,  or  oxonium  salt,  which  is  extremely 
unstable.  A  rise  in  temperature  leads  to  dissociation  of  the  onium 
compound,  and  equation  2  represents  the  equilibrium  which  pre- 
dominates. 

+  (1) 

CH3\       /H  <=>    CH3OH     +     CH3C1 

+       >0<_  (2) 

CH-/    \C1  <=±     (CH3)20    +     HC1 

The  well-known  Zeisel  method  for  the  determination  of  methoxy 
groups  is  a  practical  illustration  of  this  phenomenon,  and  in  terms  of 
the  "  onium  "  theory  the  reaction  may  be  formulated  as  follows: 

+    (1) 

H     «=>    ROH         +     CH3I 

(2) 

<=»    ROCH3     +     HI 

-     (3) 

I     <=>    CH3OH    +     RI 

1  Falk  and  Nelson,  Jour.  Am.  Chem.  Soc.,  37,  1732  (1915). 


114  THEORIES  OF  ORGANIC  CHEMISTRY 

The  hydrogen  iodide  first  produces  the  oxonium  salt,  which  may  disso- 
ciate theoretically  in  three  ways,  but  follows  that  course  represented 
by  equation  1,  by  reason  of  the  excess  of  hydriodic  acid  used  and  the 
velocity  of  this  reaction. 

Application  of  the  "  onium  "  theory  to  the  interaction  of  an  alcohol 
and  an  acid  may  be  shown  as  follows: 

+  (1) 

Hv       /H  <=>    H2O     +     RX 

+  >O<  _  (2) 

R/      XX  <=±    ROH    +     HX 

The  addition  of  alcohol,  or  HX,  or  the  removal  of  H20  or  RX  causes 
the  reaction  to  proceed  according  to  equation  1. 

The  usual  method  for  the  preparation  of  acid  anhydrides  is  inter- 
preted according  to  the  "  onium  "  theory  as  follows: 

+  +  (1) 

CH3COV      /COCHs  <=*    HC1     +     (CH3CO)2O 

+  >0<  _  (2) 

H/     XC1  <±    CH3COOH     +     CH3COC1 

In  this  case  an  excess  of  organic  acid  is  used  and  the  hydrochloric 
acid  is  removed  by  heat,  thereby  leading  to  conditions  favorable  for 
equilibrium  and  the  production  of  acetic  anhydride. 

The   action  of  hydrogen  chloride  as  a  catalytic  agent,   in  both 
esterification  and  hydrolytic  reactions,  may  be  explained  on  the  basis 

of  "  onium  "  compound  formation.     Until  recently,  the  hydrogen  H, 

and  hydroxyl  OH  ions  of  acids  and  bases  have  been  assumed  to  be  the 
active  catalysts  in  such  changes,  but  experimental  evidence  points 
to  the  importance  of  considering  the  catalytic  effect  of  unionized 
molecules  in  these  reactions.  The  application  of  the  "  onium " 
theory  to  the  reaction  possibilities  of  ethyl  acetate,  water,  acetic  acid 
and  alcohol  is  expressed  by  the  following  equations: 

n 


CHgCOOH  4-  HC1   ^=±         OH8CO^H  H 

H     Cl 


THE  ELECTRON  CONCEPTION  OF  VALENCY  115 

According  to  Falk  and  Nelson, l  this  scheme  accounts  for  the  following 
facts: 

1.  The  hydrogen  chloride  bears  the  same  relation  to  the  acid  and 
the  ester.     It  therefore  catalyzes  the  reaction  in  both  directions. 

2.  The  onium  compound  formation  depends  on  the  strength  of 
the  acid  catalyst.     This  is  measured  by  the  degree  to  which  it  breaks 
up  in  solution  to  form  ions,  a  physical  property  similar  to  the  chemical 
property  of  onium  addition,  which  controls  the  rate  of  reaction  with 
water  or  alcohol. 

3.  Increasing  the  concentration  of  water  favors  the  production  of 
acid;    increasing  the  concentration  of  alcohol  favors  the  production 
of  ester. 

On  the  basis  of  this  primary  formation  of  addition  compounds, 
it  is  possible  to  formulate  reactions  such  as  sulphonation,  nitration, 
aldol  condensation,  coupling,  etc. 

Classification  of  Organic  Acids 

Falk  has  suggested  a  classification2  of  organic  acids  which  cor- 
relates the  additive  effect  of  the  direct  valences  of  the  a-carbon  atoms 
with  the  ionization  constants  of  the  acids,  (J^XlO5),  as  calculated  from 

?/2 

OstwakTs  dilution  law,  K=-T-^ — r.      The  a-carbon  atom  is  considered 

v(l-y) 

as  exerting  the  greatest  influence  on  the  ionization  constants  of  organic 
acids.  An  arbitrary  classification  may  therefore  be  made  on  the  basis 
of  the  direction  of  the  valences,  by  which  the  a-carbon  atom  is  com- 
bined with  the  atoms  connected  with  it,  the  arrangement  of  the  elements 
in  the  periodic  system  serving,  in  general,  to  indicate  the  electrical 
relations  of  the  elements  to  each  other  in  the  production  of  a  bond. 

The  four  classes  of  acids  which  result  from  the  above  conception 
may  be  formulated  as  follows : 

1 
3  C— COOH 

Three  electro-positive  atoms  connected  with  the  a-carbon  atom. 

2 
2  C— COOH 

1  Loc.  cit. 

2  Jour.  Am.  Chem.  Soc.,  33,  1140  (1911). 


116  THEORIES  OF  ORGANIC  CHEMISTRY 

Two  electro-positive  atoms  and  one  electro-negative  atom  connected 
with  the  a-carbon  atom. 

3 

£  C—  COOH 

One  electro-positive  atom  and  two  electro-negative  atoms  connected 
with  the  a-carbon  atom. 

4 
£  C—  COOH 

Three  electro-negative  atoms  connected  with  the  a-carbon  atom. 

Their  ionization  constants  increase  in  the  order  1,  2,  3,  4.  Repre- 
sentative acids  of  the  first  group  are  acetic,  caprylic,  caproic,  isobutyric, 
butyric,  etc.,  whose  ionization  constants  are  less  than  .01.  Class  2 
includes  acids  like  a-brompropionic,  cyanacetic  acid,  etc.,  or  those 
possessing  ionization  constants  between  .1  and  .4,  while  Class  3  includes 
acids  having  ionization  constants  above  2,  such  as  a-a-dibrompropionic 
acid,  and  dichloracetic  acid.  Group  4  comprises  acids  which  are  too 
highly  ionized  to  give  satisfactory  dissociation  constants,  as  for 
example,  trichloracetic  acid.  Of  the  saturated  dibasic  acids,  malonic 
acid  is  placed  in  Class  2,  and  succinic  acid  in  Class  1.  With  regard 
to  unsaturated  acids,  fumaric,  on  the  basis  of  its  ionization  constant, 
is  shown  to  be  in  Class  2  and  acrylic  in  Class  1,  while  maleic  acid,1 
depending  upon  the  carboxyl  group  involved,  functions  either  in  accord 
with  the  acids  of  Class  1,  or  Class  3.  The  same  principles  have  been 
applied  to  the  acids  of  the  benzene  series,  benzoic  acid  being  a  repre- 
sentative of  Class  1,  while  salicylic  acid  is  placed  by  Falk  in  Class  2. 

A  criticism  of  the  above  classification  has  been  made  by  Fry.  He 
concludes  that,  if  the  ionization  constants  of  acids  are  dependent  upon 
the  direction  of  the  valences  of  the  a-carbon  atom;  all  four  and  not 
three  valences  of  the  a-carbon  atom  should  be  taken  into  considera- 
tion.1 That  is  to  say,  the  valence  of  carboxyl,  assumed  as  constant 
by  Falk,  may  function  either  positively  or  negatively.  Therefore, 
eight  rather  than  four  classes  of  organic  acids  are  theoretically 
possible,  and  Fry  represents  them  electronically  as  follows: 


1.  ^C-^COOH  5.  3C 

2.  ^C-^COOH  6.  :3C<-COOH 

3.  ^C-^COOH  7.  ^C<-COOH 

4.  ^C-*COOH  8.  ^C^ 

i  Jour.  Am.  Chem.  Soc.,  34,  664  (1912). 


THE  ELECTRON  CONCEPTION  OF  VALENCY  117 

Fry  has  furnished  evidence  in  support  of  the  theory  of  the  positivity 
and  negativity  of  carboxyl,  and  has  also  shown  that  the  properties  1 
of  acids  are  decidedly  influenced  by  the  polarity  of  this  radical. 

Nascent  State  and  Nascent  Action 

Fry  2  advances  the  electronic  conception  of  oxidation  and  reduction 
to  explain  the  phenomena  of  nascent  state  and  nascent  action.  The 
term  nascent  action  connotes  all  those  phenomena  in  which  a  substance 
at  the  moment  of  its  liberation  from  compounds,  performs  reactions 
of  which  it  is  incapable  in  its  ordinary  condition.3  In  electronic 
terminology,  oxidation  corresponds  to  a  loss,  and  reduction  to  a  gain 
of  negative  electrons  or  charges  by  an  atom.  Ordinarily,  hydrogen 
functions  positively,  and  chlorine  negatively.  Each  atom,  however, 
may  under  special  conditions  function  in  the  opposite  polarity,  negative 
hydrogen  thereby  becoming  a  reducing  agent,  losing  negative  electrons 
and  reverting  to  positive  hydrogen,  and  positive  chlorine  acquiring 
negative  electrons,  thus  reverting  to  negative  chlorine  and  becoming 
an  oxidizing  agent.  The  natural  tendency  of  negative  hydrogen  and 
positive  chlorine  to  revert  to  the  positive  and  negative  polarity, 
respectively,  is  illustrated  below.  The  symbol  ©  represents  one  unit 
of  negative  electricity  and  ®  one  unit  of  positive  electricity. 

1.  H->H+20 

2.  C1-+C1+20 

Nascent  state  has  been  defined  by  Fry  as  "an  unstable  condition 
of  a  substance,  which  manifests  an  adaptability  and  a  tendency  to 
lose  or  to  gain  electrons  and  thereby  revert  to  a  more  stable  condition." 
All  reactions  attributable  to  nascent  action  are  apparently  either 
oxidizing  or  reducing  in  their  nature. 

Application  of  the  Electron  Theory  of  Positive  and  Negative  Valence 
to  Some  Reactions  of  the  Aromatic  Series 

Application  of  the  electronic  conception  of  positive  and  negative 
valency  to  the  atoms  constituting  the  benzene  molecule  leads  to  the 

1  Loc.  cit.,  Jour.  Am.   Chem.  Soc.,  36,257  (1914);  Hanke  and  Koessler,  Jour. 
Am.  Chem.  Soc.  40,  1727  (1918.). 

2  Fry,  Jour.  Am.  Chem.  Soc.,  36,  270  (1914). 
3Freurid,  Study  of  Chemical  Composition,  p.  327  (1904). 


118  THEORIES  OF  ORGANIC  CHEMISTRY 

following  formula  for  benzene  !  in  which  the  hydrogen  atoms  in 
positions  1,  3,  5,  function  negatively,  and  those  in  positions  2,  4,  6, 
function  positively. 

H 


I 


C 

+     -/\       + 
H— C        C— H 


H— C        C— H 

v 


This  formula  presents  a  structural  basis  for  the  similarity  in  behavior 
of  the  ortho-  and  para-positions  in  contradistinction  to  the  meta-posi- 
tion.  The  distinction  between  the  1,  3,  5,  positions  and  the  2,  4,  6,  is 
quite  in  accord  with  Collie's  space  formula.  When  a  given  substituent 
in  the  nucleus  is  positive,  a  hydrogen  atom  or  substituent  in  the  meta- 
position  is  of  the  same  polarity.  If  the  substituent  is  negative,  the 
hydrogen  atom  or  substituent  in  the  ortho-  or  para-positions  is  positive. 
This  formula  therefore  requires  that  the  hydrogen  atoms  of  the  benzene 
ring  function  alternately  as  positive  and  negative,  which  is  not  illogical 
in  the  light  of  J.  J.  Thomson's  statement  that:  "  atoms  of  one  and  the 
same  kind  may  become  either  positively  or  negatively  electrified  by 
the  loss  or  gain  of  corpuscles,"  2  and  that  "  those  with  charges  of 

opposite  sign  would  combine  to  form  a  diatomic  molecule."     There- 

+ 

fore  molecular  hydrogen  becomes  H — H  and  molecular  chlorine 
+ 

Cl — Cl.  Evidence  of  the  existence  of  positive  chlorine  has  been 
furnished  by  Noyes 3  and  Stieglitz.4  Noyes 5  has  also  presented 
evidence  bearing  on  the  positivity  of  iodine  in  diiodacetylene  and, 
in  general,  the  positivity  of  halogen  in  the  =C-Hal.  linkages.  The 
positivity  of  chlorine  in  hypochlorous  acid  may  be  shown  in  the 

1  Fry,  Zeitschr.  physikal.  Chemie,  76,  385  (1911);  76,  398;  76,  591. 

2  Electricity  and  Matter  (Scribner's,  1907),  p.  139. 

3  Jour.  Am.  Chem.  Soc.,  23,  460  (1901);  35,  767  (1913). 

4  Jour.  Am.  Chem.  Soc.,  23,  797  (1901). 
6  Jour.  Am.  Chem.  Soc.,  42,  991  (1920), 


THE  ELECTRON  CONCEPTION  OF  VALENCY       119 

following    transformation?,1    illustrating   the   interaction   of   chlorine 
and  water: 

C12  =  C1—  Cl  <=*  Cl  +  Cl 
H2O=H—  O—  H  <=»  H  +  6~H 
Cl  +  H  *±  H—  Cl 

Cl  +  0~H  <=±  H—  0—  Cl  ?=>  H  +  CIO 

If  the  valence  of  an  element  is,  n2,  it  may  function  in  n+1  different 
ways.  For  example,  chlorine  whose  valency  is  1,  may  function  in  two 
ways,  and  carbon  with  a  valency  of  4,  in  five  ways,  as  is  shown  in  the 
following  scheme,  representing  the  successive  stages  of  the  oxidation 
of  methane,  to  carbon  dioxide  ; 


H  H  H 

+  -L  +       *  --1-  +       -U- 

H—  C—  H    -»    H—  C—  O—  H    -*      C=O 


Methane  Methylalcohol  Formaldehyde 

+      -+   --      +  +--++    --     +  --++   -- 

H—  C—  O—  H    -»    H—  O—  C—  0—  H    ->    O=C=O 


O 

Formic  acid  Carbonic  acid  Carbon  dioxide 

It  is  evident,  therefore,  that  the  direction  of  the  valencies  of  the  carbon 
atom  in  its  compounds  can  not  be  determined  3  by  the  position  of 
carbon  in  the  periodic  system,  but,  on  the  contrary,  depends  solely  upon 
the  polarity  of  the  atoms  (or  radicals)  which  are  connected  with  a  given 
carbon  atom.4 

Examples  of  the  application  of  the  electronic  formula  of  benzene 
and  its  derivatives  will  now  be  considered.  Why  is  it  that  chloroben- 
zene  undergoes  nitration  in  the  ortho-  and  para-positions  whereas 

1  Fry,  Zeitschr.  physikal.  Chemie,  76,  388  (1911). 

2  Ibid. 

3  Fry,  Jour.  Am.  Chem.  Soc.,  34,  669  (1911). 

4  In  contradistinction  to  Falk,  loc.  cit. 


120  THEORIES  OF  ORGANIC  CHEMISTRY 

nitrobenzene   is  attacked    by   chlorine    in    the    raeta-position?      The 
electronic  equations  1  which  follow  are  explanatory  of  this  fact: 


+     --  +  +  + x^o 
+  H— 0—  N  C  -  - 

IH  +++  >  o 

(H0-N02) 


H 


2  +  ci— 0  H      -+    J  +  H— OH 

'V 

In  equation  1,  the  electro-positive  N(>2  group  has  replaced  one  of  the 
two  electro-positive  hydrogen  atoms  (ortho  or  para  positions).  Since 
water-free  chlorine  will  not  act  upon  nitrobenzene.  Fry  considers  that 
hypochlorous  acid,  with  its  positive  chlorine,  is  the  reagent  which 
functions  in  reaction  2.  If,  therefore,  halogen  be  positive  it  must 
replace  a  hydrogen  atom  meta  to  the  positive  N02  group.  These 
facts  reaffirm  Fry's  statement  to  the  effect  that  when  substituents 
are  of  the  same  sign  or  polarity  they  occupy  positions  which  are  meta 
to  each  other,  whereas  if  two  substituents  are  of  opposite  sign  or 
polarity,  they  will  occupy  positions  either  ortho  or  para  to  each  other.2 

The  positivity  of  the  halogen  in  w-nitrochlorbenzene,  as  compared 
with  the  negativity  of  halogen  in  p-nitrochlorbenzene  or  o-nitro- 
chlorbenzene,  is  likewise  shown  in  the  stability  of  the  former  and  the 
reactivity  of  the  two  latter  compounds  with  alkali. 

Evidence  of  the  positivity  of  chlorine  in  positions  2,  4,  6,  of  the 
benzene  nucleus  is  also  shown  in  the  characteristic  rearrangements 
of  the  nitrogen  substituted  chloranilides.3  For  example,  acetanilide 
when  treated  with  hypochlorous  acid  gives  phenylacetyl-nitrogen  chlo- 
ride, which  is  readily  transformed  into  p-chloracetanilide.  Successive 
treatments  with  hypochlorous  acid,  followed  by  rearrangement,  will 

1  Fry,  Zeitschr.  physikal.  Chemie,   76,  384   (1911);  Jour.  Am.  Chem.  Soc.,  36, 
248  (1914). 

2  Jour.  Am.  Chem.  Soc.,   36,  248   (1914);   37,   2368   (1915).     See   criticism  by 
Brunei,  Jour.  Am.  Chem.  Soc.,  37,  709  (1915). 

3Chattaway  and  Orton,  Jour.  Chem.  Soc.,  75,  1046;  Ber.,  32,  3572  (1899). 


THE  ELECTRON  CONCEPTION  OF  VALENCY 


121 


eventually  give  2,  4,  6-trichlorphenylacetyl-nitrogen  chloride,  which  is 
incapable  of  further  rearrangement.     These  reactions  are  shown  below: 

NH-COCH3  N(C1)COCH3        NHCOCH3  N(C1)COCH3 

HOC1 


NHCOGHa 


HOC1 


N(C1)COCH3 
31 


N(C1)COCH3 


Explanation  of  the  positions  occupied  by  the  labile  halogen  atoms,1 
as  well  as  the  impossibility  of  introducing  further  halogen  groups  into 
the  nucleus,  is  given  in  the  fact  that  the  amido  group,  NHo,  in  aniline 
is  negative,  and  must  therefore  occupy  the  position  of  a  negative 
hydrogen  atom  in  the  benzene  nucleus.  If,  therefore,  positive 
chlorine  from  hypochlorous  acid  replaces  one  positive  hydrogen  atom 
of  the  amido  group,  this  positive  labile  halogen  atom,  by  reason  of  its 
polarity,  can  exchange  positions  with  the  positive  hydrogen  atoms 
(2,  4,  6)  of  the  nucleus: 


H— N— H 

A 

H— C         C— H 

-     +  1         l+     - 
H— C        C— H 

v 


H— N— Cl 


HO— Cl 


H— C         C— H 

\/ 

c- 


H— N— H 

C+ 

H— C        C— Cl 

+  1          1+    - 
H— C        C— H 

v 


These  facts  further  substantiate  the  positive  equality  of  the  2,  4,  6, 
positions  in  the  benzene  nucleus  and  also,  indirectly,  the  negativity 
of  the  1,  3,  5,  positions,  on  account  of  the  inability  of  2,  4,  6,  trichlor- 
phenylacetyl-nitrogen  chloride  to  suffer  rearrangement,  through  the 
replacement  of  another  hydrogen  of  the  nucleus  with  positive  halogen. 
Proof  of  the  alternate  negativity  and  positivity  2  of  the  nucleus 

iFry,  Jour.  Am.  Chem.  Soc,  34,  667  (1912). 
2  Jour.  Am.  Chem.  Soc.,  38,  1324  (1916). 


122  THEORIES  OF  ORGANIC  CHEMISTRY 

hydrogen  atoms,  or  substituent  groups,  is  also  afforded  in  the  case 
of  hydrolysis  of  polynitro-compounds  of  the  following  types,  in  each 
of  which,  one  of  the  nitro  groups  functions  either  positively  or  nega- 
tively. A  negative  nitro  group  is  susceptible  to  the  action  of  water 
alkalies,  or  ammonia,  due  to  the  replacement  of  the  negative  nitro 

group  by  negative  hydroxy  (OH)  or  amino  (NH2)  radical: 

OH 

N02f  \NO2     H-OH      ^vy2.       ,i>v72 
1.  ->  +     H-NO< 


HI      JNO2 
NO2 


)NO2     Na-OH        ixi       i^vy2  + 

2.  ~*  +     Na-N02 


CH3— N— CH3  CH3— N— CH3 


H-NH2  J-AJ  Ii>vy2  +     - 

H-N02 


It  has  been  stated  above  that  eight  classes  of  organic  acids  may 
theoretically  be  derived  from  a  consideration  of  the  directive  valences 
of  their  a-carbon  atoms.  This  assumption  presupposes  the  possibility 
of  positive  as  well  as  negative  carboxyl  groups.  According  to  Fry, 
this  receives  corroboration  in  the  oxidation  of  the  A3?5 -,-A2>4 -,  and 
A2i6-,-dihydrophthalic  acids,  where  benzoic  acid  is  formed  in  each 
case  as  the  final  product  of  reaction. 

H  H  H 

C  C  C 

/N  /\  /\ 

H2C        C-COOH  HC        C-COOH  HC        CH 

H2C        C-COOH  HC        C-COOH  HC        C-COOH 

\/  \>  \> 

C  C  C 

H  H 


THE  ELECTRON  CONCEPTION  OF  VALENCY  123 

Since  groups  on  adjacent  carbon  atoms  must  possess  opposite  polari- 
ties, it  follows  that  one  carboxyl  group  must  be  positive  and  the  other 
negative.  Referring  to  the  electronic  formulas  of  formic  and  carbonic 
acids,  it  will  be  observed  that  the  carboxyl  group  in  the  former  is  nega- 
tive, and  in  the  latter  positive, 

H—  C—  O—  H  H-^0—  €—  ~i  0—  H 

I  I 

Formic  acid  Carbonic  acid 

In  formic  acid  three  of  the  valences  of  carbon  are  positive  and  the 
fourth  negative,  while  in  carbonic  acid,  all  four  valences  are  positive. 
Carbonic  acid  is  therefore  electronically  capable  of  losing  carbon 
dioxide,  whereas  formic  acid  is  not.  The  characteristic  oxidation 
reaction  of  the  dihydrophthalic  acids  is  therefore  explained  on  the 
basis  of  the  difference  in  polarity  of  the  two  carboxyl  groups.1  The 
above  explanation  is  also  applicable  to  the  fact  that  o-  and  p-hydroxy- 
benzoic  acids  are  unstable,  when  heated  with  water  or  aniline,  whereas 
ra-hydroxybenzoic  acid  is  stable  under  these  conditions.2  Fry  has 
generalized  this  type  of  decomposition  in  the  following  words:  "a 
carboxyl  radical,  either  ortho  or  para  to  a  negative  hydroxyl,  is  positive, 
and  therefore  unstable,  yielding  carbon  dioxide  when  heated  with 
water  or  aniline.  On  the  other  hand,  a  carboxyl  radical,  meta  to  a 
negative  hydroxyl  radical,  is  negative,  and  therefore  stable,  not 
yielding  carbon  dioxide  when  heated  with  water  or  aniline."  These 
conclusions  are  confirmed  by  the  recent  work  of  Hemmelmayr.3 

The  assumption  that  atoms  or  groups  of  atoms  may  function 
either  positively  or  negatively,  leads  to  the  possibility  of  the  existence 
of  electronic  isomers  or  "  electromers."  4  For  example,  one  can 


conceive  of  two  forms  of  chlorobenzene,  namely  CeHsCl  and 

+  + 

or  derivatives  of  C6H5H  and  CeHsH  respectively.  Although  definite 
evidence  has  not  yet  been  obtained  concerning  electromers  of  this 
halide,  Fry  concludes  that  the  electromeric  forms  of  benzene  sul- 
phonic  acid,  CeHsSOaH,  must  be  assumed  to  exist  because  hydrolysis 

1  Baeyer,  Annalen  der  Chemie,  269,  178  (1892)  ;  Bruhl,  Jour,  prakt.  Chemie  (2), 
49,  229  (1894);  Cohen,  Organic  Chem.  (1907)  p.  461. 

2  Fry,  Jour.  Am.  Chem.  Soc.,  36,  257  (1914). 
"Monatsh.  Chemie,  34,  365  (1913). 

«  Zeitschr.  physikal.  Chemie,  76,  387  (1911). 


124  THEORIES  OF  ORGANIC  CHEMISTRY 

of  this  compound  in  alkaline  solution  is  productive  of  phenol  and 
sulphurous  acid,  while  in  acid  solution,  or  with  superheated  steam, 
the  products  of  reaction  are  benzene  and  sulphuric  acid. 1 

1.  C6H5— S03H+H— OH=C6H5— OH+H— SO3H 

Alkaline  hydrolysis 

2..  C6H5— SO3H+H— 6H=C6H5— H+HO— SO3H 

Acid  hydrolysis 

Fry  has  offered  some  interesting  speculation  on  the  mechanism 
of  halogenation  in  the  benzene  nucleus  and  side  chain.2  The  factors 
which  are  known  to  promote  nucleus  substitution  are  the  following: 

1.  Presence  of  moisture. 

2.  Low  temperatures. 

3.  Absence  of  sunlight. 

4.  Presence  of  halogen  carriers. 

These  are  the  conditions  which  are  peculiarly  favorable  for  the  for- 
mation of  hypochlorous  or  hypobromous  acids.  Therefore,  when 
toluene  is  halogenated  under  the  above  conditions,  the  presence  of  the 
electro-negative  methyl  group  directs  the  positive  chlorine  of  hypo- 
chlorous  acid  to  the  positive  ortho  or  para  positions, 


fi-c-fi 


HO-C1 


pTi- 
Hs 


HI     JH  HI     JH  HI      JH 


or         _|        _       +      H-OH 


Furthermore,  the  fact  that  these  nuclear  positive  halogens  are  firmly 
bound  under  conditions  of  hydrolysis  is  a  proof  of  their  polarity. 

With  regard  to  side-chain  substitution,  it  is  evident  that  the  sub- 
stituent  halogen  atom  is  negative,   in  that  it  is  easily  capable  of 

!Fry,  Jour.  Am.  Chem.  Soc.,  36,  265  (1914);   Jones,  Am.  Chem.  Jour.,  48,  26 
(1912);  Bray  and  Branch,  Jour.  Am.  Chem.  Soc.,  35,  1445  (1913). 
2  Jour.  Am.  Chem.  Soc.,  36,  1035  (1914). 


THE  ELECTRON  CONCEPTION  OF  VALENCY  125 

hydrolysis.     For  example,  the  hydrolysis  of  benzyl  chloride  is  repre- 
sented by  the  following  electronic  equation: 

+ 
H 

-U      +-  -U-  + 

C6H5—  €—  Cl  +  H-OH    -»    C6H5—  C—  OH  +  H-C1 

T 

H 


Moreover,  the  reaction  is  reversible  in  the  presence  of  an  excess  of 
hydrochloric  acid.  The  mechanism  of  side-chain  substitution  is 
apparently  more  complex  than  that  of  nuclear.  This  reaction  is 
facilitated  by  heat,  light,  and  the  absence  of  water.  The  substituting 
agent,  molecular  halogen,  dissociates  into  positive  and  negative  halo- 

gen atoms,  of  which  the  latter  combines  with  a  positive  hydrogen 

+  - 

atom  of  the  methyl  group,  forming  hydrogen  chloride,  HC1.  This 
necessitates  the  conclusion  that  the  remaining  positive  halogen  atom 
acts  as  an  oxidizing  agent,  converting  the  negative  valence  of  carbon 
to  a  positive  valence,  and  being  itself  reduced  to  negative  chlorine. 
These  changes  may  be  represented  as  follows  : 


C12  =  C1  — Cl    -*    Cl  +  Cl 


H 


C6H5—  C—  H  +  Cl    -»    CflHs—  C-  +  H-C1 

H 
H  H 

I 
C6H5—  C—  +  (C1  =  C1  +  20)     -» 


Heat  and  the  photochemical  action  of  light  effect  the  conversion  of 
positive  to  negative  halogen. 

The  electronic  formula  of  benzene  presents  an  explanation  of  the 

Brown  and  Gibson  rule,1  which  is  stated  by  these  investigators  as 

follows:    "  when  X  is  naturally  to  be  regarded  as  a  derivative  of  HX, 

then  CeHsX  gives  ortho  and    para    di-derivatives;    and  when  X  is 

i  Jour.  Chem.  Soc.,  61,  366  (1892). 


126 


THEORIES  OF  ORGANIC  CHEMISTRY 


naturally  to  be  regarded  as  a  derHative  of  HOX,  then  e^  gves 
meta  di-derivatives."  As  previously  shown,  X  in  compounds  such  as 
HX  functions  negatively,  and  X  in  compounds  like  HO  X,  positively, 
as  for  example,  Cl  in  HOC1.  Therefore  a  substituent  X,  which  is  a 
natural  derivative  of  H  0  X,  or  CcH^X,  occupies  the  position  of  a 
positive  hydrogen  atom  in  the  benzene  nucleus,  while  a  substituent 
X  which  is  a  natural  derivative  of  H  X,  or  CeHsX,  occupies  the  posi- 
tion of  a  negative  hydrogen  atom,  i.e., 


In  nuclear  substitution  it  may  be  assumed  that  the  entering  sub- 
stituent is  positive,  this  being  evidenced  by  the  fact  that  either  H  OH, 
or  some  binary  compound  of  the  formula  H  B  is  always  eliminated  dur- 
ing this  type  of  reaction.  It  is  evident,  therefore,  that  X  must  govern 
the  me ta  position  and  X,  the  ortho  and  para  positions  in  accord  with 
the  previously  stated  electronic  rule  for  benzene  substitution.  It 
should  be  emphasized  that  the  entrance  of  Y  as  a  positive  substituent 
does  not  preclude  its  reversion  to  the  radical  of  opposite  polarity. 

The  following  equations  will  serve  to  illustrate  the  action  of  Y  OH 
on  both  C6H5X  and  C6H5X: 


+     YOH 


or 


YOH 


A  mixture  of  ortho,  para  and  meta  derivatives  is   often   produced 
in  a  given  reaction.1     This  has  been  explained  on  the  basis  of  elec- 
1  Fry,  Jour.  Am.  Chem.  Soc.,  37,  864  (1915). 


THE  ELECTRON  CONCEPTION  OF  VALENCY 


127 


tronic  tautomerism,  which  is  shown  in  the  following  equations  by  the 
partial  transformation  of  a  negative  substituent  X  into  an  atom  of 
opposite  polarity.  In  this  case,  the  introduction  of  a  second  sub- 
stituent would  produce  a  preponderance  of  ortho  and  para  derivatives, 
together  with  a  small  amount  of  the  meta  di-derivative. 


X 


H 


+ 


C6HsX 


X 

Hr 

> 

+    - 

J 

L 

+     Y-OH 

m 

¥H 

H 


H 


The    interpretation    of    the    hypothesis    of    Holleman  l    regarding 

benzene   substitution  from  the  electronic   standpoint  2  is  illustrated 

+ 
by  the  following  formulas  showing  the  action  of  a  reagent  OH  •  Y  on  a 

+ 
derivative  CeHs-X,  whereby   an    addition   product  is   first    formed. 


The  elimination  of  H  •  OH  from  the  latter  produces  the  final  product. 
M  eta  and  para  substitution  can  be  similarly  shown. 


OH 


H-OH 


This  electronic  formulation   of  substitution  has  met  with  criticism 
from  Holleman.3 

1  Rec.  trav.  chim.  des  Pays-Bas,  33,  1  (1914). 

2  Holleman,  Jour  Am   Ch^m    Sorv,  36,  2495  (1914);    of.,  Fry,  Jour.  Am.  Chem. 
Soc.,  37,  883  (1915). 

3  Loc.  cit. 


128  THEORIES  OF  ORGANIC  CHEMISTRY 


Valence  Number 

In  applying  the  electronic  theory  to  explain  the  phenomenon  of 
valence,  two  outstanding  ideas  are  embodied  in  the  term  valence  num- 
ber of  an  atom.  Bray  and  Branch  l  have  suggested  that  this  be  differ- 
entiated by  the  terms  polar  number  and  total  valence  number.  For 
example,  in  ammonium  chloride, 


the  total  valence  number  of  nitrogen  is  5,  while  the  polar  number, 
representing  the  algebraic  sum  of  the  valences,  is  —3.  The  valence  of 
nitrogen  in  ammonium  chloride  is  therefore  represented  as  (  —  3,  5), 
the  polar  number  being  written  first.  Similarly  the  valence  of  carbon 
in  methane  would  be  (  —  4,  4)  and  in  carbon  tetrachloride  (+4,  4). 

Bray  and  Branch  express  doubt,  however,  regarding  the  application 
of  this  electronic  conception  of  valence  number,  in  all  cases,  citing 
in  particular  the  saturated  hydrocarbons,  concerning  which  there  is 
frequently  a  question  as  to  the  actual  polar  valency  of  the  individual 
carbon  atoms.  Moreover,  methane  and  carbon  tetrachloride  do  not 
offer  the  striking  contrast  in  properties  which  might  be  expected  from 
their  respective  polar  valences  (  —  4,  4)  and  (+4,  4).  This  is  in 
decided  contrast  to  the  analogous  case  of  ammonia  (  —  3,  3)  and  nitro- 
gen trichloride  (+3,  3).  In  view  of  these  facts,  they  favor  using  the 
ordinary  organic  structural  formulas  and  altering  them  only  where 
there  are  definite  evidences  of  polar  unions.  This  theory  permits  of 
the  appearance  of  both  polar  and  non-polar  bonds  in  the  same  mole- 
cule, the  former  being  one  in  which  an  electron  has  passed  from  one 
atom  to  another  and  the  latter,  one  in  which  there  is  no  movement 

of  electrons.2     A  polar  bond  is  represented  by  the  arrow  — >  (Falk), 

•+-      - 

or  by  polarity  symbols,  H (Fry) ;  for  example,  H  — •>  Cl,  or  H — Cl. 

1  Jour.  Am.  Chem.  Soc.,  36,  1440  (1913). 

2  Attention  should  be  called  here  to  Langmuir's  "  octet  "  theory  of  valency  which 
has  been  applied  to  the  structure  of  inorganic  compounds  and  certain  organic  nitrogen 
compounds,  and  which  gives  an  interesting  theoretical  explanation  of  their  formation. 
For  complete  information  see  the  following  publications:  G.  M     Lewis,   Jour.  Am. 
Chem.  Soc.,  38,  762  (1916);  Langmuir,  Jour.  Am  Ch«m.  Soo.,  41,  868  (1919);  ibid., 
41,  1543;  Proc.  Nat.    Acad.  Sci.,  5,    252  (1919);   Jour.  Ind.  Eng.  Chem.,  12,  385 
(1920);   Jour.  Am.  Chem.  Soc.,  42,  274  (1920). 


THE  ELECTRON  CONCEPTION  OF  VALENCE  129 

Two  groups  of  compounds  can  be  made  on  the  basis  of  the  above 
assumptions.  The  first  group  comprises  those  compounds  whose 
valence  bonds  are  chiefly  polar,  and  the  second  group,  those  whose 
valence  bonds  are  mainly  non-polar.  The  first  class  is  characterized 
by  high  dielectric  constants,  the  property  of  forming  ions,  and  in  gen- 
eral by  high  chemical  activity.  The  opposite  properties  are  possessed 
by  the  second  class.  In  other  words,  the  first  class  corresponds  to  the 
inorganic  class  of  compounds  and  the  second  to  that  of  the  organic 
compounds. 

In  a  commentary  on  the  speculations  of  Bray  and  Branch,  G.  N. 
Lewis,1  has  further  developed  the  conception  of  polar  and  non-polar 
compounds,  and  has  contributed  the  following  scheme,  which  may 
serve  to  show  the  salient  differences  between  the  polar  and  non-polar 
types  of  compounds; 

Polar  Non-Polar 

Mobile  Immobile 

Reactive  Inert 

Condensed  structure  Frame  structure 

Tautomerism  Isomerism 

Electrophiles  Non-electrophiles 

Ionized  Non-ionized 

Ionizing  solvents  Non-ionizing  solvents 

High  dielectric  constants  Low  dielectric  constants 

Molecular  complexes  No  molecular  complexes 

Association  No  association 

Abnormal  liquids  Normal  liquids 

It  should  be  pointed  out  that  the  differentiation  as  to  polar  or 
non-polar  compounds  is  not  necessarily  a  sharply  defined  one,  for, 
as  Lewis  states,2  "  it  must  not  be  assumed  that  any  one  compound 
corresponds  wholly  and  at  all  times  to  either  one  type." 

Falk  and  Nelson'3  do  not  consider  the  non-polar  valence  view  as 
a  sufficiently  broad  basis  upon  which  to  classify  valence  phenomena 
in  organic  chemistry.  In  the  following  series  of  transformations  they 
point  out  that  the  non-polar  view  would  represent  the  replacement  of 

CH4  -»  CH3OH  -»  CH20  -»  CH2O2  -*  CO2 

1  Jour.  Am.  Chem.  Soc.,  36,  1448  (1913);  38,  762  (1916). 

2  Loc.  cit. 

•  Jour.  Am.  Chem.  Soc.,  36,  210  (1914). 


130  THEORIES  OF  ORGANIC  CHEMISTRY 

hydrogen  by  oxygen  or  hydroxyl,  with  no  change  in  the  electrical 
charge  on  the  carbon  atom,  taking  no  cognizance  of  the  fact  that  oxida- 
tion is  involved,  and  that  this  reaction  is  generally  interpreted  on  an 
electronic  basis.  Furthermore,  the  chlorination  of  methane  would 
consist  in  the  mere  replacement  of  hydrogen  by  chlorine. 

A  classification  of  tautomeric  changes  made  on  the  basis  of  the 
new  conception  of  polar  number  and  total  valence  number  l  leads  to  two 
groups  of  reactions,  namely: 

1.  Tautomeric  changes  in  which  no  change  in  polar  number  is 
involved. 

(a)  No  change  in  total  valence  number. 

(b)  Change  in  total  valence  number. 

2.  Tautomeric  changes  in  which  the  polar  numbers  of  two  elements 
are  altered. 

Reactions  characteristic  of  Class  1,  are  those  represented  by  the 
well-known  examples  of  a,  /3  or  (1,  2),  and  a,  7  or  (1,  3)  tautomerism, 
which  involve  a  shifting  of  a  hydrogen  atom  from  one  part  of  the 
molecule  to  another.  An  illustration  of  a,  /3-tautomerism  (Class  16), 
is  that  of  sulphurous  acid,  while  an  example  of  a,  7-isomerism  (Class 
la),  is  shown  in  the  tautomerism  of  acetamide.  These  equilibria  are 
expressed  as  follows: 


H\ 


H/C  .^  H/C 


H-*  O  * 

S 


(+  4,  4  ")  -*—       —  i          S  (+  4, 


Benzene   sulphonic   acid,   CeHsSOaH,   is  an  excellent  example   of 
Class  2,   which  includes  tautomeric  changes  involving  a  change  of 

and  Branch,  Jour.  Am.  Chem.  Soc.,  35,  1444  (1913). 


THE  ELECTRON  CONCEPTION  OF  VALENCY  131 

polar  number  of  two  constituent  elements.  This  tautomerism  of 
benzene  sulphonic  acid  is  manifested  in  its  difference  in  behavior  on 
hydrolysis  in  acid  and  alkaline  solution,  the  former  producing  benzene 
and  sulphuric  acid,  the  latter,  phenol  and  sulphurous  acid. 

-H  C6H5 >•  S  *-0<    H 


C6H5  and    S 


The  Beckmann  Rearrangement 

Reference  will  be  made  to  the  Beckmann  rearrangement  in  the 
chapter  on  Molecular  Rearrangements  where  the  various  theories 
which  have  been  proposed  to  explain  the  mechanism  of  this  change 
have  been  discussed.  According  to  Stieglitz,  the  molecular  rearrange- 
ments of  halogenated  acid  amides  RCONH.  Hal.,  hydroxamic  acids 
RCONHOH,  dihydroxamic  acids  RCONHOCOR,  acid  azides  and 
ketoximes  are  all  induced  by  the  presence  of  univalent  nitrogen,  which 
results  from  a  dissociation  of  the  molecule  undergoing  change,  whereby 
the  valence  of  nitrogen  is  reduced  from  3  to  1.  The  predisposing 
cause  of  rearrangement  is,  therefore,  the  free  valency  of  univalent 
nitrogen.  His  conclusions  are  summarized  in  a  paper  published  by 
Stieglitz  and  Stagner. 1 

Jones2  correlates  Stieglitz'  theory  with  the  electronic  hypothesis, 
and  calls  attention  to  the  fact  that  in  all  the  reactions,  classified  under 
the  Beckmann  rearrangement,  the  transformation  is  accompanied 
by  a  process  of  intramolecular  oxidation  and  reduction.  That  is  to 
say,  there  is  a  tendency  of  the  system  to  revert  to  one  in  which,  elec- 
tronically speaking,  the  carbon  atom  is  as  fully  oxidized  as  possible 
and  the  nitrogen  atom  as  fully  reduced  as  possible.  In  other  words, 
where  Stieglitz  attributes  the  rearrangement  to  the  potency  of  the 
free  valencies  of  univalent  nitrogen,  according  to  this  electronic  con- 
ception the  change  is  dependent  on  the  potency  of  a  carbon  atom  to 
lose  negative  electrons  and  of  the  nitrogen  atom  to  acquire  them. 

1  Jour.  Am.  Chem.  Soc.,  38,  2064  (1916). 

2  Am.  Chem.  Jour.  50,  440  (1913);  Jones  and  Sneed,  Jour.  Am.  Chem.  Soc.,  39, 
674  (1917), 


132  THEORIES  OF  ORGANIC  CHEMISTRY 

The  rearrangement  of  a  hydroxamic  acid  is  expressed  electronically  as 
follows: 


R    H  R 

-  J_    .    .    __+      -H-OH     -  +L 

0=C-N—  OH     —      -»      O=C—  N         ->     O=C—  N—  R 


This  view  of  the  Beckmann  transformation  leads  to  a  classification 
of  this  type  of  reaction  into  groups  determined  by  the  state  of  oxidation 
which  the  carbon  atom  shows  prior  and  subsequent  to  rearrange- 
ment. In  all  of  these  rearrangements  the  nitrogen  atom  is  considered 
to  be  in  a  state  of  oxidation  corresponding  to  that  of  nitrogen  in 
hydroxylamine. 

GROUP  1.  Azides,  monosubstituted  hydroxylamines,  such  as  tri- 
phenylmethylhydroxylamine,  and  mono-bromoamines  such  as  tri- 
phenylamine  bromide: 

CE     and     N=     ^arrange  to     cz     and    Nl 

+  + 

Alcohol         Hydroxylamine  Aldehyde  Ammonia 

GROUP  2.     Aldoximes  and  ketoximes: 

CI     and    N=     rearrange  to    c+     and    N= 

+  +  + 

Aldehyde          Hydroxylamine  Acid  Ammonia 

GROUP  3.  Hydroxamic  acids,  their  salts  and  esters,  amidoximes, 
acid  azides,  and  monohalogen  amides: 

C+     and    N=     ^arrange  to     c|     and    N  = 

+  + 

Acid  Hydroxylamine  Carbonic  acid  Ammonia 

The  statement  under  each  symbol  —  alcohol,  hydroxylamine,  etc., 
indicates  the  state  of  oxidation  as  compared  with  that  of  the  atom  in 
compounds  where  its  polarity  is  similar.  With  regard  to  carbon,  the 
stages  of  oxidation  referred  to  are  expressed  electronically  as  follows: 

H  H  0~  0" 


H—  C—  O  H  C=O  H—  C—  0  H  H  O—  C—  0  H 


H 

Methyl  alcohol  Formaldehyde  Formic  acid  Carbonic  acid 


THE  ELECTRON  CONCEPTION  OF  VALENCY  133 

At  this  point  mention  should  be  made  of  the  fact  that  Stieglitz,1 
in  support  of  his  theory,  was  unable  to  bring  about  rearrangement  of 
stereoisomeric  chlorimides,  RR'C=NC1,  thereby  showing  that  these 
compounds  are  not  intermediate  products  of  a  Beckmann  rearrangement 
of  oximes.  According  to  their  electronic  constitution,  these  chlorides 
of  Stieglitz'  should  be  capable  of  undergoing  rearrangements  on  account 
of  the  positive  polarity  of  the  halogen  atom.  That  the  halogen  in  such 
compounds  is  positive  is  apparent  from  consideration  of  the  electronic 
equation  showing  their  formation  from  imidoketones  and  hypo- 
chlorous  acids.  More  recent  investigations  have  led  Stieglitz  2  to  con- 

R\  ++    ---    +          _       +  K  \  +  +    ---    +  -i-       _ 

>C  :  N—  H  +  HO-C1    ->         >C  :  N—  Cl  +  H-OH 
R'/  R'/ 

sider  a  third  electromer  of  the  chlorimidoketone,  an  assumption  which 
is  of  broad  significance  concerning  the  mechanism  of  the  Beckmann 
rearrangement,  and  which  is  an  extension  of  his  original  theory.  The 
mechanism  of  rearrangement  of  an  oxime  by  the  action  of  hydrochloric 
acid  may  be  expressed  electronically  as  follows:  3 

OH 


+         +  -  R\-+  ---  +    —  H 

C=N-OH  +  H-C1    ->          >C=N—  H 

R'/-  - 


C+— ?~C1    ~*     R'— C=N— Cl     -*    R'C-C1:(NR) 

++  --  + 

(B) 

According  to  this  interpretation,  the  hydroxyl  group  in  the  oxime 
is  considered  to  be  positive.4  Its  tendency,  therefore,  is  to  change 
to  a  stable  negative  condition,  the  nitrogen  atom  furnishing  the  two 
requisite  electrons  to  the  oxygen  atom.  Water  is  then  eliminated  with 
the  production  of  the  chloride  B.  According  to  Stieglitz,  this  product 
is  an  ammonium  salt  with  two  of  the  nitrogen  valencies  unsaturated 

»Ber.,  43,  782  (1910). 
2  Jour.  Am.  Chem.  Soc.,  36,  288  (1914.) 

2  See  criticism  by  A.  Michael,  Jour.  Am.  Chem.  Soc.,  42,  787,  1232  (1920). 
4  The  views  of  Jones  and  Stieglitz  are  not  at  variance.     Jour.  Am.  Chem.  Soc., 
36,  288  (1914). 


134  THEORIES  OF  ORGANIC  CHEMISTRY 

in  the  same  way  as  they  are  unsaturated  in  univalent  nitrogen  deriva- 
tives. It  is  these  unsaturated  valencies  which  are  considered  to  be  the 
direct  cause  of  the  rearrangement,  two  electrons  passing  from  the  carbon 
atom  to  the  nitrogen  to  give  this  a  normal  and  stable  charge  such  as 
it  has  in  ammonium  salts.  With  the  change  of  electronic  forces,  the 
positive  radical  R,  nearest  to  the  field  of  force,  is  lost  by  the  now 
positive  carbon  and  carried  to  the  now  negative  nitrogen.  Such  a 
series  of  changes  accounts  for  the  nature  and  action  of  the  reagents 
used  to  accomplish  the  rearrangement  (acid  dehydrating  agents), 
and  it  gives  a  rational  picture  of  the  electrical  forces  in  play  in  the 
rearrangement  of  the  valencies  of  the  molecule.  Such  a  course  would 
also  account  for  the  influence  of  stereoisomerism  on  the  rearrangement, l 
if  such  an  influence  should  finally  be  established  as  beyond  doubt — 
the  radical  (R  in  the  above  illustration)  nearest  to  the  electrical  fields 
of  force,  produced  by  the  migration  of  electrons  from  carbon  to  nitrogen, 
passing  under  the  influence  of  this  force  to  the  nitrogen.  According 
to  this  view  the  rearranging  product  B  would  be  a  third  electromer  of 
of  the  chlorimido  ketones  of  Stieglitz  and  Peterson  2  and  would  repre- 
sent a  logically  active  form  of  the  chloride  which  Hantzsch  assumed 
as  the  intermediate  product  in  the  rearrangement  of  ketoximes  by  the 
action  of  hydrochloric  acid.  This  fundamental  difference  between 
the  latter's  theory  and  the  one  now  developed  should  be  carefully 
noted. 

Hydrogen  and  Hydrocarbo  Bases 

Franklin,3  Kraus  and  others  have  developed  a  new  system  of  acids, 
bases  and  salts,  the  derivaties  of  which  have  been  termed  "  ammono/' 
in  contradistinction  to  the  "  aquo  "  or  ordinary  oxygen  acids  or  bases. 
For  example,  the  neutralization  phenomena  in  both  types  may  be  con- 
trasted as  follows: 

CHsCOOH     +     KOH       =    CH3COOK        +     H20 

Aquo  acid  Aquo  base  Aquo  salt 

CH3CONH2     +     KNH2     =    CH3CONHK    +     NH3 

Ammono  acid  Ammono  base  Ammono  salt 

On  the  basis  of  comparison,  Jones4  has  suggested  that  certain 
metallic  hydrides,  NaH,  etc.,  and  metallic  alkyls,  such  as  zinc  alky  Is, 
be  termed  "  hydrogen  bases  "  and  "  hydrocarbo  bases  "  respectively. 
The  reactions  of  either  type  show  a  similarity  to  those  of  the  ammono 

^Schroeter,  Ber.,  44  (1911). 

2  LOG.  cit. 

3  Am.  Chem.  Jour.,  46,  291  (1912). 

4  Jour.  Am.  Chem.  Soc.,  40,  1259  (1918). 


THE  ELECTRON  CONCEPTION  OF  VALENCY  135 

and  aquo  derivatives.  For  example,  the  negative  alkyl  group  of  the 
metallic  alkyls  (hydrocarbo  bases)  enters  into  a  large  number  of  reactions 
involving  the  displacement  of  the  same  with  other  negative  groups. 
This  is  also  true  to  a  more  l.'mited  extent  with  regard  to  the  hydrogen 
bases.  A  few  examples  will  serve  to  show  their  chemical  behavior: 


NaH  +  HX  =   NaX  +  H-H 
Zn(CH3)2  +  2HC1   =   ZnCl2  +  2H.CH3 
ZnR2  +  2HOH   =   Zn(OH)2  +  2R-H 

ZnR2  +  2NH3   =  Zn(NH2)2  +  2RH 

Of  interest  from  an  electronic  standpoint  is  the  fact  that  both  the 
hydrogen  bases  and  the  hydrocarbo  bases  are  reducing  agents,  a  prop- 
erty sharply  distinguishing  them  from  the  ammono  and"  aquo  bases. 
This  property  is  due  to  the  tendency  of  negative  hydrogen  and  nega- 
tive alkyl  to  lose  negative  electrons,  and  assume  positive  charges,  i.e., 

— elec.  — elec.     + 

H  -          -->  H  -          -»  H 

— elec.                — elec.     + 
CH3  >  CH3  A  CH3 

The  interaction  of  carbon  dioxide  and  sodium  hydride  is  an  excellent 
example  of  this  property. 

a=<>=0"  +  Na-H  =  0~=C— O— Na    ->    0=C— ~ O— Na 

H 

+ 

Moreover  the  well-known  synthetic  reactions,  utilizing  the  zinc  alkyls, 
or  Grignard  reagents,  probably  involve,  apart  from  the  usual  additive 
reactions,  processes  of  reduction,  effected  by  the  negative  alkyl  radicals 
of  the  metallic  organic  compounds.  Thus,  in  the  .following  reactions, 

the  carbon  atom  is  reduced  from  C  to  C  by  the  change  of  CH3 
toCH3: 

"  +      /CH3          HX_+XCH3  H20 

:=o  +  Mg<_     =    +>c<(__++_   — ^ 

*•  +    XBr  H/-+  \0   MgBr 


f 

— h  /CH3 

C<__    +     +     MgBr-OH 

-  +  \n TT 


0— H 


136  THEORIES  OF  ORGANIC  CHEMISTRY 

The  mercury  and  lead  alkyls  afford  an  interesting  and  very  apparent 
proof  of  electromeric  possibilities,  in  the  fact  that  the  former  are  hydro- 
lyzed  by  acetic  acid,  giving  one  molecule  of  hydrocarbon  and  one 
molecule  of  alcohol,  while  the  latter  give  three  molecules  of  hydro- 
carbon and  one  molecule  of  alcohol.  In  the  first  case,  one  alkyl  group 
functions  positively  and  the  other  negatively,  while  in  the  tetra-alkyl 
lead  complex,  one  alkyl  functions  positively  and  three  negatively. 
These  facts  may  be  expressed  as  follows: 

1.  Hg/+  +  H-OH  =  Hg  +  R-OH  +  R-H 

Nft 


2.  >Pb<       +  H-OH  +  2CH3COOH 

W  +~  \R 

=  3RH  +  ROH  +  Pb(C2H302)2 

The  Electronic  Constitution  of  Certain  Acids 

It  has  been  shown  by  Fry  that  the  carboxyl  group  may  function 
either  positively  or  negatively.  A  further  confirmation  of  this  possi- 
bility has  been  given  in  the  deductions  concerning  the  electronic  con- 
stitutions of  acetoacetic  and  citric  acids,  and  some  of  their  derivatives.  l 
The  electronic  formula  of  citric  acid  may  be  represented  as  follows, 
the  position  of  bonds  of  undetermined  polarity  being  shown  by  the  heavy 
black  lines: 

fi      o 

-+1   Jl-  + 

H—  C  -  C—  O-H 

+ 

b~ 

HO—  C  -  ~C~—  ~6—  H 

I'd" 
Ji  -  * 

H—  C  —  C—  O—  H 


When  citric  acid  is  treated  with  fuming  sulphuric  acid  at  60°-70° 
the  central  carboxyl  group  is  eliminated  as  carbon  monoxide,  with 
formation  of  acetone  dicarboxylic  acid: 

iHanke  and  Koessler,  Jour.  Am.  Chem.  Soc.,  40,  1727  (1918). 


THE  ELECTRON  CONCEPTION  OF  VALENCY  137 

CH2COOH  CH2COOH 

HO-C— COOH     =    CO  +     CO     +     H20 

CH2COOH  CH2COOH 

This  reaction  speaks  for  a  difference  in  polarity  between  this  radical 

and  the  two  extreme  carboxyl  groups.     The  formula  of  active  carbon 

-+  -- 
monoxide  is  C=O;    therefore   the  bond  uniting  the  middle  carboxyl 

++ 

group  in  citric  acid  would  logically  be, 

0~ 

(+  -         -)    or    C— C— OH 

while  the  two  extreme  carboxyl  groups  must  be, 

++    -    \ 
C— OH 


I 

This  latter  view  is  supported  by  the  fact  that  the  two  carboxyl  groups 
of  acetone  dicarboxylic  acid  may  be  eliminated  as  CO2  and  not  as  CO 
by  heating  with  mineral  acids  or  alkalis.  In  other  words,  only  carboxyl 

groups  containing  carbon  in  its  highest  state  of  oxidation,     C  ,   are 

electronically  capable l  of  losing  CO2  in  chemical  reactions.  The 
formula  of  citric  acid  may  now  be  written  as  follows: 

H    ~0 

H- C— C+— OH 

!   "o 

HO— C— €+— OH 

o" 

H— C— C+— 6  H 

--  ++ 


r,  Jour.  Am.  Chem.  Soc.,  34,  664  (1913);  36,  256  (1915). 


138  THEORIES  OF  ORGANIC  CHEMISTRY 

The  fact  that  acetone  dicarboxylic  acid  can  be  transformed  into 
acetone  makes  it  evident  that  if  the  electronic  formula  of  the  latter 
could  be  established,  the  polarity  of  the  doubtful  bonds  in  the  former, 
and  also  citric  acid,  would  thereby  be  determined.  Turning  to  aceto- 
acetic  acid,  the  following  formula  presents  an  electronic  view  of  its 
structure: 


H—  C 


By  "  ketonic  "  hydrolysis  the  carboxyl  group  may  be  eliminated  as 
carbonic  acid 

O~ 

+  -A-  + 

H  O—  C—  O  H 

++ 

or  CO  2  and  acetone  formed.     Bond  3  must  therefore  be  (—  +)  and  the 
formula  of  acetoacetic  acid  may  be  electronically  expressed  as  follows  : 


Acid  hydrolysis  of  acetoacetic  acid  is  productive  of  two  molecules 

+    - 
of  acetic  acid,  the  hydrogen  atom  of  H  •  OH  going  to  the  a-carbon  atom 

of  the  acid.     Bond  2  may  therefore  be  written  (+  — ),  and  the  struc- 
tures of  acetoacetic  acid,  acetone  and  acetic  acid  become  respectively : 

0 

I          II 
++ 

-OH, 


Since,  however,  two  molecules  of  acetic  acid  result  from  the  acidic 
hydrolysis  of  acetoacetic  acid,  it  follows  that  the  electronic  formula 


THE  ELECTRON  CONCEPTION  OF  VALENCY 
of  acetone  should  be  the  symmetrically  constituted, 

O~ 


139 


The  formula  of  acetic  acid  receives  futher  confirmation  in  the  light  of 

its  formation  from  ketene,  CH2=CO  and  water,  H  OH. 

The  formula  of  acetone  now  being  established,  the  polarity  of  the 
doubtful  bonds  in  acetoacetic,  acetone  dicarboxylic  acid,  and  citric 
acid  becomes  apparent.  Their  electronic  fo  mulas  may  now  be 
represented  as  follows : 


C— 0— H 


Acetoacetic  acid 


H 


Citric  acid 


Acetone  dicarboxylic  acid 


The  Electronic  Constitution  of  Normal  Carbon  Chain  Compounds 

It  is  difficult  to  assign  definite  electronic  formulas  to  acyclic  or  chain 
hydrocarbons.     The  difficulty  arises  in  the  fact  that  the  proper  dis- 


140  THEORIES  OF  ORGANIC  CHEMISTRY 

tribution  of  polarity  on  the  adjacent  carbon  atoms  cannot  be  made. 
Guy  l  has  recently  called  attention  to  the  fact  that  the  electronic 
formulas  of  acetoacetic,  citric,  acetone  dicarboxylic  and  acetic  acids, 
derived  by  Hanke  and  Koessler,2  show  a  striking  regularity  with  regard 
to  the  alternate  positivity  and  negativity  of  the  carbon  atoms.  Con- 
sidering only  the  carbon  atoms  and  their  polarity,  the  simplified 
formulas  of  these  acids  may  be  given  as  follows: 

Acetic  acid,  C C 


Acetone, 
Acetoacetic  acid, 
Acetone  dicarboxylic  acid, 
Citric  acid, 


It  is  assumed  by  Cuy  that  carbon  atoms  in  a  chain  have  a  natural 
tendency  to  appear  alternately  positive  and  negative,  whenever  possible. 
For  example,  the  positive  and  negative  charges  in  an  homologous 
series  of  the  paraffines  are  presumably  distributed  as  follows: 

1.  C~ 

2.  ~C +C 


6.     C- 

1  Jour.  Am.  Chem.  Soc.,  42,  503  (1920). 

2  Jour.  Am.  Chem.  Soc.,  40,  1726  (1918). 


THE  ELECTRON  CONCEPTION  OF  VALENCY  141 

The  compounds  containing  an  even  number  of  carbon  atoms  form 
an  homologous  series  and  those  containing  an  odd  number  of  carbon 
atoms  form  another  homologous  series,  since  Class  1  differs  from  Class  3 

by  C C,  and  Class  3  differs  from  Class  5  by  the  same  C C  group. 

Similarly  Class  2  differs  from  Class  4,  and  Class  4  from  Class  6  by 

C-   — C.     But  1  differs  from  2  by  a    C    group,    and   2   from   3   by 

C C  1^1  Similar  formulas  may  be  written  and  com- 
pared in  the  case  of  other  homologous  series  with  the  same  result. 
In  other  words,  the  members  containing  an  even  number  of  carbon 
atoms  and  those  containing  an  odd  number  give  two  distinct  homologous 
series  from  an  electronic  view  point. 

This  tendency  to  assume  alternately  positive  and  negative  charges 
is  reflected  in  the  various  physical  properties  of  the  compounds,  such 
as  melting  points  and  boiling  points.  Moreover,  this  hypothesis  admits 
of  interesting  and  logical  explanations  of  various  addition-reactions 
of  hydrogen  halides  to  unsaturated  hydrocarbons.  For  example, 
Markownikoff's  rule  states:  "when  unsymmetrically  constructed 
hydrocarbons  of  the  series  Cn  H2n  combine  with  hydrogen  iodide,  the 
iodine  is  added  to  the  least  hydrogenated  carbon  atom."  The  addition 
of  hydrogen  iodide  to  propylene,  which  follows  Markownikoff's  rule, 
is  also  in  accord  with  the  above  described  theory  of  alternating  polarity. 
The  least  hydrogenated  carbon  atom  of  propylene  would  be  electro- 
positive in  comparison  with  the  extreme  carbon  atom.  Therefore, 
the  positive  hydrogen  of  hydrogen  iodide  adds  to  the  latter,  by  reason 
of  the  free  negative  valence  of  this  atom,  while  negative  iodine  becomes 
attached  to  the  intermediate  carbon  atom,  through  the  medium  of  the 
free  positive  valency  of  the  latter.  These  facts  are  expressed  in  the 
following  electronic  equation: 


HI=CH3 


Kinetic  Hypotheses  Accounting  for  Chemical  Combination 

The  reactions  which  have  hitherto  been  formulated  on  the  basis 
of  the  electronic  conception  have  been  largely  viewed  from  the  stand- 
point of  the  theory  that  chemical  combination  involves  the  transfer 


142  THEORIES  OF  ORGANIC  CHEMISTRY 

of  valence  electrons  from  one  atom  to  another,  on  the  basis  of  the  original 
suggestion  of  J.  J.  Thomson.  Ramsay,1  however,  has  discussed  the 
possibility  of  the  electron  taking  a  position  between  any  two  atoms 
held  in  combination,  and  has  developed  this  conception,2  showing 
through  models  the  magnetic  effects  which  would  serve  to  bring  about 
combination  between  two  given  atoms,  by  reason  of  the  rotation  of 
electrons  in  adjoining  parts  of  the  atoms.  Bohr  3  has  also  considered 
the  possibility  of  atomic  combination  through  the  result  of  electrons 
rotating  about  a  path  joining  the  positive  nuclei  of  two  atoms.  An 
exposition  of  this  "  magneton  "  conception  of  atomic  structure  has 
been  given  by  Parsons.4  Interesting  electronic  theories  in  explana- 
tion of  isomerism  have  been  suggested  by  Garner,5  on  the  basis  of 
Bohr's  hypothesis  and  by  Allen  6  on  the  basis  of  Parson's  magneton 
theory. 

Recently  W.  A.  Noyes  has  offered  a  kinetic  hypothesis  explanatory 
of  the  function  of  electrons  in  producing  chemical  combination  between 
atoms.  He  writes  as  follows:7  "  let  us  suppose  that  two  atoms  which 
have  an  affinity  for  each  other  are  brought  close  together.  A  valence 
electron  which  is  rotating  around  a  positive  nucleus  in  the  first  atom 
may  find  a  positive  nucleus  in  the  second  atom  sufficiently  close,  so 
that  it  will  include  the  latter  in  its  orbit,  and  it  may  then  continue 
to  describe  an  orbit  about  the  positive  nuclei  of  the  two  atoms.  During 
that  portion  of  the  orbit  within  the  second  atom,  that  atom  would 
become,  on  the  whole,  negative  while  the  first  atom  would  be  positive. 
During  the  other  part  of  the  orbit,  each  atom  would  be  electrically 
neutral,  and  the  atoms  might  fall  apart.  When  we  remember,  however, 
the  tremendous  velocity  of  the  electrons,  and  the  relatively  sluggish 
motions  of  the  atoms,  it  seems  evident  that  the  motion  of  an  electron 
in  such  an  orbit  might  hold  two  atoms  together.  In  ionization  the 
electron  would,  of  course,  revolve  about  the  nucleus  of  the  negative 
atom  leaving  the  other  atom  positive.  It  seems  impossible  to  explain 
ionization  otherwise  than  on  the  supposition  of  the  complete  transfer 
of  the  electron.  This  complete  transfer  in  ionization  is  one  of  the 
strongest  arguments  against  the  magneton  theory  as  the  only  explana- 
tion of  chemical  combination." 

1  Jour.  Chem.  Soc.,  93,  774  (1908). 

2  Proc.  Roy.  Soc.,  (A)  92,  451  (1916). 

3  Phil.  Mag.,  26,  1,  476,  857  (1913). 

4  Smithsonian  Miscellaneous  Collections,  65,  No.  11,  (1915.) 

5  Nature,  104,  661  (1920). 

6  Nature,  105,71,  (1920). 

7  Jour.  Am.  Chem.  Soc.,  39,  880  (1917). 


CHAPTER  VIII 

THE  SO-CALLED  NEGATIVE  NATURE  OF  ATOMIC 
GROUPS  OR  RADICALS 

IT  became  apparent  very  early  in  the  history  of  chemistry  that 
the  forces  acting  between  the  atoms  could  not  be  simple  attractive 
forces  like  gravity.  As  has  been  pointed  out,  the  analogy  between 
chemical  and  electrical  phenomena  was  observed  and  for  a  time  received 
international  recognition  among  chemists,  but  following  the  rise  of  the 
Type  Theory  the  idea  that  chemical  action  involved  an  exchange  of 
electricity  was  almost  completely  ignored.  Nevertheless,  even  during 
the  period  when  Kekule's  views  were  most  in  the  ascendant,  C.  W. 
Blomstrand  (1869)  succeeded  in  weaving  the  threads  of  Berzelius' 
electrochemical  theory  into  the  very  texture  of  the  theories  of  Frank- 
land,  Kolbe  and  even  of  Kekule.  It  is  true  that  these  developments 
commanded  very  little  attention  at  the  time,  but  a  later  generation 
appreciated  the  soundness  of  this  classic  work.  Blomstrand 's  concep- 
tion of  diazonium  salts  as  compounds  which  contain  both  trivalent 
and  pentavalent  nitrogen  has  for  the  past  dozen  years  superseded  the 
long  prevailing  views  of  Kekule,  and  his  writings  contain  the  kernel 
of  many  other  ideas  which  are  of  paramount  importance  to-day.  It 
was  Blomstrand  who  first  expressed  the  idea  that  the  carbon  in  • — NC 
is  bivalent,  a  fact  which  was  later  proven  experimentally  by  Nef. 
In  1869,  however,  organic  chemistry  was,  in  general,  impenetrable 
to  electrochemical  conceptions,  and  even  such  an  original  work  as 
van't  Hoff's  "  Ansichten  iiber  Organischen  Chemie  "  was  for  many 
years  entirely  without  influence  upon  theoretical  developments  in  this 
field. 

Yet  even  at  this  time  certain  specific  relationships  which  exist 
between  different  atoms  were  manifest.  It  was  known  that  a  metallic 
atom  coupled  much  more  easily  with  oxygen  than  with  carbon,  and 
furthermore  that  in  the  condensation  of  hydrogen  cyanide  with 
aldehydes  and  ketones,  the  hydrogen  always  combined  with  oxygen 
and  the  cyanogen  with  carbon,  and  that  the  opposite  arrangement 
was  never  observed.  But  while  these  and  a  large  number  of  similar 
phenomena  were  noted,  all  attempts  to  explain  them  failed  to  reveal 

143 


144  THEORIES  OF  ORGANIC  CHEMISTRY 

fundamental  causality.  It  must  be  confessed,  however,  that  such 
atomic  relationships  are  not  always  simple  and  may  at  times  be  very 
complex.  In  order  to  avoid  misunderstanding  in  the  course  of  the 
further  development  of  this  theme,  it  will  be  necessary,  therefore,  to 
pause  at  this  point  and  to  consider  both  the  differences  and  the  similari- 
ties which  exist  between  the  so-called  negative  nature  of  atomic  groups 
and  the  electrochemical  character  of  the  elements. 

Characteristic  differences  in  the  properties  of  saturated  and  unsatu- 
rated  organic  compounds,  as  well  as  such  gradations  in  unsaturation 
as  are  observed  in  the  case  of  ethylene  and  benzene  derivatives,  were 
first  brought  into  prominence  through  the  investigations  of  A.  von 
Baeyer  and  his  students.  These  researches  were  primarily  concerned 
with  the  thermal  relationships  of  these  substances,  but  it  soon  became 
apparent  that  the  influence  of  the  unsaturated  condition  upon  physico- 
chemical  properties  was  very  general.  A  study  of  molecular  volumes, 
electrolytic  conductivity,  and  especially  of  molecular  refractions  all 
showed  the  same  influences,  acting  always  in  the  same  direction  and 
in  conformity  to  natural  and  fundamental  laws.  It  remained  for  F. 
Henrich  1  to  demonstrate  that  all  of  the  remarkable  series  of  reactions, 
which  had  hitherto  been  referred  to  the  so-called  negative  nature  of 
certain  radicals,  were  actually  due  to  their  unsaturated  character. 

When  Victor  Meyer  2  discovered  the  nitroparaffines,  he  observed 
that  they  were  capable  of  forming  salts  with  alkali,  a  phenomenon 
which  could  not  at  that  time  have  been  foreseen  from  theoretical 
considerations.  Further  investigation  revealed  the  following  regu- 
larity in  the  behavior  of  nitroparaffines  up  to  nitro-butane.  If  one  of 
two  hydrogen  atoms  in  union  with  carbon  is  replaced  by  a  nitro  group, 
the  properties  of  the  remaining  hydrogen  atom  are  affected  in  the  sense 
that  it  becomes  more  acidic  or  negative  in  character  and  is  now  capable 
of  being  replaced  by  a  metal.  Similar  observations  were  made  in  the 
case  of  ethyl  acetoacetate  and  diethyl  malonate,3  and  later  in  the  case 
of  nitrocinnamyl  ketone  and  benzoyl  acetone.4  In  these  latter  cases 
the  remarkable  behavior  of  the  hydrogen  of  the  methylene  group  was 
attributed  to  the  influence  of  the  organic  radicals  CHsCO — , 
— COOC2H5,  CeHsCO — ,  etc.  New  compounds  were  easily  formed 
by  treatment  of  these  metallic  derivatives  with  alkyl  halides.  Later 
it  was  discovered  that  these  supposedly  acid  hydrogen  atoms  were 

1  Ber.,  32,  668  (1889);  also  Habilitationsschrift,  Erlanger,  1900,  Junge  and  Son. 

2  Ber.,  5,  404  (1872);  Annalen  der  Chemie,  171,  1  (1874);  175,  88  (1875);  180, 
111  (1876). 

3  Annalen  der  Chemi^,  186,  182  (1877). 

4  Ber,  16,  33  and  2239  (1883);  18,  2132  (1885). 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS        145 

capable  of  reacting  not  only  with  metals  but  with  halogen,  nitrous 
acid,  aldehydes,  salts  of  diazobenzene,  etc.  Thus  on  the  basis  of 
experimental  evidence  the  rule  governing  the  effect  of  nitro-substi- 
tutions  was  extended  to  include  the  substitution  of  other  so-called 
negative  groups  and  may  be  stated  as  follows:  "  if  one  of  two  or  more 
hydrogen  atoms  in  union  with  carbon  is  replaced  by  unsaturated 
groups  such  as  — NO2,  — COCH3,  — COC6H5  and  — COOC2H5  the 
remaining  hydrogen  atoms  will  show  acid  properties  and  increased 
chemical  reactivity."  1  As  the  result  of  the  researches  of  Haller, 
Held,  Henry  and  others,  the  cyanogen  group  was  subsequently  added 
to  this  list. 

The  first  systematic  investigation  of  this  problem  was,  however, 
undertaken  by  Victor  Meyer  in  1887,  when  he  began  the  publication 
of  his  series  of  important  papers  on  the  "  negative  nature  "  of  organic 
radicals.2  He  defined  an  acid  radical  as  a  salt-forming  group,  or,  in 
other  words,  as  a  group  capable  of  decreasing  the  basicity  of  amines, 
and  he  supplemented  the  list  of  those  already  mentioned  by  the  addition 
of  the  phenyl  and  thionyl  groups.  At  the  same  time  he  pointed  out  a 
difference  in  the  action  of  different  acid  radicals  upon  methylene 
and  methine  hydrogen,  cyanogen  being,  for  example,  decidedly  more 
negative  than  carbethoxy  — COOC2H5.  Claisen  has  since  shown 
that  — COCH3  and  — COC6H5  are  also  more  negative  than  —  COOC2H5. 
Later  vinylene,  CH=CH,3  came  to  be  recognized  as  a  negative  group 
following  the  researches,  dating  from  1889  on,  of  von  Baeyer,4  W. 
Markwald,5  Claisen 6  and  F.  Henrich.7  The  investigations  of 
Thiele  8  on  ethyl  phenyl-acetoacetate  and  of  W.  Wislicenus  9  on  indene 
and  fluorene  support  this  view. 

The  number  of  negative  radicals  continued  to  be  increased  as  a 
result  of  investigation  and  came  to  include  the  following: 


X 

X)H 


)C2H5 


!Annalen  der  Chemie,  204,  198  (1880). 

2  Ber.,  20,  534  and  2944;  21,  1295,  1306,  1316,  1331,  1344,  etc.  (1888). 

s  Compare  O.  Hinsberg,  Jour,  prakt.  Chemie,  84,  180  (1911);  85,  337  (1912). 

'Annalen  der  Chemie,  251,  267  (1889). 

5  Annalen  der  Chemie,  279,  9  (1894)  ;  Ber.,  28,  1501  (1895). 

6  Annalen  der  Chemie,  297,  14-16  (1897). 

7  Ber.,  31,  2103  (1898);   32,  670  (1899);   33,  668,  851  (1900);  Monatsh.  Chemie, 
20,  539  (1899). 

s  Annalen  der  Chemie,  306,  114  (1899);  Ber.,  33,  666,  851,  3359  (1900);  34,  68 

(1901). 

9  Ber.,  33,  771  (1900). 


146  THEORIES  OF  ORGANIC  CHEMISTRY 

^O  .0  .0 

-c<      ,    -c/      ,       -c/ 

XCH3  XC6H5  X  COOC2H5 


—NO,    N02,    —  CH=CH—  ,    =C=CX2, 
C=C,    C=N,    —  C6H5    —  Ci0H7    and    C4H3S. 


These  groups  possess  one  characteristic  in  common,  as  was  pointed 
out  by  F.  Henrich  in  1899,  viz.,  they  consist  of  multivalent  atoms, 
whether  of  the  same  kind  or  of  different  kinds,  which  are  bound 
together  by  means  of  double  or  triple  bonds.  That  the  negative 
character  of  the  group  varies  with  the  degree  of  unsaturation  of  its 
atoms  is  shown  by  the  strongly  negative  properties  of  NO2  and  C=N. 
Further,  the  degree  of  unsaturation  may  be  regarded  as  corresponding 
to  a  relatively  high  energy  content,  as  shown  by  the  heats  of  com- 
bustion, molecular  refraction,  etc.,  of  the  substances.  According 
to  Henrich  it  is  not  only  characteristic  of  negative  radicals,  but  essen- 
tial to  their  very  existence  that  they  contain  homogeneous  or  hetero- 
geneous atoms  in  dense  groupings,  that  is  to  say,  bound  together  by 
double  or  triple  bonds. l 

If  this  statement  is  correct  it  should  follow  that  all  unsaturated 
groups  possess  negative  properties,  and  this  is  in  fact  the  case. 
Henrich  has  already  pointed  out  that  the  group  CH=N  is  negative 
in  character,  and  that  it  is  due  to  its  influence  that  the  hydrogen  of  the 
imido  group,  NH,  in  the  following  compounds  for  example,  is  replace- 
able by  metals. 

— |NH  C6H5 iNH 


'\S 
N  CeH5 

Dimethylbenzimidazol  Diphenylacetamidine 

These  substances  and  their  derivatives  have  been  investigated  by 
Bamberger,  Berle  and  Lorenzen.  This  acidic  character  of  the  NH 
group  is  not  only  observed  in  the  acyclic  and  cyclic  amidine  combina- 
tions but  also  in  the  cyclic  hydrazines,  as  represented,  for  example, 
by  the  following  compounds  where  the  imido  hydrogen  is  readily  replace- 
able by  metals.  It  may  be  noted  that  the  unsaturated  grouping 
— CH=N  functions  in  both  classes  of  compounds, 

CH N  CH CH 

JH      in       and      Ai     1 


* 
Her.,  32,  673  (1899). 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS  147 

Bamberger  and  Berle  l  made  the  further  discovery  that  dimethyl- 
benzimidazol  is  capable  of  forming  condensation  products  with 
aldehydes.  A  study  of  this  interesting  reaction  revealed  the  fact 
that  only  one  of  the  two  methyl  groups  present  in  the  molecule  was 
able  to  react  in  this  way.  It  was  identified  as  the  methyl  group  in 
direct  union  with  the  carbim  radical  (CH=N),  the  other  methyl  being 
entirely  inert  toward  aldehydes.  This  marked  difference  in  chemical 
activity  was  naturally  interpreted  as  due  to  the  influence  of  the  nega- 
tive group  CH=N.  The  reaction  thus  becomes  directly  comparable 
to  a  normal  ketone  condensation, 

RCCH'Ha+OCH-CHs    ->    RCCH  :  CHCH3+H2O 


and  may  be  expressed  by  means  of  the  following  equation: 

RCCH-Hs+OCHCHs      -»      RCCH  :  CHCH3+H2O 


Similar  condensations  have  been  noted  in  the  case  of  a-methyl  pyridine 
and  a-methyl  quinoline  and  other  cyclic  combinations  of  similar 
structure, 


and  may  be  interpreted  as  offering  additional  proof  of  the  negative 
influence  of  the  carbim  group,  — CH  =  N.  The  negative  character 
of  this  grouping  has  also  been  strongly  commented  upon  by  E.  Erien- 
meyer,  Jr.,2  who  in  this  way  explains  the  tendency  of  hippuric  acid 
to  condense  with  aldehydes.3  It  has  been  observed  that  methyl  groups 
in  the  7  position  to  nitrogen,  as  in  methylacridine, 


1  Annalen  der  Chemie,  273,  277  (1893). 

2  Jour,  prakt.  Chemie,  62,  145  (1900). 

3  Annalen  der  Chemie,  337.  219  (1904). 


148  THEORIES  OF  ORGANIC  CHEMISTRY 

also  show  increased  chemical  reactivity  and  play  the  part  of  acidic  or 
negative  groups.  Here  we  have  complete  saturation  of  the  mole- 
cule, and  consequently  the  increased  reactivity  cannot  be  explained 
in  terms  of  the  above  hypothesis.  It  must  be  assumed  to  be  due  to 
other  causes,  as,  for  example,  the  very  dense  grouping  of  the  atoms 
in  the  molecule  l  or  to  other  molecular  or  structural  influences  which 
exert  the  same  effect  as  unsaturation.  In  this  connection  it  may  be 
noted  that  dense  groupings  of  the  atoms,  such  as  are  common  to  the 
various  types  of  ring  structure,  tend  to  produce  increased  chemical 
activity.  A  comparison  of  the  following  compounds  demonstrates 

CH2  CH2 

and 


Diphenylpropylene 
I 


and 


COOH 

Homophthalic  acid  Homophthalic  anhydride 

III  IV 

according  to  W.  Dieckman  2  that  the   closing  of  the   ring  materially 
increases  the  reactivity  of  the  methylene  group. 

In  1894  Bamberger  3  pointed  out  the  distinctly  negative  character 
of  the  azo  group,  N=N,  in  organic  combinations.  Thus  in  diazo- 
amidobenzene  and  related  compounds  the  imido  groups  possess  weakly 
acid  properties,  but  these  are  greatly  strengthened  by  the  introduction 
of  nitro  groups  into  the  benzene  nucleus.  For  example,  the  acidity  of 
diazoamidobenzene  is  increased  to  such  an  extent  by  introduction  of 
two  nitro  groups  into  the  benzene  nucleus  that  the  resulting  compound, 
NO2C6H4N==N-NHC6H4NO2,  dissolves  even  in  weak  alkali  solutions 
to  form  salts.  Still  later  O.  Dimroth  4  was  able  to  show  that  even 
diazo-amino  compounds  of  the  fatty  series  possess  pronounced  acid 
properties.  The  azo  group  is  ordinarily  only  weakly  negative  in 
character.  When,  however,  both  of  its  free  affinities,  —  N=N — ,  are 


1  Compare  Ber.,  39,  3046  (1906). 
2Ber.  39,  3046  (1906);  47,  1428  (1904). 
3  Ber.,  27,  2511  (1894). 


4  Ber.,  39,  3905  (1906). 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS  149 

satisfied  by  union  with  a  common  atom,  as  is  the  case  in  hydrazoic  acid, 
HN3,  in  which  the  azo  group  may  be  regarded  as  replacing  two  of  the 
hydrogen  atoms  in  ammonia,  the  atomic  density  of  the  grouping  causes 
a  marked  increase  in  acid  properties.  In  other  words,  the  cyclic  group, 

Nv 
N 

is  characterized  by  its  strongly  negative  properties  and  is  even  capable 
of  independent  existence  as  an  ion. 

Th.  Curtius  1  explained  the  action  of  metallic  sodium  and  potas- 
sium upon  ethyl  diazoacetate  by  assuming  that  the  hydrogen  of  the 
methine  group  is  replaceable  by  metals  with  the  formation  of  car- 
bonium  salts.  A.  Hantzsch  2  has  shown,  however,  that  this  interpre- 
tation is  incorrect,  and  that  the  ethyl  diazoacetate  reacts  in  its  tauto- 
meric  form  with  metals  giving  nitrogen  salts  of  the  general  formula : 

Me-Nv 

I   >C'COOC2H5 


The  reactivity  of  the  imido  hydrogen  in  this  case  may  be  interpreted 
as  due  in  part  to  the  influence  of  the  double  bond  between  the  carbon 
and  nitrogen  (carbim  grouping),  and  in  part  to  the  dense  grouping 
of  atoms  in  the  molecule  which  results  from  ring  formation. 

The  negative  character  of  the  group  SO2  in  organic  combination 
(sulphone)  has  been  the  subject  of  extended  investigation.  It  was 
originally  regarded  as  a  negative  radical  and  was  included  among  the 
so-called  negative  groups  by  Haller.  Later  Victor  Meyer  pointed 
out  that  phenyl  benzyl  sulphone,  CeHsSC^CH^CeHs,  shows  none  of 
the  reactions  typical  of  the  ketone  Cells  •  CO  •  CH2  •  CeH^,  and  for  a 
long  time  after  this  the  sulphone  grouping  (862)  ceased  to  be  included 
in  the  list  of  acid  radicals.  In  1899,  however,  Michael  3  succeeded 
in  demonstrating  the  reactivity  of  the  methylene  group  in  phenylsul- 
phonic  acid  esters,  as  is  represented  in  CeHsSC^  •  CHkCOCX^Hs.  In 
the  same  year  Autenrieth  and  Wolff,4  and  a  year  later  A.  Kotz  5  pointed 
out  the  mobility  of  hydrogen  in  cyclic  sulphones  possessing  the  atomic 
grouping, 

-S02-CH2-S02- 

!Ber.,  17,  956  (1884);  Jour,  prakt.  Chemie,  38,  410  (2),  (1888). 
2Ber.,  34,  2508  (1901);  46,  1657  (1912). 
3  Jour,  prakt.  Chemie,  60,  96  (1899). 
4Ber.,  32,  1381  (1899). 
6Ber.,  33,  1120  (1900). 


150  THEORIES  OF  ORGANIC  CHEMISTRY 

The  reactivity  of  the  methylene  hydrogen  in  the  following 
combinations 

SO 
H2C/NCH2 

OsiJsO 

CH2  CH2 

Trimethylene  trisulphoxide  Trimethylenetrisulphone 

I  II 

is  shown  by  the  fact  that  both  substances  are  distinctly  acidic  in 
character.  The  former  is  soluble  in  aqueous  sodium  hydroxide. 
It  also  reacts  with  aldehydes,  diazo  compounds,  etc.  The  latter  is  a 
stronger  acid  and  dissolves  readily  in  sodium  carbonate,  but  does  not 
react  with  aldehydes,  etc.1 

It  has  already  been  observed  that  methylene  hydrogen  is  more 
strongly  influenced  by  such  groups  as  CH=CH  and  COCH=CH 
than  by  Cells;  and  it  now  appears  that  o-carbethoxyphenyl 
C2H5OOC-C6H4  —  also  belongs  to  the  former  class.  This  follows 
from  the  researches  of  Dieckmann,2  who  has  discovered  that  homo- 
phthalic  ester  reacts  readily  with  benzylbromide  in  the  presence  of 
potassium  ethoxide  to  give  benzylhomophthalic  ester; 


4  •  CH  - 


C2H5OOC  •  C6H4  •  CH  -  COOC2H5 

and  with  benzaldehyde  in  the  presence  of  sodium  ethylate  to  give 
benzalhomophthalic  acid 

CH  -Cells 
HOOC-C6H4-C-COOH 

Dieckmann  has  pointed  out  in  this  connection  that  this  observed 
reactivity  of  the  CH2  group  is  in  marked  contrast  to  the  behavior  of 
this  group  in  ethyl  phenylacetate. 

Further  generalizations  along  these  lines  have  resulted  from  the 
investigations  of  D.  Vorlander,3  who  calls  attention  to  the  fact  that 
the  acid  character  of  the  hydrogen  atom  present  in  the  carboxyl  group 
of  acids  is  not  due  to  the  influence  of  either  —  CO  or  —  OH  alone,  but 

iHinsberg,  Jour,  prakt.  Chemie  86,  337,  351  (1912). 
2Ber.,  47,  1428  (1914). 

3  Abhandlungen  der  naturforschen  Gesellschaft  zu  Halle,  21,  235  (1899);  Ber., 
34,  1632  and  1637  (1901);  Annalen  der  Chemie,  320,  66  and  99  (1901). 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS        151 

to  the  combined  influence  of  both  groups.  If  the  symbol  E  be  made 
to  represent  a  non-metallic  element,  then  the  expressions 

H-E-E=E;        H-E-E^E;        H-E=E=E 

1234  1234  1234 

represent  the  atomic  groupings  common  to  most  organic  and  inorganic 
acids.  Vorlander  calls  the  unsaturated  atoms  occupying  positions 
3-4  the  reactive  group,  and  points  out  that  this  group  is  both  physically 
and  chemically  active.  The  mobility  of  the  hydrogen  in  any  such  com- 
plex depends  (1)  upon  the  nature  of  the  non-metallic  atom  with  which 
the  hydrogen  is  in  direct  union,  and  (2)  upon  the  nature  and  relative 
degree  of  unsaturation  of  the  non-metallic  atoms  3  and  4.  In  order  to 
increase  the  mobility  of  hydrogen  above  that  observed  in  the  types 
HC1,  H2O,  H^S,  NHs  and  CH4,  a  double  bond  must  be  introduced  in 
the  position  3-4.  This  is  shown,  for  example,  by  the  strongly  acid 

OH 

properties  of  such  compounds  as  chloric  acid  0=C1=0  and  hydrox- 
OH 

amic  acid  HO-N=C-R.  A  "  reactive  group"  in  other  positions,  as 
in  the  combinations 

H-E=E        and        H-E-E-E=E 

123  12345 

has  no  influence  upon  the  mobility  of  the  hydrogen.  This  is  obvious 
from  a  consideration  of  the  chemical  properties  of  aldehydes,  chloral 
hydrate,  and  also  from  a  study  of  the  behavior  of  triphenylcarbinol. 
It  has  been  observed,  for  example,  that  the  hydroxyl  group  in  the 
latter  compound  is  no  more  strongly  acidic  than  in  the  aliphatic 
alcohols.  The  fact  that  the  presence  of  three  strongly  negative  groups 
do  not  increase  the  acidity  of  this  substance  may  be  explained  by 
reference  to  the  following  structural  formula; 

(C6H5)2— C-OH 


when  it  becomes  apparent  that  the  hydrogen  atom  occupies  an  unfavor- 
able position  with  reference  to  the  unsaturated  vinylene  groups.  The 
fact  that  this  substance  differs  from  the  aliphatic  alcohols  in  its  ability 


152  THEORIES  OF  ORGANIC  CHEMISTRY 

to  split  off  hydroxyl  and  so  to  exhibit  basic  properties  is  explained  by 
Vorlander  1  as  due  to  the  favorable  position  of  the  oxygen: 


It  may  be  added  that  the  mercapto  group  in  triphenylthiocarbinol 
shows  a  similar  tendency  to  undergo  analogous  changes. 

Vorlander  explains  the  fact  that  nitrogen  bases  are  readily  oxi- 
dized in  alkaline  and  not  in  acid  solution,  by  supposing  that  the  free 
bases  are  unsaturated  compounds  while  their  salts  are  saturated. 
Trivalent  nitrogen  thus  belongs  to  the  same  general  category  as 
C=C,  C=O,  etc.,  and  the  following  pairs  of  reactions  may  be  regarded 
as  strictly  analogous  in  character: 

/H 

R3N=+HBr     <±    R3N< 

\Br 

R2C=CR2+HBr    ^±    R2C— CR2 


H3N=+H2O    ^±    NH4OH 
CClaCH  :  O+H2O    ^±    CC13-CH(OH)2 

Thus  the  relative  reactivity  of  the  nitrogen  is  measured  by  the  basicity 
of  its  compounds.  The  analogy  is  complete  when  the  influence  of 
trivalent  nitrogen  upon  the  mobility  of  hydrogen  is  considered.  This 
is  the  same  as  in  the  case  of  ethylene,  as  may  be  seen  from  a  compari- 
son of  the  following  formulas: 

H-E-E=E        and        H-E-N= 

1234  123 

I  II 

in  both  of  which  hydrogen  occupies  the  favorable  3-position  with 
reference  to  the  unsaturated  element. 

Many  phenomena  observed  in  the  oxidation  of  amines  may  be 
explained  on  the  basis  of  these  assumptions.  Thus  ammonia  NHs, 
azobenzene  C6H5N==NC6Hs,  and  similar  substances  are  very  stable  in 
the  presence  of  oxidizing  agents,  while  aliphatic  amines,  CH3NH2 

iBer.,  46,  3450  (1913). 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS  153 

hydroxylamine  NH2OH,  hydrazine  and  similar  combinations  are 
readily  oxidized.  Although  substances  belonging  to  the  first  class 
of  compounds  contain  reactive  nitrogen,  this  is  not  in  a  position  to 
affect  the  mobility  of  the  hydrogen  atom.  Substances  belonging  to 
the  second  class  of  bodies,  on  the  other  hand,  contain  the  combination 
H-E-N:,  and,  according  to  Vorlander,  should  therefore  be  reactive. 

Two  ethylhydroxylamines  are  theoretically  possible  according  to 
whether   the   alkyl   group   is   attached   to   oxygen   or  nitrogen,    i.e., 

II  II 

C2H50-NH2  (a)        and    HONH-C2H5  (ft) 

Of  these  the  former  should  be  stable  and  the  latter  unstable  toward 
oxidizing  agents.  In  1880  Gorke1  was  able  to  show  that  a-ethyl 
hydroxylamine  is  not  attacked  even  by  boiling  alkali  solutions  of 
copper,  while  its  isomer  reduces  Fehling's  solution  in  the  normal 
way.  Aniline,  Cells -NH2,  and  dimethyl  aniline,  Cells- N(CH3)2, 
on  the  other  hand,  are  both  readily  oxidized  since  both  contain  hydro- 
gen in  the  correct  structural  relation  to  an  unsaturated  group,  as  is 
represented  by  the  following  expressions 


•C=C—        and         (CH3)2N- 


H2N-C=C—        and  (CH3)2N.C6H5 

respectively.     The  following   pairs   of  substances  also  show  marked 
analogy  in  their  chemical  properties: 

(CH3)2N-C6H5        and        CH3C-C6H5 


C6H5NH.CH2COOH        and        C6H5C  •  CH2COOH 


The  unsaturated  character  of  nitrous  acid  is  shown  by  the  ease 
with  which  it  passes  into  nitric  acid  on  oxidation  with  permanganates. 
Sodium  nitrite  and  amyl  nitrite,  on  the  other  hand,  resist  oxidation 
because  they  possess  no  mobile  hydrogen.  Vorlander  explains  the  fact 
that  the  methyl  group  in  a-picoline, 

H 
C 


H< 


v/ 

N 


C-CH3 


1  Annalen  der  Chemie,  205,  277  (1880). 


154  THEORIES  OF  ORGANIC  CHEMISTRY 

is  distinctly  more  reactive  than  in  toluene, 


by  assuming  that  the  former  compound  contains  unsaturated  nitro- 
gen. In  toluene  the  carbon  atoms  of  the  benzene  ring  exercise  their 
maximum  valency. 

The  addition  of  the  unsaturated  elements,  nitrogen,  oxygen,  and 
sulphur,1  to  the  general  category  of  unsaturated  (i.e.,  so-called  negative) 
groups,  affords  a  new  point  of  contact  between  organic  and  inorganic 
chemistry.  Vorlander  is  of  the  opinion  that  the  same  general  relations 
hold  between  saturated  and  unsaturated  compounds,  whether  organic 
or  inorganic  and  whether  compounds  of  the  positive  metals  or  of  the 
negative  non-metallic  elements. 

It  should  be  added  that  Vorlander  has  succeeded  in  demonstrating 
experimentally  that  one  and  the  same  radical  may  be  positive  or 
negative  in  character,  depending  upon  the  form  of  chemical  combina- 
tion in  which  it  is  found.  He  even  shows  that  the  effect  of  a  so-called 
negative  radical  may  be  to  strengthen  instead  of  weaken  the  basic 
properties  of  a  given  compound.  For  example,  Vorlander  and  Nolte  2 
have  recently  discovered  that  trimethylamine  will  combine  with 
benzene  sulphonchloride  to  give  a  salt-like  body.  The  product  is 
neutral  in  aqueous  solution  and  may  be  readily  separated  in  the  form 
of  the  chlorplatinate.  Analysis  shows  that  it  is  formed  by  the 
addition  of  one  molecule  of  benzene  sulphonchloride  to  one  molecule 
of  trimethylamine,  and  it  is  regarded  by  Vorlander  and  Nolte  as 
representing  the  first  member  of  a  new  type  of  quaternary  ammonium 
salt  in  which  the  acid  radical  CeHsSC^  —  takes  the  place  of  an  alkyl 
residue,  viz., 


1  Hinsberg,  Ber.,  43,  901  (1910);   Jour,  prakt.  Chemie,  84,  184  (1911);   Pummerer, 
Ber.,  43,  1376  (1910). 

2  Ber.  ,46,  3215  (1913). 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS         155 

Now,  benzensulphonic  acid  is  even  more  strongly  acidic  than  sulphuric 
acid  and  it  would,  therefore,  seem  reasonable  to  expect  such  a  strongly 
acid  residue  as  CeHsSC^ —  to  decrease  the  stability  of  the  resulting  com- 
pound. As  a  matter  of  fact,  this  salt  is  not  hydrolyzed  by  water.  It 
is  acted  upon  by  moist  silver  oxide  with  the  formation  of  the  free  base 
which  then  undergoes  immediate  decomposition. 

These  facts  would  seem  to  indicate  that  the  salt-forming  and  essen- 
tially basic  properties  of  ammonia  are  not  entirely  destroyed  by  the 
introduction  of  a  negative  group  like  C6H5S02 —  into  the  so-called 
ammonium  complex.  In  such  a  case  as  this,  where  a  distinctly  nega- 
tive group  does  not  function  in  a  negative  manner,  it  becomes  neces- 
sary to  reconsider  the  whole  matter,  if  possible,  from  a  different  angle. 
According  to  Vorlander  and  Nolte,  the  fact  that  ammonia,  aniline, 
nitraniline  and  diphenylamine  show  differences  in  their  ability  to  form 
bases  by  the  addition  of  water,  and  salts  by  the  addition  of  acids, 
may  be  explained  upon  the  assumption  of  differences  in  what  they  call 
the  aminity  of  the  substances  and  not  of  differences  in  their  basicity. 
The  degree  of  aminity  depends  upon  the  unsaturated  character  of  the 
nitrogen  atom  and  not  primarily  upon  the  positive  or  negative  character 
of  the  groups  in  union  with  it.  In  other  words  the  ordinary  conception 
of  strong  and  weak  ammonia  bases1  must  be  abandoned,  and  the 
strength  of  a  given  amine  must  be  measured  in  terms  of  the  free 
affinity  of  its  nitrogen  as  expressed  in  the  ability  of  the  substance 
to  enter  into  addition  reactions. 

In  applying  this  idea  to  radicals  containing  multivalent  elements 
other  then  nitrogen,  Vorlander  assumes  the  existence  of  a  "  dual 
nature "  which  depends  primarily  upon  the  positive  or  negative 
character  of  the  element  in  question  and  secondarily  upon  the  satu- 
rated or  unsaturated  condition  of  the  molecule  in  which  that  element 
is  found.2  The  facts  may  be  briefly  summarized  by  saying  that  the 
reactivity  of  the  hydrogen  present  in  — OH,  NH,  CH,  etc.,  depends 
upon  the  presence  of  unsaturated  atoms  or  groups  in  favorable  posi- 
tions. 

The  conceptions  of  Henrich  and  Vorlander  have  recently  been 
extended  by  O.  Hinsberg3  as  the  result  of  a  study  of  ionization 
phenomena.  A  comparison  of  the  chemical  properties  of  the  three 
types  of  compounds  represented  by  formulas  I,  II  and  III, 

1  Jour,  prakt.  Chemie,  87,  90  (1913). 

2  Compare  Vorlander,  Ber.,  37,  1646,  1651  (1904).     Annalen  der  Chemie,  341,  p. 
1  and  following;  345,  155,  251  (1906). 

3  Jour,  prakt.  Chemie,  84,  169  and  following  (1911):   A  more  recent  treatment 
in  Jour,  prakt.  Chemie,  86,  337  (1912)  is  valueless  at  present. 


156  THEORIES  OF  ORGANIC  CHEMISTRY 

000 

C6H5-C— H  C6H5-C— OH  C6H5-C-0-OH 

a  /3      a  7  ft    a 

I  II  III 

leads  to  the  discovery  of  striking  differences  in  behavior.  While  all 
three  possess  reactive  hydrogen,  the  influence  of  the  carbonyl  group 
on  the  hydrogen  atoms  in  the  a,  ft  and  7  positions  respectively  is 
markedly  different.  Thus  the  hydrogen  present  in  I  is  non-ionizable 
and  is  not  replaceable  by  metals.  It  is,  however,  capable  of  being 
replaced  by  Cl,  O-COCH3,  etc.,  and  its  reactivity  is  stimulated 
by  the  presence  of  potassium  cyanide.  The  hydrogen  present  in  II 
and  III,  on  the  other  hand,  shows  distinctly  acid  properties,  III 
being  a  very  weak  acid  whose  salts  are  decomposed  by  carbon  dioxide 
and  II  being  so  decidedly  acid  as  to  show  ionizable  hydrogen.  Thus 
the  carbonyl  group  is  capable  of  acting  in  two  ways:  it  may  simply 
intensify  the  mobility  of  the  hydrogen  atom  so  that  the  usual  reactions 
take  place  more  readily  as,  for  example,  in  the  case  of  I,  or  it  may  act 
to  change  the  character  of  the  hydrogen  so  that  it  is  no  longer  capable 
of  the  usual  reactions  but  exhibits  typically  acid  properties,  as  in  II 
and  III.  In  general  it  has  been  found  that  other  unsaturated  groups, 
in  the  a,  ft  and  7  positions  respectively,  exercise  the  same  influence 
upon  hydrogen  as  the  carbonyl  group.  The  rule  is  not  perfectly 
general,  however,  since  certain  atoms  in  the  a  position  are  capable 
of  bringing  about  the  ionization  of  hydrogen,  as  for  example,  Cl  in 
HC1  and  S  in  HSH.  Hinsberg  classifies  such  atoms  as  lonogens  of 
the  First  Order,  while  atoms  and  groups  in  the  ft  position  are  classified 
as  lonogens  of  the  Second  Order. 

A  comparison  of  the  properties  of  the  three  organic  substances 
represented  below  shows  that  the  acid  hydrogen  atom  in  II  is  much 
more  strongly  ionized  than  that  in  I  by  the  presence  of  carbonyl  in  the 
ft  position: 

0  O  OH 

CH3— C— OH         C13CC-OH  ClsCCH-OH 

ft      a 

1  II  III 

This  marked  difference  in  properties  is  obviously  due  to  the  substi- 
tution of  chlorine  in  the  methyl  radical,  but  that  this  does  not  wholly 
explain  the  phenomenon  is  shown  by  the  fact  that  chloral  hydrate, 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS         157 

III,  is  neutral  in  its  properties.  The  primary  condition  for  the 
ionization  of  hydrogen  thus  seems  to  be  the  presence  of  the  carbonyl 
group  in  the  ft  position.  If  such  an  unsaturated  group  is  present  in 
the  molecule  chlorine  may  exercise  a  secondary  influence  by  increasing 
the  degree  of  ionization.  Hinsberg  has  formulated  these  facts  as 
follows:  lonogens  of  the  first  order  have  an  acidifying  influence 
upon  hydrogen  atoms  present  in  organic  molecules  only  when  these 
are  simultaneously  acted  upon  by  ionogens  of  the  second  order. 

If  the  acidity  of  a  substance  is  increased  by  the  presence  of  certain 
groups  in  its  molecule,  it  should  also  be  possible  to  decrease  the 
negative  properties  or  even  to  change  the  whole  character  of  the 
substance  from  acidic  to  basic  by  the  introduction  of  other  groups. 
This  is  in  fact  true  as  the  following  example  shows:  sulphur  may  be 
assumed  to  possess  originally  only  negative  ionogen  valencies,  since 
its  hydrogen  derivative,  H^S,  is  a  weak  acid.  If  sulphur  is  in  union 
with  three  methyl  groups,  however,  its  character  is  completely  changed 
from  acidic  to  basic,  and  the  resulting  trimethyl  sulphonium  radical 
(CH^Ss-  ~,  is  found  to  possess  one  positive  (~~)  ionogen  valence. 
Hinsberg  calls  such  radicals  "  commuting  groups  "  ("  Kommutierende 
Atomgruppen  ")  and  defines  them  as  groups  which  strengthen  or  weaken 
ionogen  valencies  without  being  able  themselves  to  create  such 
valencies. 

The  behavior  of  the  unsaturated  vinylene  group,  — CH=CH — , 
is  very  remarkable  in  that  it  seems  to  have  the  power  of  acting  in  two 
capacities,  viz.,  it  is  capable  of  bringing  about  the  ionization  of  hydro- 
gen atoms  occupying  the  ft  position  with  reference  to  it,  in  the  sense 
of  forming  either  cations  or  anions.  Thus,  for  example,  it  evidently 
plays  the  role  of  a  negative  ionogen  of  the  second  order  in  both  cyclo- 
pentadiene  and  indene, 

CH— CH 

and 


CH     CH 


CH2 

Cyclopentadiene 

since  the  hydrogen  of  the  methylene  group  is  very  reactive  in  both  of 
these  substances.  On  the  other  hand,  it  functions  as  a  positive  ionogen 
of  the  second  order  when  occupying  the  ft  position  with  relation  to 
such  anions  as  Cl,  — OSOsH,  — ONO2,  etc.  The  basic  character  of  the 
following  oxide  and  hydroxide  is  clearly  demonstrated  by  their  ability 
to  form  salts  in  which  the  anion,  chlorine,  is  in  the  ft  position  to  two 
or  more  vinylene  groups: 


158 


THEORIES  OF  ORGANIC  CHEMISTRY 


CH  CH  +  2HC1 

I!  II 

CH— CO— CH 

X    /\     /" 


CH 

II 

CH— CCls 


I 
CH 

II 
-CH 


J    \/    V 


+HC1 


Cl 


These  apparently  opposite  functions  of  the  vinylene  group  may  be 
readily  interpreted  in  terms  of  Vorlander's  theory  in  regard  to  the  dual 
nature  of  radicals.1 

lonogens   of   the    second    order   not    only   exercise    an    acidifying 

influence  upon  hydrogen  but  may  affect  its  reactivity  in  .other  ways. 

Thus,  for  example,  if  the  presence  of  an  ionogen  increases  the  tendency 

of  a  substance  to  undergo  intramolecular  rearrangement  in  the  sense 

CH2CO >    CH=COH 


it  is  obvious  that  the  reactivity  of  the  hydrogen  will  be  indirectly 
affected.  That  the  enol  modification  is  in  fact  more  reactive  than 
the  corresponding  keto  form  has  been  demonstrated  experimentally  by 
Kurt  H.  Meyer  2  who  has  shown  that  it  alone  is  capable  of  entering 
into  reactions  with  aldehydes,  nitrous  acid,  diazo  and  nitroso  com- 
pounds. The  groups  — CH2N02  and  — CH2CH=NH  show  a  similar 
tendency  to  rearrange  into  the  more  reactive  atomic  groupings 


_CH=Nf         and 
X)H 


_ CH=C-NH2. 


If  this  conception  is  correct  and  ionogens  actually  do,  in  some  cases, 
induce  intramolecular  rearrangements,  it  would  serve  to  explain  the 
somewhat  remarkable  relationships  observed  in  the  case  of  trimethy- 
lenetrisulphoxide  and  trimethylenetrisulphone.  It  may  be  recalled 
that  of  these  two  substances  the  latter  is  more  strongly  acid  than  the 
former  but  does  not  react  with  aldehydes,  diazo  compounds,  etc.  This 

1  A.   Kotz,    "  Betrachtungen   iiber  die   Reaktivitat  schwefelhal tiger  Atomgrup- 
pierungen."     Nernst-Festschrift,  bei  W.  Knapp  in  Halle,  p.  227. 

2  Annalen  der  Chemie,  379,  37  (1911);  380,  212  (1911) 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS  159 

is  explained  by   Hinsberg,1  who  supposes  that  rearrangements  in  the 
sense 

A 


-CH2—  S02  --  >  —  CH=Sf  — 


)H 

do  not  take  place  readily  and  that  quite  specific  conditions  are  neces- 
sary in  order  to  bring  them  about. 

It  has  been  observed  that  under  different  conditions  one  and  the 
same  group  may  function  in  either  of  two  diametrically  opposite 
ways.  The  reenforcing  or  activating  influence  of  the  phenyl  group 
has  already  been  referred  to,  but  that  phenyl  does  not  always  function 
in  this  way  is  seen  from  the  fact  that  the  presence  of  a  number  of 
phenyl  groups  in  union  with  ethylene  carbon  may  actually  retard  addi- 
tion to  the  ethylene  bond.  For  example,  tetraphenyl-ethylene  does 
not  react  with  bromine  despite  the  fact  that  it  contains  an  unsaturated 
double  linkage.  This  is  also  true  of  dinitrodiphenyl-ethylene.2 


NO2 

The  character  of  both  of  these  substances  is,  however,  such  that 
it  is  difficult  to  say  whether  this  effect  is  due  to  the  chemical  nature 
of  the  substituents  or  to  stereochemical  influences.3  In  order  to 
decide  this  question,  H.  Biltz  4  prepared  a  number  of  substances  in 
which  the  phenyl  groups  in  union  with  the  ethylene  carbon  atoms 
were  replaced  by  halogen  atoms.  His  results  were  not  conclusive, 
although  he  was  able  to  show  that  while  unsaturated  compounds 
heavily  loaded  with  halogen  will  still  add  halogen,  they  react  much 
less  readily  than  when  not  so  laden. 

Other  experiments  which  have  been  undertaken  with  the  same 
end  in  view  have  given  equally  equivocal  results.  For  example,  Emil 
Fischer  and  G.  Giebe  5  discovered  that  acetals  could  readily  be  pre- 
pared according  to  Fischer's  esterification  method  6  by  treating  alde- 
hydes with  1  per  cent  hydrochloric  acid  in  alcohol  solution. 

C6H5CHO+2HOC2H5    -» 


1  Jour,  prakt.  Chemie,  85,  351  (1912). 

2  ,1   Schmidt,  Ber.,  34,  619  (1901). 

8J.  Schmidt,  "  Ahren&  Sarnml.  Chem.  und  Chem.  Techn.  Vortrage,"  7,  vols.  9 
and  10  (1902) 

4  H.  Biltz,  Annalen  der  Chemie,  296,  231,  263  (1897). 

5  Ber.,  34,  619  (190U 

6  Fischer,  Ber  ,  30,  3053  (1897). 


160  THEORIES  OF  ORGANIC  CHEMISTRY 

In  applying  this  reaction  to  substituted  aromatic  aldehydes  they  found 
that  o-nitrobenzaldehyde  reacted  to  form  the  corresponding  acetal 
with  greater  ease  than  did  benzaldehyde  itself.1  This  was  also  true 
in  the  case  of  the  following  substituted  aldehydes: 

Cl 

CHO 
Cl    NO2 

In  other  words,  electro-negative  substituents,  although  in  the  ortho- 
position  and  of  high  molecular  weight,  accelerate  the  reaction.2  This 
is  exactly  the  opposite  of  what  might  be  expected,  at  least  in  the  case 
of  mononitrodichlorbenzaldehyde  on  the  basis  of  Victor  Meyer's  law 
of  esterification  as  applied  to  acids. 

Recent  experiments  by  Rupe  and  Labhardt  do,  however,  seem  to 
show  in  a  fairly  conclusive  manner  that  the  relatively  great  extension 
of  phenyl  groups  in  space  does  not  in  itself  affect  the  reactivity  of 
adjacent  carbonyl  linkages.  These  investigators  have  prepared  phenyl- 
oxytriazoles  by  treating  /3-acylphenylhydrazines  with  carbamyl  chloride : 

NH-CO-R  NH-CO-R 

+  C1CONH2  =   HC1  +  | 

C6H5  •  NH  C6H5  •  N  •  CO  •  NH2 

N=C-R 

=  H2O  + 

C6H5-N— C— OH 

In  applying  this  reaction  to  a  great  number  of  cases  they  discovered 
that  triazoles  were  formed  only  when  the  radical  R  was  not  unsatu- 
rated  or,  in  the  old  sense,  negative.  For  example,  no  triazole  could  be 
obtained  in  which  R  equals  — CeHs  although  the  corresponding 


C6H5N— C— OH 

compound  could  be  prepared. 

In  order  to  determine  whether  the  extension  of  the  phenyl  group 
in  space  interferes  with  the  formation  of  diphenyloxytriazole,  Rupe 
and  Metz  investigated  the  action  of  carbamyl  chloride  upon  /3-hexa- 

^er.,  31,  545  (1898). 

1  H.  Biltz,  Annalen  der  Chemie,  296,  231,  263  (1897). 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS         161 

hydrobenzoylphenylhydrazine.     In    this    case    the    reaction    proceeds 
smoothly  and  1-phenyl  3-hexahydrophenyl-5-oxytriazole 

N=CC6H11 


C6H5-N—  C—  OH 

is  formed.  This  result  would  seem  to  eliminate  any  explanation  of  the 
phenomenon  based  upon  steric  hindrance,  since  a  triazole  in  which 
R  equals  —  CeHn  can  be  prepared,  although  the  corresponding  com- 
pound in  which  R  equals  —  CeHs  cannot.  It  would  therefore  seem  to 
follow  that  the  reaction  is  actually  inhibited  by  the  unsaturated  charac- 
ter of  the  phenyl  group.  This  conclusion  is  sustained  by  the  results 
obtained  in  a  series  of  experiments  in  which  R  equals  —  CH^CH^CHs, 
_  CH=CH-CH3,  -CH2CH2C6H5,  -CH=CH  C6H5  and  where 
the  observed  decrease  in  the  reactivity  of  the  hydrazine  corresponded 
to  the  degree  of  unsaturation  of  the  radical. 

In  the  process  of  acetylating  amines  l  the  effect  of  electro-negative 
groups  is  directly  opposite  to  that  in  the  instances  just  mentioned. 
The  relations  are,  however,  more  complicated,  and  it  has  been 
observed  that  substituents  in  the  ortho  and  para  positions  behave 
differently. 

In  the  following  pages  a  number  of  illustrations  will  be  given  to 
show  the  way  in  which  some  of  the  more  important  so-called  negative 
groups  function  under  different  conditions  and  in  different  forms  of 
combination. 

A  very  exact  investigation  by  P.  Jacobson  and  his  students,2  in 
regard  to  the  influence  exercised  by  various  substituents  in  deter- 
mining the  course  of  intramolecular  rearrangements  among  the 
aromatic  hydrazines,  has  led  to  the  discovery  of  a  number  of  interesting 
facts.  If  hydrogen  is  present  in  both  para-positions  of  the  benzene 
ring  diamido-diphenyl  derivatives  (the  so-called  benzidine  bases) 
result,  as  is  represented  by  the  following  equation: 


>-NH-NH-<r  >H-»H2N-<:  V<  >-NH2 


If,  however,  either  one  of  these  positions  is  occupied  by  some  other 
substituent  than  hydrogen,  the  course  of  the  transformation  is  altered, 

iBer.,  27,  93  (1894). 

2Ber.,  25,  992;  26,  681,  688,  699;  27,  2700;  28,  2541;  29,  2680;  31,  890;  36, 
3841,  3857,  4069,  4093;  Annalen  der  Chemie,  287,  97  (1895);  303,  290  (1898). 


162  THEORIES  OF  ORGANIC  CHEMISTRY 

and  the  main  products  of  the  reaction  are  now  monoamido  deriva- 
tives of  diphenylamine  : 

R 


(o-Semidine) 

NH2 

(p-Semidine) 

In  this  case,  side  reactions  also  take  place  resulting  in  benzidine 
rearrangements  and  in  decompositions  into  monamines.  Indeed, 
in  the  reduction  of  azo-compounds  by  means  of  zinc  and  hydro- 
chloric acid,  all  of  these  processes  may  take  place  simultaneously  as, 
for  example,  in  the  case  of  chlorazobenzene.  In  general  it  may  be 
said  that  the  character  and  amount  of  the  reaction  product  in  any 
given  instance  depends  upon  the  chemical  nature  of  the  substituent 
and  also  upon  its  relative  position  in  the  molecule.  Jacobson  deter- 
mined the  influence  of  nine  substituents  in  the  p-position,  namely, 
Cl,  Br,  I,  —  OC2H5,  —  OCOCH3?  —  N(CH3)2,  —  NHCOCH3,  —  CH3 
and  —  COOH.1  His  results  show  that  substitution  of  hydrogen  in 
hydroxyl  or  amido  groups  in  the  para-position  to  the  azo  group 
by  alkyl  or  acyl  radicals  has  a  very  marked  effect  upon  the  course 
of  the  rearrangement.  If  hydrogen  is  replaced  by  an  alkyl  group 
as,  for  example,  in  p-ethoxyhydrazobenzene,  rearrangement  takes 
place  exclusively  in  the  sense  of  II  and  semidine  is  formed,  the 
presence  of  the  alkyl  group  being  unfavorable  to  diphenyl  rearrange- 
ment as  in  I.  The  reverse  happens  if  the  hydrogen  of  hydroxyl  is 
replaced  by  acetyl  as  in  p-acetoxyhydrazobenzene,  where  a  diphenyl 
base  forms  the  principal  product  of  rearrangement.  In  general  it  may 
be  said  that  alkyl  and  acyl  groups  produce  directly  opposite  effects  in 
influencing  rearrangements  of  this  type.  Such  rearrangements  are 
not  always  in  the  sense  which  has  just  been  described,  however.  Thus, 
while  in  the  case  of  derivatives  of  p-hydroxyazobenzene,  alkyl  groups 
favor  semidine  and  acyl  groups  favor  diphenyl  rearrangements,  the 
reverse  holds  in  the  case  of  derivatives  of  p-amidoazobenzene,  where 
the  presence  of  alkyl  groups  favor  diphenyl  and  acyl  groups  semidine 
rearrangements.  Free  amido-  and  hydroxyazo-compounds  do  not 
themselves  rearrange,  but  decompose  with  the  formation  of  two 
amines. 

The  effect  of  the  substituents  CH3,  Cl,  Br,  I  is  not  so  specific  nor 
so  definite  as  is  the  case  with  —  OC2H5,  —  OCOCH3,  —  N(CH3)2 
1  Annalen  der  Chemie,  303,  296  (1898). 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS         163 

and  — NHCOCH3.  The  presence  of  a  methyl  group  in  the  para- 
position  seems  to  favor  simultaneously  both  semidine  and  diphenyl 
rearrangement.  Furthermore,  it  has  been  demonstrated  with  certainty 
that  halogen  derivatives  of  hydrazobenzene  show  approximately  the 
same  inclination  to  semidine  as  to  diphenyl  rearrangement. 

It  frequently  happens  that  in  the  course  of  these  rearrangements 
the  substituent  group  R  becomes  detached  from  the  aromatic  nucleus. 
A  study  of  this  phenomenon  shows  that  the  substituents  which  are 
most  firmly  bound  to  the  benzene  ring  are  — OC2H5,  — CHs,  — N(CHs)2 
and  — NHCOCHa.  This  is  shown  by  the  fact  that  these  four  groups 
are  not  split  off  in  the  course  of  rearrangements.  This  stands  in 
marked  contrast  to  the  behavior  of  — OCOCH3,  —  COOH,  Cl  and 
Br, which  separate  with  more  or  less  ease.  Indeed  the  group — O  •  COCHs 
is  so  unstable  that,  in  the  reduction  of  acetoxyazobenzene  in  the 
presence  of  zinc  and  hydrochloric  acid,  the  formation  of  acetic  acid 
proceeds  more  rapidly  than  does  the  true  reduction  reaction.  It  is 
interesting  to  note  that,  although  free  hydroxy-azobenzene  might  be 
expected  to  rearrange  to  give  a  benzidine  base,  specially  conducted 
experiments  have  shown  that  no  trace  of  benzidine  is  formed,  while 
the  acetyl  derivative  under  identical  conditions  gives  considerable, 
quantities  of  this  compound.  The  conclusion  follows  that  it  must 
be  the  specific  influence  of  the  negative  acetyl  group  upon  the  oxygen 
of  the  hydroxyl  so  weakens  the  affinity  for  oxygen  for  carbon  of  the 
benzene  ring,  that  hydrolysis  can  be  brought  about  under  the  extremely 
mild  conditions  of  an  experiment  which  takes  place  at  a  temperature 
below  40°  C.1 

An  observation  of  F.  Kehrmann  2  referring  to  the  transformation 

C6H402     <±    C6H4(OH)2 

Quinone  Hydroquinone 

is  of  especial  interest  in  this  connection,  and  may  be  stated  as  follows: 
the  lower  the  homologue  and  the  more  negative  the  substituents 
in  the  molecule,  the  more  easily  quinones  take  up  hydrogen  and 
pass  into  hydroquinones,  while  the  reverse  reaction  involving  oxida- 
tion is  favored  by  high  molecular  weight  and  by  an  accumulation  of 
positive  groups.  This  explains  why  a  mixture  of  hydrothymoquinone 
and  benzoquinone  rearranges  to  give  thymoquinone  and  hydrobenzo- 
quinone,  and  why,  as  has  been  observed  by  C.  Graebe,  tetrachlorhy- 
droquinone  is  oxidized  at  the  expense  of  thymoquinone,  giving  chloranil 

^nnalen  der  Chemie,  303,  300  (1898). 
2Ber.,  31,  979  (1898). 


164  THEORIES  OF  ORGANIC  CHEMISTRY 

and  trichlorhydroquinone.  Hydroxyhydroquinone  and  aminohydro- 
quinone  are  oxidized  by  the  oxygen  of  the  air  with  such  energy  that 
it  is  difficult  to  prepare  them.  Yet  the  corresponding  acyl  deriva- 
tives are  usually  stable,  since  the  positive  character  of  the  — OH 
and  — NH2  groups  is  decreased  by  substitution  of  the  negative  acyl 
radical.  Thus,  for  example,  4-amino-l,  2-hydronaphthaquinone, 
one  of  the  most  unstable  substances  of  this  class,  gives  an  acetyl  deriva- 
tive which  is  stable  in  the  air.  Moreover,  while  pyrocatechol  is 
stable  in  the  air,  j3-hydronaphthoquinone  oxidizes  slowly  and  hydro- 
phenanthroquinone  oxidizes  rapidly  to  the  corresponding  quinones 
upon  standing  in  the  air. 

The  researches  of  F.  Henrich  1  throw  further  light  upon  this  gen- 
eral problem.  Henrich  found  that  the  hydrazone  of  benzazogluta- 
conic  ester,  which  is  formed  from  diazobenzene  and  glutaconic  ester, 
splits  off  alcohol  more  or  less  readily  and  passes  into  a  1-2-diazine 
derivative  (pyridazone) : 


C2H5OCO-C=N— N 

CH 

II 


H 


C6H5  C2H5OCO— C=N 

=  C2H5OH  +  CHN-C6H5 

I 


Aryl— N=N— C— CO  |OC2H5|  Aryl— N=NC— CO 

I  II 

The  relative  ease  with  which  this  type  of  reaction  takes  place  was 
observed  to  depend  upon  both  the  chemical  nature  of  the  substituent 
and  upon  its  relative  position  in  the  benzene  ring.  Thus  negative 
groups  such  as  Cl,  Br,  N02,  etc.,  in  the  meta-  and  para-positions 
accelerate  the  reaction. 

After  Kurt  H.  Meyer  and  Lenhardt 2  had  made  the  surprising  dis- 
covery that  phenol  ethers  possess  the  same  ability  as  phenols  to  couple 
with  diazo  compounds  and  thus  to  form  the  corresponding  oxyazo- 
ethers,  they  proceeded  to  investigate  the  influence  of  substituents 
upon  this  type  of  reaction.3  Results  show  that  mono-  and  di-nitro- 
diazobenzenes  couple  much  more  energetically  than  azobenzene  itself 
and  that  this  is  also  true  of  the  diazo  derivatives  of  the  halogenated 
anilines.  In  brief,  "  the  more  negative  the  substituent  in  the  diazo 
compound  the  more  readily  do  these  compounds  react  with  the  phenol 
ethers."  On  the  other  hand,  negative  substituents  in  the  phenol  ethers 
produce  the  opposite  effect  and  tend  to  retard  the  reaction.  For 

1  Annalen  der  Chemie,  376,  131  (1910). 

2  Annalen  der  Chemie,  398,  74  (1913). 

3Ber.,  47,  1741  (1914);  also  Auwers,  Ber.,  47,  1275  (1914). 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS         165 

example,  p-nitroanisol,  and  also  nitro,  sulphonic  acid,  and  carboxyl 
derivatives  of  naphthol  ethers  either  fail  to  couple  or  else  react  much 
more  slowly  than  the  corresponding  unsubstituted  ethers. 

The  sodium  salt  of  diethylmalonate,  when  treated  with  succinyl 
chloride,  interacts  according  to  Schreiber  to  give  a  derivative  of  cyclo- 
pentane  of  the  formula: 

CH2-CO      COOC2H5 


H2-CO      COOC2H5 
I 

In  other  words,  both  acyl  groups  of  the  succinic  acid  link  themselves 
to  the  methylene  group  in  diethylmalonate.  When,  however,  this 
investigator  l  modified  the  reaction  by  substituting  the  sodium  salt 
of  ethyl  acetoacetate  for  the  sodium  salt  of  diethylmalonate,  he 
obtained  an  acyclic  or  open  chain  compound,  namely  ethyl  succinyl 
acetoacetate, 

/CO  OC2H5 
CH2.CO-CH 

CH2-COOH 
II 

and  all  attempts  to  transform  the  latter  compound  into  the  corre- 
sponding pentane  derivative, 

CH2-CO      COCH3 


CH2-CO      COOC2H5 
III 

by  the  elimination  of  water  were  unsuccessful.  The  stability  of  the 
compound  I  as  compared  with  II  and  III  affords  another  illustration 
of  the  disintegrating  influence  of  the  more  negative  acetyl  group  when 
substituted  for  the  less  negative  carbethoxy  group. 

Brojendra  Nath  Ghosh  2  has  recently  published  a  very  interesting 
article  in  which  he  points  out  the  important  influence  which  constitu- 
tion has  upon  the  basicity  of  oxygen.  According  to  the  observations 
which  he  has  made,  it  would  seem  as  if  compounds  which  contain  one 
or  more  oxygen  atoms  in  a  ring  are  very  much  influenced  in  their 

iBer.,  44,  2423  (1911). 

2  Jour.  Chem.  Soc,  107,  1588  (1915). 


166  THEORIES  OF  ORGANIC  CHEMISTRY 

ability  to  form  salts  with  acids  by  the  unsaturated  or  negative  char- 
acter of  the  substituent  groups. 

Many  other  illustrations  might  be  mentioned.1  In  fact  it  may  be 
said  that  a  majority  of  the  reactions  in  organic  chemistry  show  either 
the  restraining  or  accelerating  influences  of  negative  as  compared 
with  positive  radicals.  The  subject  will  be  considered  again  later 
from  a  somewhat  different  angle  in  the  chapter  on  Tautomerism  and 
Desmotropism.  Every  important  reaction  should  be  systematically 
studied  from  this  point  of  view. 

The  term  "  negative  radical  "  has  undoubtedly  merely  an  historical 
significance  as  applied  to  the  various  groups  which  have  just  been 
considered.  In  other  words,  this  term  no  longer  exactly  expresses 
the  condition  which  it  attempts  to  describe.  Because  of  this  fact 
Vorlander  has  suggested  the  substitution  of  the  term  "  reactive  group  " 
to  express  the  condition  represented  by  the  unsaturated  elements  3-4 
in  the  atomic  complex.2 

HEE=E 

123         4 

Vorlander  has  also  urged  the  use  of  the  term  "  reactive  influence  " 
("  reaktive  Wirkung")3  to  describe  action  of  an  accelerating  or 
disintegrating  nature.  Jacobson  and  Stelzner,4  on  the  other  hand, 
think  that  the  term  reactive  group  may  be  more  suitably  aplied  to  that 
part  of  a  molecule  (methylene  group,  for  example)  whose  chemical 
reactivity  is  actually  increased  by  the  introduction  of  a  given  sub- 
stituent. They  further  suggest  that  the  terms  reenforcing  or  activating 
group  ("  reaktivierende  Gruppe  ")  and  reenforcing  or  activating  influ- 
ence ("  reaktivierende  Wirkung"  )  be  used  instead  of  the  corresponding 
expressions,  negative  group  and  negative  influence. 

Since,  however,  unsaturated  atoms  or  groups  of  atoms  exercise  a 
reenforcing  influence  only  when  they  occupy  certain  definite  positions 
in  the  molecule,  as  typified,  for  example,  in  the  systems, 

H  E  E=        and        HE  E=E 

123  1234 

it  may  be  assumed  that  this  whole  complex  is  actually  responsible 
for  the  increased  chemical  reactivity  of  the  given  part  and  that,  there- 

1  Wieland,  Habilitationsschrift,  p.  9,  Munich,  V,  Hoflung,  1904. 
2Ber.,  34,  1633  (1901). 

3  Annalen  der  Chemie,  320,  112  (1902). 

4  V.   Meyer  and  Jacobson's   "  Lehrbuch  der  Organischen  Chemie,"   2nd  Ed., 
vol.  1,  419. 


NEGATIVE  NATURE  OF  ATOMIC  GROUPS  167 

fore,  the  term  reactive  group  applies  to  the  system  as  a  whole,  the  hydro- 
gen atom  itself  being  simply  "  replaceable." 

All  of  the  rules  which  have  been  mentioned  thus  far  as  governing 
the  action  of  unsaturated  groups,  are  purely  empirical  in  character. 
They  lead  directly  to  the  assumption  that  in  9,11  cases  the  reactivity 
of  the  hydrogen  atom  is  due  to  the  presence  of  a  given  atom  or  radical 
in  the  molecule.  The  question  as  to  the  mechanism  of  the  change 
involves  a  consideration  of  the  phenomena  of  tautomerism,  desmo- 
tropism  and  molecular  rearrangements,  so  that  its  detailed  discussion 
must  be  omitted  for  the  present. 

In  conclusion  it  may  be  said  that  so  equivocal  are  the  terms 
positive  and  negative  that  Vorlander  recommends  that  they  should 
never  be  used  except  in  cases  where  they  are  employed  to  describe 
the  nature  of  the  chemical  elements  themselves.  Even  when  applied 
to  individual  chemical  atoms,  these  terms  must  be  used  with  discretion 
since  many  elements  may  appear  to  be  positive  under  certain  condi- 
tions and  negative  under  others.  Ordinarily  the  positive  or  negative 
character  of  a  given  element  can  be  determined  approximately  from 
its  position  in  the  periodic  system,  from  the  basic  or  acidic  proper- 
ties of  its  respective  oxygen  and  hydrogen  derivatives,  and  from  its 
electrochemical  behavior.  In  certain  instances  where  an  element 
appears  to  be  positive  at  one  time  and  negative  at  another,  it  has 
been  observed  that  the  variation  in  these  properties  seems  to  depend 
upon  its  degree  of  saturation.  For  example,  the  exercise  of  its  higher 
valencies  often  tends  to  make  a  metal  appear  more  negative,  and  a 
non-metal  more  positive.  Under  these  conditions  there  is  obviously 
no  sharp  line  of  demarcation  between  these  two  groups  of  elements. 

Vorlander  l  assumes  that  potential  differences  actually  exist 
between  the  elements  present  in  a  chemical  compound  and  that  such 
differences  are  relatively  small  in  the  case  of  stable  forms  of  combination 
and  relatively  great  in  the  case  of  unstable  linkages.2  For  purposes 
of  convenience  the  existence  of  a  condition  of  tension  within  a  given 
molecule  may  be  roughly  represented  by  means  of  plus  and  minus 
signs.  These  considerations  are  of  increasing  importance  in  inter- 
preting reactions  in  the  field  of  Organic  Chemistry  and  will,  therefore, 
be  developed  in  some  detail  in  the  succeeding  chapter. 

1  Ber.,  52,  263  (1919).  2  Ber.,  37,  1646  (1904). 


CHAPTER  IX 

RECENT  THEORIES  IN  REGARD  TO   THE   MECHANISM    OF 
CHEMICAL  REACTIONS 

THE  direct  union  of  molecules  was  recognized  very  early  in  the 
development  of  chemical  theory.  To  this  class  of  reactions  belong 
not  only  additions  of  Ck,  HC1,  etc.,  to  molecules  containing  doubly 
bound  carbon  atoms,  but  also  substitutions  of  hydrogen  by  halogen 
and  other  acid  radicals.  Kekule,  in  his  "  Textbook  of  Organic  Chem- 
istry," conceives  the  matter  in  the  following  way:  "  in  any  chemical 
action  the  two  reacting  molecules  are  first  drawn  together  by  the  mutual 
exercise  of  chemical  affinity  and  then  ultimately  become  attached 
to  each  other.  In  this  closer  association,  individual  atomic  attractions 
make  themselves  felt,  with  the  result  that  atoms  which  had  previ- 
ously been  present  in  two  different  molecules  come  into  very  close 
proximity.  Such  rearrangements  may  finally  lead  to  the  disruption 
of  the  whole  complex  and  the  formation  of  entirely  new  molecules." 


1~ 

bi 


II  III 


Kekule's  conception  of  the  mechanism  of  chemical  combinations 
is  attracting  renewed  attention  at  the  present  time,  because  it  is 
needed  to  explain  the  formation  of  intermediate  products.  In  the 
past  only  a  few  addition  products  of  this  type  were  known,  largely 
because  they  are  usually  very  unstable  and  are  therefore  difficult 
to  isolate.  Since,  however,  the  existence  of  such  substances  has  been 
postulated  with  increasing  frequency  in  the  course  of  the  development 
of  modern  theory,  methods  and  reagents  l  have  been  devised  by  means 
of  which  it  is  now  possible  to  recognize  and  also  frequently  to  isolate 
these  products.  In  addition  to  the  well-known  optical  methods,  methods 

1  C(NO2)4,  SnCL,  SnBr4,  etc.,  have  been  used  successfully.  See  Annalen  der 
Chemie,  376,  286  (191Q). 

168 


THE  MECHANISM  OF  CHEMICAL  REACTIONS  169 

of  thermo  analysis  have  been  perfected  by  R.  Kremann,  Ph. 
Guye,  von  Holleman,  Schmidtlin  and  others,  so  that  they  can  now 
be  applied  successfully  in  the  field  of  organic  chemistry.  As  a  result 
H.  Wieland,  P.  Pfeiffer,  G.  Reddelien  and  others  have  been  able  to 
identify  a  number  of  intermediate  products  and  to  study  them  much 
more  closely  than  had  previously  been  possible. 

For  a  long  time  the  constitution  of  these  compounds  remained 
an  open  question.  It  was  debated  as  to  whether  they  should  be  con- 
ceived in  terms  of  Thiele's  theory,  in  which  case  disruption  of  the 
bromine  molecule,  for  example,  must  be  supposed  to  have  taken  place, 

Br  Br 

i      I 

:  C=C  : 

or  in  terms  of  simple  molecular  combinations,  in  which  case  the 
exercise  of  free  affinity  may  be  depicted  in  any  of  the  following  ways: 

Br2 

I      |  v  /H          /OH 

>C=C    ;  >C=O......HX;        C=0<      ;      C< 

/  NX       xx 

The  solution  of  this  problem  has  finally  been  arrived  at,  and  a 
satisfactory  picture  of  the  mechanism  of  addition  reactions  has  been 
evolved  through  the  efforts  of  P.  Pfeiffer 1  and  others  co-operating 
with  him.  As  a  result  of  a  very  thorough  and  systematic  investiga- 
tion it  has  been  demonstrated  that  substances  which  contain  carbonyl — 
such  as  aldehydes,  ketones,  acids,  esters,  amides,  etc. — react  with 
metallic  chloride  (MeX4)  and  with  acids  (HX)  to  give  addition 
products  which  closely  resemble  each  other  in  composition  and  in  the 
ease  with  which  they  decompose.  The  general  behavior  of  these 
substances  is  most  readily  explained  on  the  assumption  of  a  similarity 
in  constitution,  and  it  is  supposed  that  the  addend  (MeX4  and  HX) 
is  first  joined  to  the  oxygen  of  the  carbonyl  group  by  means  of  a  small 
residual  valence,  viz., 

T>  TD 

\C=O......MeX4        and  \C=O......HX 

R'/  R'/ 

In  the  course  of  time  a  greater  fraction  of  the  total  affinity  of  the 
oxygen  may  be  exercised  in  this  union,  the  actual  amount  varying  in 
different  instances  and  depending,  in  general,  upon  the  nature  of 
the  addendum.  In  every  case  it  follows,  however,  that  a  proportionally 
great  fraction  of  the  total  affinity  of  carbon  will  be  set  free,  and  this 
1  Annalen  der  Chemie,  376,  285  (1910);  383,  92  (1911);  404,  13  (1914). 


170  THEORIES  OF  ORGANIC  CHEMISTRY 

is  represented  graphically  by  Pfeiffer  by  means  of  an  arrow  directed 
downward : 


C=O MeX4        and          >C=O HX 

Rf/  | 

Such  an  increase  in  the  free  energy  of  the  carbon  atom  makes  itself 
manifest  in  the  form  of  increased  chemical  reactivity. 

By  means  of  this  conception  it  is  possible  to  understand  the 
catalytic  action  which  is  so  frequently  attributed  to  metallic  salts  and 
acids,  and  to  explain  many  other  phenomena  in  the  most  diverse 
fields.  It  is  frequently  referred  to  as  "  Pfeiffer's  Theory  of  Halo- 
chromism "  because  it  was  originally  advanced  to  explain  the  phenom- 
enon of  halochromism,  and  will  be  referred  to  later,  not  only  in  this 
connection  but  also  in  connection  with  a  discussion  of  "  Walden's 
rearrangement." 

In  pursuing  the  present  development,  G.  Reddelien's  l  investiga- 
tions must  now  be  considered.  These  remarkable  discoveries  are 
based  upon  Pfeiffer's  theory  and  are  concerned  with  the  addition 
products  which  are  formed  by  the  action  of  nitric  acid  upon  carbonyl 
groups  in  organic  combinations.  Many  of  these  compounds  have  been 
isolated,  but,  of  the  innumerable  substances  which  Reddelien  describes, 
only  a  few  need  be  referred  to  for  purposes  of  discussion,  viz., 
C6H5CHO-HN03,  C6H5COCH3-HN03,  CeHsCOCeHs-HNC^,  and 
CioHie-HNOs  (camphor  nitrate).  It  seems  probable  that  the  nitric 
acid  reacts  in  such  cases  in  the  sense  indicated  by  Pfeiffer's  formulas. 

The  application  of  this  reaction  has  been  extended  by  Reddelien, 
who  has  discovered  that  substances  containing  such  groups  as 
— C=N — ,  and  N==N  are  also  capable  of  forming  well-defined  but 
not  very  stable  addition  products  with  HNOs.  Substances  with  the 
grouping  C=C,  on  the  other  hand,  fail  to  give  products  which  can  be 
isolated,  although  there  are  numerous  indications  of  addition  reactions 
even  in  such  cases.  In  later  investigations,  and  after  he  had  observed 
that  the  oxidizing  action  of  HNOs  and  other  of  its  properties  seem  to 
interfere  with  the  purification  of  its  addition  products,  Reddelien 
substituted  picric  acid  for  it.  This  substance  is  also  a  very  strong 
acid  and  gives  addition  products  with  carbonyl,  carbimine-  and  azo- 
compounds,  viz., 

C6H5CHO  •  HOC6H2(N02)3 ;  C6H5COCH3  •  HOC6H2  (NO2)3,  etc. ; 
(C6H5)2C=N-  Cells  -HO  C6H2(N02)3,  (benzophenoneanil  picrate)  and 
C6H5N=NC6H5-HOC6H2(N02)3,  (azobenzene  picrate).2  These  com- 

1  Jour,  prakt.  Chemie,  91,  213  (1915). 

2  Jour,  prakt.  Chemie,  91,  214  (1915). 


THE  MECHANISM  OF  CHEMICAL  REACTIONS  171 

pounds  are  assumed  to  possess  constitutional  formulas  which  are 
analogous  to  those  which  have  already  been  assigned  to  the  corre- 
sponding nitrates. 

Minor  differences  in  properties  have  been  observed  in  the  case  of 
these  two  classes  of  addition  products.  Thus  the  nitrates  are  very 
unstable  and,  even  in  the  presence  of  water,  are  broken  down  either 
wholly  or  partially  into  their  components: 

C6H5CHO  •  HNO3  +  H20    -»     C6H5CHO  +  HNO3  •  H2O 

In  the  preparation  of  these  substances  it  is,  therefore,  necessary  to 
maintain  a  constant  concentration  of  the  nitric  acid,  since  a  too 
strong  acid  tends  to  nitrate  the  original  material  and  a  too  weak  acid 
tends  to  decompose  the  product.  The  picrates  behave  analogously, 
but  in  their  case  solution  tension  (i.e.,  osmotic  pressure)  is  greater 
than  the  affinity  of  the  components.  If,  for  example,  a  given  picrate 
is  dissolved  in  a  solvent  in  which  one  component  is  very  soluble  and  the 
other  only  slightly  soluble,  decomposition  will  take  place  and  will  be 
partial  or  complete  according  to  the  conditions  of  equilibrium  in  the 
system.1  All  such  compounds  show  increased  chemical  reactivity, 
which  may  be  accounted  for  readily  in  terms  of  Pfeiffer's  theory  of 
halochromism,  viz., 

T>  "D 

\C=O HN03 ;  Nc=N HN03 ; 

R//      I  T?'/      I 

•I  .TV        4, 

R 

RN=N HN03;  R2C=C HOC6H2(N02)3,  etc. 

I       I  I 

R,  R2 

Reddelien  made  the  further  discovery  that  substances  which 
contain  conjugate  systems  of  double  bonds  give  addition  products 
which  are  markedly  less  stable  than  those  described  in  the  preceding 
cases.  For  example,  while  benzophenone  nitrate  (C6H5)2CO  •  HN03, 
and  fluorenone  nitrate  (C6H4)2CO-HNO3  are  fairly  stable  and  can  be 
kept  from  one-half  to  two  days,  benzil  nitrate  (C6H5CO)2HNO3,  on 
the  other  hand,  decomposes  almost  immediately  and  phenanthra- 
quinone  nitrate  (C6H4CO)2-HNO3  breaks  down  in  less  than  half 
an  hour.  The  same  general  relations  have  been  observed  to  hold 

1  Jour,  prakt.  Chemie,  91,  216  (1915);  also  Behrend,  Jour.  Physikal.  Chemie,  10, 
278  (1892), 


172  THEORIES  OF  ORGANIC  CHEMISTRY 

in    the    case    of    the    addition    products    of    benzophenone-anil  and 
benzil-dianil, 

N -Cells  C6H5N  N-C6H5 

and  ||  || 

— C 


j. 


An  important  exception  to  this  general  rule  has  been  observed 
in  connection  with  substances  in  which  ethylene  forms  the  second 
group  in  a  conjugate  system  of  double  bonds,  as  for  example: 

0=C-C=C;    N=C-C=C;    C=C-C=C 

In  such  cases  the  stability  of  the  addition  product  is  greatly  increased, 
as  is  shown  by  the  fact  that  the  nitrate  and  picrate  of  cinnamic  alde- 
hyde can  be  kept  for  weeks,  while  the  corresponding  products  with 
benzaldehyde  decompose  after  a  few  hours.  Benzalacetone,  benzal- 
acetophenone  and  dibenzalacetone  show  the  same  general  relation 
to  acetophenone  and  benzophenone.  By  means  of  these  reactions, 
Reddelien  was  even  able  to  distinguish  numerically  between  the 
relative  affinity  of  such  groups  as  O=C  •  C=0  ;  O=C  ;  and  O=C  •  C=C. 
This  was  accomplished  by  decomposing  the  corresponding  nitrates 
with  dilute  nitric  acid  and  measuring  the  amount  of  hydrolysis.  Results 
show  this  to  be  70,  36,  and  6  per  cent  in  the  case  of 
CeHsCO-COCeHs-HNOs,  CeHsCOCeHs-HNOs,  and  C6H5CH= 
CHCO  C6H5-HNO3  respectively. 

The  constitution  of  addition  products  formed  by  the  action  of 
nitric  acid  upon  substances  which  contain  conjugate  systems  may  be 
represented  as  follows  : 

C6H5  •  CH=CH—  CH=0 

HN03 

The  above  figure  shows  free  affinity  on  the  carbon  atom  in  the 
4-position  with  respect  to  oxygen  of  the  carbonyl  and  suggests  a  condi- 
tion which  is  similar  to  one  which  has  been  referred  to  earlier  in  this  text. 
It  may  be  recalled  that  Staudinger  and  Kon  have  used  an  analogous 
conception  in  interpreting  addition  reactions  in  the  case  of  ketenes.1 

Quantitative  differences  in  the  relative  reactivity  of  such  unsatu- 
rated  systems  as 


R—  •C"   ~*—  CH2-CH=CH2  and 


^S 


1  Annalen  der  Chemie,  306,  102;  also  "  Die  Ketene,  "  p.  108,  Stuttgart. 


THE  MECHANISM  OF  CHEMICAL  REACTIONS  173 

have  been  observed  by  both  Bruni  and  Tornani  *  and  by  Thiele  and 
Henle,2  who  point  out  that,  of  the  two  systems,  only  the  latter  gives  addi- 
tion products  with  picric  acid.  From  this  it  follows  that  the  free  affinity 
present  in  a  given  compound  must  reach  a  certain  value  before  com- 
bination with  picric  acid  can  take  place.  This  minimum  has  appar- 
ently been  reached  in  the  case  of  the  latter  but  not  in  the  case  of  the 
former  substance.  Reddelien  applies  this  general  conception  to  explain 
the  deeply  colored  picrates  of  benzene,  naphthalene  and  anthracene,  viz., 


The  nitrates  differentiate  themselves  from  the  corresponding 
picrates  in  the  tendency  which  they  show  to  undergo  chemical  rearrange- 
ments. Such  transformations  result  in  the  formation  of  nitro  com- 
pounds and  may  be  supposed  to  take  place  according  to  the  following 
scheme  : 

R-CH-+  R-CHOH  R-CH 

II  I  =  H20  +        || 

R-CH  ......  HN03  R-CHN02  R-C-N02 

Such  a  process  may  be  regarded  as  analogous  to  the  addition  of 
bromine  to  unsaturated  ethylene  compounds  as  formulated  by  von 
Reich:4 

R-CH  R-CH->  R-CHBr      R-CH 

II      +Br2=        ||  1=1      +HBr 

R-CH  R-CH......Br2        R-CHBr     R'-CBr 

Molecular  compounds  of  the  above  type  are  frequently  met 
with  and  are  characterized  not  only  by  the  fact  that  they  are  relatively 
unstable  but  also  by  their  tendency  to  undergo  intramolecular  rearrange- 
ment and  so  to  pass  into  the  corresponding  substitution  products.5 
Thus,  according  to  Reddelien,  dibromcamphor, 

Br2 

C.....Br2  C=0-» 

or    C8H14<   | 
XCH2 

1  Atti.  R.  Acad.  Lincii,  5,  13,  II,  184  (1904). 

2  Annalen  der  Chemie,  347,  295  (1906). 

3  Compare  this  formula  with  that  of  Pfeiffer,  Annalen  der  Chemie,  404,  13  (1914). 

4  Jour,  prakt.  Chemie,  90,  177  (1914). 

6  Compare  H.  Wieland,  Ber.,  40,  4260  (1907);  43,  699  (1910). 


174  THEORIES  OF  ORGANIC  CHEMISTRY 

loses  its  bromine  on  standing  in  the  air  and  passes  back  into  free 
camphor,  but  if  heated  rapidly  in  a  closed  tube  it  loses  HBr  and  is 
transformed  into  monobromcamphor.  All  such  molecular  compounds 
may  therefore  be  regarded  as  representing  the  primary  products  which 
usually  form  in  substitution  processes  and  as  the  actual  forerunners 
of  the  final  substitution  products.  They  may  be  formulated  in  the 
most  general  terms  by  means  of  the  expression 


A  ......  M 

where  A  represents  an  atom  and  M,  a  molecule. 

In  general  it  may  be  said  that  if  the  various  types  of  doubly-bound 
atoms, 

O=N,         0=O,         N=N,        C=C. 
I  II  III  IV 

are  compared,  it  appears  that  addition  products  form  most  readily 
in  the  case  of  substances  which  contain  I  and  least  readily  in  the  case 
of  substances  which  contain  IV.  This  order  is  reversed,  however, 
if  the  reactivity  of  the  resulting  compound  is  considered.  It  has  been 
observed  for  example,  that  addition  products  which  possess  unsatu- 
rated  ethylene  linkages  (IV)  show  the  greatest  tendency  to  undergo 
intramolecular  rearrangement  and  to  pass  over  into  the  corresponding 
substitution  products. 

The  role  which  such  molecular  compounds  play  in  the  processes 
of  substitution  and  orientation  will  be  considered  more  fully  in  the 
next  chapter. 


CHAPTER  X 
THE   QUESTION  AS  TO   THE   CONSTITUTION    OF  BENZENE 

THE  well-known  method  for  determining  crystalline  structure  which 
was  discovered  by  M.  von  Laue  and  which  depends  upon  the  measure- 
ment of  the  interference  caused  during  the  passage  of  Rontgen  rays 
through  crystals  of  various  kinds,  has  been  applied  recently  by  P.  Debye 
and  P.  Scherrer  to  a  study  of  the  behavior  of  liquid  benzene.  From 
the  results  which  have  been  obtained  in  this  way  they  have  been  able 
to  construct  a  Rontgen  diagram  and  from  this  to  calculate  that  the 
space  which  is  occupied  by  the  carbon  atoms  of  the  ring  (without  regard 
to  that  required  for  valence-electrons)  is  bounded  by  a  circle  whose 
diameter  equals  12.4  XlO~8  and  has  a  thickness  of  at  most  1.19X10"8 
cm.  On  the  basis  of  these  figures  it  would  seem  reasonable  to  con- 
clude that  the  six  carbon  atoms  of  the  benzene  nucleus  probably  all 
lie  in  a  common  plane. 

This  discovery  is  in  general  agreement  with  Kekule's  theory  in  regard 
to  the  constitution  of  benzene  according  to  which  "  the  six  carbon 
atoms  are  bound  together  in  a  perfectly  symmetrical  ring  structure." 
This  hypothesis  was  advanced  by  its  author  in  an  effort  to  give  the 
most  comprehensive  formulation  to  the  chemical  behavior  of  benzene, 
but  although  it  has  been  very  generally  accepted  by  chemists  it  has 
never  been  wholly  free  from  certain  objections.  Even  in  very  early 
times  the  non-existence  of  isomeric  ortf/io-disubstitution  products 
made  it  necessary  for  Kekule  to  add  to  his  original  conception  the 
assumptions  which  are  embodied  in  his  "Oscillation  Hypothesis"; 
but,  as  H.  Pauly l  has  again  recently  pointed  out,  this  explanation 
was  never  very  satisfactory  and  has  long  since  been  discredited.  Even 
Thiele's  partial  valency  formula  in  its  usual  form  fails  to  account  for 
the  notable  discrepancy  between  fact  and  theory  which  has  arisen 
through  the  failure  in  any  single  instance  to  isolate  isomeric  ortho 
derivatives  of  benzene. 

In  1911  the  problem  involved  in  the  formulation  of  a  satisfactory 
expression  for  the  constitution  of  benzene  entered  a  new  phase  as  the 

1  Jour,  prakt.  Chemie.,  98,  110  (1918). 
175 


176  THEORIES  OF  ORGANIC  CHEMISTRY 

result  of  the  discovery  of  cyclo-octatetraene  by  R.  Willstatter  and  E. 
Waser.1     A  consideration  of  the  formula  of  this  substance 

CH—  CH 


HC  CH 

HC  CH 

/ 

c 


H       H 

shows  that  it  differs  from  the  Kekule  formula  for  benzene  only  in 
being  composed  of  4  instead  of  3  conjugate  systems  of  double  bonds. 
This  similarity  in  structure  should  correspond  to  a  similarity  in  chemical 
behavior  between  the  two  substances,  but  as  a  matter  of  fact  cyclo- 
octatetraene  has  all  the  properties  of  a  decidedly  unsaturated  compound 
and  in  this  respect  stands  in  marked  contrast  to  benzene.  To  quote 
Willstatter:2  "  cyclo-octatetraene  is  absolutely  different  from  benzene 
in  its  chemical  properties.  That  it  must  be  regarded  as  a  true  cyclo- 
olefine  follows  from  the  facts:  (1)  that  it  readily  adds  4  molecules  of 
hydrogen  in  the  presence  of  platinum  as  a  catalyst  while  benzene  is 
entirely  unreactive  under  the  same  conditions;  (2)  that  it  instantly 
reduces  potassium  permanganate  and  adds  bromine;  (3)  that  it  does 
not  readily  form  substitution  products  and  does  not  react  with  nitric 
acid  to  give  nitro-derivatives,  and  (4)  that  it  is  unstable  and  tends  to 
rearrange  into  more  stable  isomers." 

Such  marked  differences  could  not  be  accounted  for  on  the  basis 
of  the  usual  formulas  assigned  to  the  substances.  It  followed,  there- 
fore, that  in  the  discussions  incident  to  these  discoveries,  Glaus'  formula 
for  benzene  and  the  centric  formula  of  Baeyer  and  Armstrong  again 
came  to  the  foreground.  Both  formulas  are  based  upon  von  Baeyer's 
conception  that  the  saturation  of  the  six  carbon  atoms  of  the  benzene 
ring  is  of  a  special  kind  and  peculiar  to  this  particular  substance. 
Claus  assumes  a  diagonal  saturation  of  the  six  valencies,  but  this 
conception  cannot  be  applied  satisfactorily  without  recourse  to  special 
hypotheses.  As  a  basis  for  their  theoretical  speculations  Willstatter 
and  Waser3  used  a  modification  of  the  Baeyer-Armstrong  centric 
formula  in  which  the  distance  of  the  carbon  atoms  from  the  center  of 
the  ring  is  emphasized  by  means  of  the  following  diagram  in  which 

iBer.,  44,  3424(1911). 
2Ber.,  44,  3428  (1911). 
3Ber.,  44,  3430(1911). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE          177 

the  six  valencies  of  carbon  are  represented  as  directed  toward  a  middle 
point : 


The  molecular  refractions  of  benzene,  cyclo-pentadiene,  and  cyclo- 
octatetraene  show  no  important  differences  and  therefore  afford  no 
data  leading  to  a  solution  of  the  problem  presented  by  the  atomic 
relationships  of  these  substances. 

In  extending  their  conception  of  the  benzene  ring  to  naphthalene 
Willstatter  and  Waser  have  discarded  Bamberger's  centric  formula: 


and  have  substituted  in  its  place; 


The  latter  was  originally  advanced  by  Harries  1  to  explain  the  action 
of  ozone  upon  naphthalene,  and  assumes  the  presence  of  only  one 
centric  ring,  the  second  ring  being  represented  as  olefine  in  char- 
acter. This  conception  affords  an  analogy  to  many  bicyclic  con- 
densation products  of  the  ortho  diamines  and  catechols.  It  also 
explains  the  reactivity  of  carbon  in  the  a-positions.  The  presence  of 
two  different  ring  structures  in  the  molecule  occasions  a  lack  of 
symmetry,  and  isomerism,  due  to  substitution  in  one  or  the  other  ring, 
becomes  theoretically  possible.  While  as  yet  no  instances  of  isom- 
erism of  this  type  have  been  observed,  the  question  must  be  raised 
in  the  case  of  each  given  substitution  product  of  naphthalene  as  to 
which  of  the  two  possible  configurations  it  represents.  Moreover,  since 
a-amino-  and  hydroxy-naphthalenes  behave  differently  from  the 
corresponding  ^-derivatives  on  reduction,  the  former  have  come  to 
be  commonly  regarded  as  aromatic,  and  the  latter  as  alicyclic  in 
character.  Thus  for  example: 

1  Annalen  der  Chemie,  343,  311,  336  (1905). 


178 


THEORIES  OF  ORGANIC  CHEMISTRY 


NH2(OH) 


H   H 


H2(OH) 


HH 


NH2(OH) 


H  H 


NH2(OHJ 
H 


HH 


That  the  solution  offered  to  the  problem  of  the  constitution  of  ben- 
zene by  the  centric  formula  is  not  wholly  satisfactory  may  be  seen 
from  the  fact  that  it  fails  to  account  for  many  of  the  observed  prop- 
erties of  the  substance.  For  example  H.  Pauly  :  has  recently  pointed 
out  that:  (1)  benzene  passes  over  into  muconic  acid  during  the  process 
of  organic  oxidation  in  the  organism  of  a  dog  or  other  canines  2 


H 
C 

/\ 

HC   CH 

I   II 
HC   CH 


v 


CH 

/\ 

HC   COOH 


HC   COOH 

v 

CH 


H 


and:  (2)  that  nitrated  p-cresol  is  transformed  into  j8-methyl-muconic 
acid  when  oxidized  at  a  temperature  of  about  100°  under  the  influ- 
ence of  sulphuric  acid :  3 


H 
C 


CH 


H3C-C     CNO2 

I       II 
HC     COH 

v 

CH 


H3C-C      COOH 


HC      COOH 

v 

CH 


Neither  of  these  reactions  can  be  accounted  for  on  the  basis  of  the 
centric  formula  for  benzene  since  on  this  assumption  the  ring  would 


1  Jour.  Prakt.  Chemie,  98,  107  (1918) . 

2  Jaffe,  Zeitschr.  f.  physiol.  Chemie,  62,  58  (1909). 


Pauly,  Gilmour  and  Will,  Annalen  der  Chemie,  403,  119  (1914);  416,  1  (1918). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         179 
be  expected  to  open  in  the  following  way 


or 


and  would  result  in  the  formation  of  butene  dicarboxylic  acids: 

CH2— C— COOH 

I  II 

CH2— C— COOH 

Substances  of  this  type  have  never  as  yet  been  observed  to  rearrange 
to  give  muconic  acids.1 

The  attention  of  chemists  was  again  directed  to  Thiele's  formulas 
for  benzene,  naphthalene  and  related  compounds  when,  following  the 
discovery  of  cyclo-octatetraene,  Reddelien  2  took  occasion  to  point  out 
that  the  observed  differences  in  the  chemical  properties  of  this  substance 
and  benzene  might  be  accounted  for  on  the  basis  of  Thiele's  theory: 


Benzene  Cyclo-octatetraene 

For  while  both  unsubstituted  compounds  may  be  regarded  as  repre- 
senting inactive  conjugate  ring  systems,  it  does  not  necessarily  follow 
that  a  system  of  three  double  bonds  will  be  affected  in  the  same  way 
as  a  system  of  four  double  bonds  by  the  action  of  new  atoms  or  groups. 
Indeed  the  differences  in  the  chemical  behavior  of  the  two  substances 
show  that  this  is  not  in  fact  the  case.  To  explain  these  differences 
on  the  basis  of  the  above  formulas  Reddelien  argues  that  if  the  action 
of  a  molecule  of  reagent  such  as  X2  is  regarded  as  being  primarily  an 
addition  reaction,  it  follows  according  to  Thiele's  theory  that  such 
addition  will  be  followed  immediately  by  a  redistribution  of  affinity,  i.e., 


1  Compare  also  Jour,  prakt.  Chemie,  98,   107  (1918),  and  Auwers,  Annalen  der 
Chemie,  415,  139  (1917). 

2  Jour,  prakt.  Chemie,  97,  225  (1917). 


180  THEORIES  OF  ORGANIC  CHEMISTRY 

In  the  case  of  benzene  this  will  correspond  to  a  tendency  on  the  part 
of  the  substance  to  form  para  derivatives.  In  the  case  of  cyclo-octa- 
tetraene  the  effect  is  somewhat  more  complicated.  As  the  above  diagram 
shows  the  system  of  eight  atoms  has  been  broken  down  into  two 
separate  conjugated  systems,  1^  and  5-8.  The  two  butadienes  which 
compose  these  systems  may  be  supposed  to  act  more  or  less  independ- 
ently if  the  above  diagram  is  correct  and  it  may,  therefore,  be  said 
that  the  affinities  of  carbon, — which  in  cyclo-octatetraene  are  equally 
distributed  and  almost  wholly  neutralized, — have  become  polar  in 
character.  This  condition  would  readily  account  for  the  unsatu- 
rated  character  of  such  a  compound  and  also  for  its  tendency  to  form 
bridge  compounds  (1-4  or  8-5). 

Thiele's  formula  for  benzene  is  of  course  open  to  the  same  objec- 
tion as  Kekule's  formula  in  that  it  presupposes  the  existence  of  isomeric 
derivatives  of  the  type: 


While  no  such  compounds  have  as  yet  been  isolated  Reddelien  is  of 
the  opinion  that  the  existence  of  such  isomers  is  possible.  They  are 
obviously  by  their  very  nature  extremely  sensitive  substances,  but 
the  development  of  a  more  delicate  technique  may,  he  thinks,  in  time 
lead  to  their  discovery.  Until,  however,  new  methods  have  been 
developed  and  either  positive  or  negative  results  obtained,  the  non- 
occurrence  of  these  isomers  offers  no  really  convincing  evidence  against 
the  Kekule-Thiele  expression  for  the  constitution  of  benzene. 

A  further  objection  to  the  Kekule-Thiele  formula  for  benzene 
has  been  urged  on  the  ground  that  it  presupposes  the  presence  of  three 
pairs  of  ethylene  linkages  while  as  a  matter  of  fact  all  aromatic  com- 
pounds behave  more  or  less  like  saturated  substances.  Reddelien 1 
meets  this  objection  by  means  of  the  following  argument:  the  fact 
that  a  given  compound  reacts  readily  in  the  presence  of  a  given  reagent 
may  be  construed  to  mean  either  one  of  two  things:  (1)  that  the 
reaction  proceeds  rapidly,  (2)  that  the  yield  is  good.  In  the  second 
case  it  must  be  assumed  that  the  conditions  of  equilibrium  are  such 
as  to  favor  the  formation  of  the  particular  reaction-product.  It  is 
obvious  that  the  rate  of  reaction  in  itself  affords  no  measure  of  affinity 
because  catalysts  and  anticatalysts  have  the  power  either  to  hasten 
or  to  retard  chemical  action  enormously.  A  direct  measure  of 

1  Jour,  prakt.  Chemie,  91,  225  (1915). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         181 

affinity  is  possible  only  when  the  conditions  of  equilibrium  favor  the 
formation  of  the  reaction-product.  The  relation  between  the  rate 
of  reaction  and  affinity  is  expressed  by  the  equation; 

Affinity 
Rate  of  Reaction  = 


Chemical  Resistance 

in  which  chemical  resistance  represents  a  very  imperfectly  known 
quantity.  Obviously  a  direct  relation  between  rate  of  reaction  and 
affinity  can  be  established  only  in  cases  where  the  chemical  resistance 
is  approximately  the  same  under  the  same  conditions. 

Reasoning  along  these  lines  Reddelien  comes  to  the  conclusion 
that  since  the  slowness  with  which  a  given  reaction  proceeds  is  not  a 
measure  of  affinity,  the  presence  of  three  pairs  of  double  bonds  in 
benzene  is  shown  by  the  simple  fact  that  benzene  reacts  with  halogen, 
hydrogen,  ozone,  etc.  The  question  as  to  whether  the  retarding  action 
of  catalysts  is  responsible  for  the  slowness  with  which  benzene  reacts, — 
and  if  so  to  what  extent, — was  then  investigated  by  Reddelien. 

An  observation  made  by  Luther  and  Goldberg  serves  to  illuminate 
this  whole  problem.  These  investigators  discovered  that  in  the 
sunlight  benzene  possesses  the  power  to  combine  instantly  with  six 
bromine  or  chlorine  atoms  provided  that  no  oxygen  is  present  during 
the  process.  It  follows  that  the  oxygen  of  the  air  has  a  retarding 
effect  upon  this  reaction,  or  in  other  words,  that  it  acts  as  an  anti- 
catalyst.  Under  these  circumstances  the  rate  of  reaction  obviously 
affords  no  measure  of  the  affinity  of  the  reacting  molecules.  The 
observed  difference  in  the  rate  with  which  additions  take  place  in  the 
case  of  benzene  and  its  derivatives  as  compared  with  unsaturated 
aliphatic  compounds,  indicates  a  difference  merely  in  degree.  That 
it  is  not  a  difference  in  kind  is  supported  by  the  fact  that  in  certain 
types  of  aliphatic  compounds  the  ethylene  linkage  is  just  as  unre- 
active  as  in  benzene  x  and  that  every  variation  between  this  and  very 
reactive  ethylene  derivatives  has  been  observed.  The  presence  of 
ethylene  linkages  is  not  proved  or  disproved  merely  by  the  behavior 
of  the  substance  in  the  presence  of  halogen. 

Benzene  as  compared  with  unsaturated  aliphatic  compounds  is 
oxidized  very  slowly  by  permanganate  solution.  This  again  may  be 
regarded  as  a  difference  in  degree  rather  than  in  kind.  In  this  case 
the  oxides  of  manganese  probably  act  as  anticatalysts  since  it  has  been 
observed  that  the  oxidation  of  cyclohexane  may  be  greatly  accelerated 
by  working  in  acid  solution.2  Moreover  benzene  reacts  with  ozone  to 

1  Bauer,  Jour,  prakt.  Chemie,  72,  201  (1905). 

2  Wieland,  Ber.,  46,  2616  (1912). 


182  THEORIES  OF  ORGANIC  CHEMISTRY 

give  a  triozonide,  and  if  as  Harries  l  believes  ozone  is  a  typical  reagent 
for  detecting  the  presence  of  ethylene  linkages  the  existence  of  three 
such  linkages  in  benzene  is  demonstrated. 

It  has  frequently  been  observed  that  benzene  does  not  add  hydro- 
gen to  any  appreciable  extent  under  the  catalytic  influence  of  Ni,  Pt, 
and  Pd,  and  that  it  therefore  differs  from  unsaturated  compounds 
of  the  aliphatic  series  which  under  the  same  conditions  are  readily 
reduced.  Recently,  however,  it  has  been  discovered  that  this  differ- 
ence in  the  behavior  of  benzene  is  due  to  the  presence  of  small  quanti- 
ties of  thiophene  which  acts  as  an  anticatalyst,  and  that  when  abso- 
lutely pure  benzene  is  treated  with  hydrogen  under  the  above  conditions 
it  is  reduced  instantly  and  completely.2 

A  consideration  of  the  above  facts  leads  to  the  conclusion  that  the 
behavior  of  benzene  is  not  on  the  whole  so  exceptional  as  has  previ- 
ously been  supposed.  For  while  benzene  does  not  show  the  character- 
istics which  might  be  expected  of  a  substance  possessing  three  pairs 
of  double  bonds,  neither  do  certain  aliphatic  compounds  which 
undoubtedly  possess  ethylene  linkages.  Reddelien,  therefore,  is  of 
the  opinion  that  Thiele's  formula  may  be  accepted  for  the  present 
as  offering  the  best  expression  of  the  constitution  of  benzene  since  it 
affords  an  exceptionally  satisfactory  explanation  of  the  heat  relation- 
ships and  also  of  the  reduction  products. 

The  question  as  to  whether  addition-reactions  and  substitution- 
reactions  represent  separate  and  distinct  types,  or  whether  so-called 
substitution  processes  may  not  always  be  regarded  as  due  to  secondary 
rearrangements  following  the  primary  addition  of  two  molecules, — 
has  long  occupied  the  attention  of  chemists.  An  important  experi- 
mental contribution  to  the  solution  of  this  general  problem  has 
recently  been  brought  forward  by  H.  Wieland3  and  since  it  has  a 
direct  bearing  upon  the  question  of  the  constitution  of  benzene  it 
may  be  considered  briefly  in  this  connection.  Wieland  in  co-operation 
with  E.  Sakellarios  undertook  an  investigation  in  regard  to  the  action 
of  nitric  acid  upon  substances  which  contained  unsaturated  ethylene 
linkages.  In  the  case  of  ethylene  itself  it  was  assumed  that  0-nitro- 
ethyl  alcohol  would  form  as  a  primary  product  by  direct  addition: 

CH2=CH2 +HO  •  NO2  -»  N02  •  CH2  •  CH2OH 

As  a  matter  of  fact  the  corresponding  nitrate,  N02-CH2CH2O-N02, 
was  isolated  along  with  glycoldinitrate,  NC^O-CH^CHoONCb,  by 

1  Harries,  Anna! en  der  Chemie.,  343,  335  (1905). 

2Compt.  rend.,  132,  211  (1901);  Willstatter  and  Hatt,  Ber.,  45,  1471  (1912). 

3Ber.,  63,  201  (1920). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE          183 

both  investigators.  The  first  of  these  substances  obviously  forms 
as  a  result  of  the  continued  action  of  nitric  acid  upon  0-nitroethyl- 
alcohol,  which  may  thus  be  assumed  to  represent  the  primary  product 
of  the  reaction.  The  important  observation  was  then  made  that  both 
/3-nitroethylalcohol  and  the  corresponding  nitrate  may  be  transformed 
into  nitroethylene,1 

CH2=CH.NO2 

by  treatment  with  a  dehydrating  agent  such  as  phosphorus  pentoxide 
or  sodium  bisulphate.  It  would,  therefore,  seem  to  follow  that  this 
reaction  may  be  added  to  the  list  of  those  which  support  the  view 
that  the  mechanism  of  nitration  consists  primarily  in  addition  reactions. 
In  the  case  of  benzene  and  its  derivatives  the  application  of  this 
conception  involves  the  assumption  that  all  nitro  compounds  represent 
secondary  products.  Thus  in  nitration,  a  molecule  of  nitric  acid  may 
be  supposed  to  add  to  the  partial  valencies  which  are  present  on  two 
ethylene  carbon  atoms  and  this  action  is  then  followed  by  the  loss  of 
one  molecule  of  water: 


+     H20 


Hi     JH 


The  whole  question  of  substitution  reactions  has  .recently  been 
further  illuminated  by  the  discoveries  made  by  H.  Wieland 2  in  con- 
nection with  a  systematic  investigation  of  the  chemistry  of  the 
hydrazines.  Wieland  found  that  tetrarylhydrazines,  as  for  example 
(p-CH3-C6H4)2N-N(C6H4-CH3-p)2,  and  also  secondary  and  ter- 
tiary amines,  such  as  (CHaOCeH^NH  and  (p-CHsCeH^aN,  give 
deeply  colored  addition-products  in  the  presence  of  acids,  halogens,  and 
metallic  chlorides.  In  some  cases  these  addition-products  have  been 
isolated,  as  for  example, 


iRer.,  62,  898  (1919). 

2  "  Die  Hydrazine,"  Enke,  Stuttgart,  1913,  p. 
699(1910). 


69;    Ber.,  40,  4260  (1907);    43, 


184 


THEORIES  OF  ORGANIC  CHEMISTRY 


All  decompose  more  or  less  readily  and  in  so  doing  yield  substitution 
products  of  benzene: 

2(p-CH3-C6H4)3NBr3  -»  (CH3(Br)C6H3)3N+(CH3C6H4)3N, 


2(CH3OC6H4)2NHBr2 


(CH3O(Br)C6H3)2NH 

+  (CH3OC6H4)2NH-HBr 

Wieland  is  of  the  opinion  that  these  colored  addition-products 
represent  primary  products  in  all  reactions  involving  substitutions 
in  the  benzene  ring  and  that  they  play  an  important  role  in  all  such 
processes.  In  the  particular  instances  which  have  just  been  cited 
his  interpretation  of  the  mechanism  of  the  transformation  is  as 
follows:  in  the  case  of  primary  and  secondary  amines  addition  takes 
place  on  the  nitrogen  atoms  and  is  due  to  the  relatively  large  fraction 
of  free  affinity  possessed  by  these  atoms.  In  the  case  of  tertiary  amines 
and  tertiary  hydrazines  there  is  relatively  less  free  affinity,  as  is  shown 
by  the  fact  that  the  nitrogen  is  unable  to  pass  into  the  pentavalent 
condition  with  the  accompanying  normal  salt  formation,  and  under 
such  circumstances  the  processes  of  halogenation,  nitration,  and 
sulphonation  cannot  be  assumed  to  take  place  as  the  result  of  direct 
additions.  On  the  other  hand,  intramolecular  rearrangements  of 
tertiary  aromatic  hydrazines  and  amines  may  lead  to  the  reappear- 
ance of  active  nitrogen  in  the  molecule,  viz., 


Ar2N 


Ar2N— < 


In  this  way  reactions  involving  direct  additions  would  again  become 
possible,  and  in  the  case  of  bromine  could  be  represented  by  the 
equation: 

(CH3C6H4)2N — \  / — CH3  -|-  Br2 

CH3 


=  (CH3C6H4)2N=< 
Br 

This  interpretation  readily  accounts  for  the  color  of  the  resulting 
quinoidal  product.  Subsequent  rearrangement  takes  place  in  the 
sense 

CH3 


Ar2N= 


Ar2N 


>CH3 


Br  Br 


~>    Ar2N— ^        V- CH3 


HBr 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE  185 

Wieland  assumes  that  similar  interpretations  may  be  applied  in  the 
case  of  all  substitutions  in  the  benzene  ring. 

Emil  Fischer  offers  an  explanation  of  the  mechanism  of  substitu- 
tion which  is  essentially  different  from  that  of  Wieland  although  it, 
too,  rests  upon  the  assumption  of  the  formation  of  intermediate 
products.  In  order  to  understand  it,  it  will  first  be  necessary  to 
consider  the  so-called  Walden  Rearrangement.1 

Chlorosuccinic  acid  and  other  similar  compounds  contain  an 
asymmetric  carbon  atom  and  therefore  exist  in  dextro,  laevo  and 
racemic  modifications.  The  difference  between  the  dextro  and  laevo 
acids  may  be  shown  by  means  of  a  diagram  in  which  atoms  are 
represented  as  occupying  positions  in  the  same  plane: 

COOH  COOH 

I  I 

H— C— Cl  Cl— C— H 

CH2COOH  (d)  CH2COOH  (I) 

Bases  such  as  AgOH,  KOH  and  NH^OH  react  with  these  substances 
to  give  d-  and  /-malic  acids  and  metallic  chlorides.  In  terms  of  the 
generally  accepted  conception  of  substitution  the  halogen  in  these 
compounds  has  been  replaced  by  hydroxyl  giving: 

COOH  COOH 

I 
H— C— OH  and        HO— C— H 

I  I 

CH2COOH  (d)  CH2COOH  (I) 

In  these  transformations  no  change  of  polarity  sign  is  brought 
about  by  the  action  of  silver  hydroxide;  d-chlorosuccinic  and  Z-chlo- 
rosuccinnic  acids  giving  d-  and  Z-malic  acids  respectively.  On  the 
other  hand,  when  stronger  alkalis  like  KOH  and  NN4OH  are  used 
deep-seated  intramolecular  changes  result  in  such  cases.  Walden  dis- 
covered that  the  transformation  involved  a  change  from  dextro-chloro- 
succinic  acid  to  laevo-malic  acid,  and  from  /ae^o-chlorosuccinic  acid  to 
dextro-malic  acid.  It  was,  moreover,  possible  to  follow  the  course  of 
this  reaction  directly  by  means  of  a  polariscope  and  to  observe  a 
gradual  change  in  the  optical  rotation  of  the  mixture  from  right  to 

^er.,  29,  133  (1896);  30,  2795,  3146  (1897);  32,  1833,  1855  (1899);  also  com- 
pare P.  Walden,  "  Optische  Umkehrerscheinungen,"  F.  Vieweg  und  Sohn,  Braun- 
schweig, 1919. 


186 


THEORIES  OF  ORGANIC  CHEMISTRY 


zero  to  left.     Thus  in  a  very  mild  chemical  reaction  a  change  of  place  is 
affected  by  atoms  in  union  with  the  asymmetric  carbon  atom : 

COOH  COOH 


H— C— Cl 


+     KOH 


CH2COOH 

d-chlorosuccinic  acid 

COOH 

Cl— C— H          + 
CH2COOH 

Z-chlorsuccinic  acid 


KOH 


HO— C— H 

I 
CH2COOH 

Z-malic  acid 

COOH 

I 
H— C— OH 

CH2COOH 

d-malic  acid 


Many  other  instances  of  a  change  in  the  nature  of  the  rotation  of 
optically  active  substances  have  been  observed  in  connection  with 
similar  reactions  and  the  process  has  come  to  be  known  as  the  Walden 
rearrangement  in  honor  of  its  discoverer,  P.  Walden.1  The  following 
scheme,  taken  from  Walden's  memoir,2  shows  the  cycle  of  changes 
possible  with  cholorosuccinic  and  malic  acids,  whereby  the  sign  of 
rotation  may  be  changed  at  will: 


Z-chlorosuccinic  acid 


f-malic  acid 


KOH   ^ 
PC1K 


KOH 

PCI* 


d  -malic  acid 


d-chlorosuccinic 
acid 


Many  new  and  interesting  examples  of  rearrangements  of  this  type 
have  been  investigated  by  Emil  Fischer  3  and  are  regarded  by  him  as 
throwing  new  light  upon  the  mechanism  of  substitution.  Up  to  this  time 
the  Walden  rearrangement  had  been  considered  abnormal.  Since,  how- 
ever, it  was  observed  so  frequently  and  took  place  so  regularly,  Fischer 
concluded  that  it  must  represent  a  normal  rather  than  an  abnormal 
process  and  that,  therefore,  this  type  of  reaction  ought  not  to  be 

1  Loc.  cit. 

2  See  Annual  Reports  of  the  Chemical  Society  (1911),  60;    The  British  Assoc. 
Report  (1912),  687,   by   McKenzie;  Presidential   Address  to  the  Cnemical  Society 
by  Frankland,  Jour.  Chem.  Soc.  103,  717  (1913). 

3Ber.,  40,  489,  1051  (1907);  41,  889,  2891  (1908);  42,  1219  (1909);  43,  2020 
(1910);  Annalen  der  Chemie,  381,  123  (1911);  386,  374  (1912). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         187 

considered  as  an  exception  but  as  representing  a  very  general  phenom- 
enon. If  this  is  true  it  follows  that  previous  views  in  regard  to  the 
process  of  substitution  must  be  false.  If  the  OH  group  actually 
takes  the  place  of  the  halogen  in  the  case  just  cited,  it  is  difficult  to 
understand  why  it  does  not  keep  this  position  instead  of  changing 
it  with  such  frequency  and  regularity.  To  correctly  interpret  this 
phenomenon  it  is  necessary  to  suppose  that  in  substitution  reactions 
the  entering  group  does  not  actually  take  the  place  of  the  group  which 
is  replaced,  but  that  it  may  assume  quite  as  readily  some  other  position 
with  reference  to  the  carbon  atom.  Such  an  explanation  is  obviously 
impossible  in  terms  of  rigid  stereochemical  conceptions  and  Emil 
Fischer,  therefore,  formulated  it  in  terms  of  Werner's  theory. 

Fischer  assumes  that  the  affinity  of  a  free  atom  is  not  a  priori 
divided  into  separate  units  (valencies),  but  that  it  operates  as  an 
attractive  force  which  radiates  equally  in  all  directions  from  the 
center  of  the  atom.  When  other  atoms  combine  with  carbon  they 
seek  to  dispose  themselves  in  such  a  way  with  reference  to  the  cir- 
cumference of  the  carbon  atom  as  to  occasion  the  greatest  possible 
saturation  of  their  respective  affinities.  Thus  a  perfectly  definite 
fraction  of  the  total  affinity  of  the  carbon  atom  is  exercised  in  holding 
each  of  the  several  atoms  in  union  with  it,  the  amount  varying  according 
to  the  nature  of  the  combining  atom.  In  this  way  the  total  affinity 
of  the  atoms  is  not  exhausted  and  residues  of  free  affinity  may  be 
imagined  as  almost  always  present  on  the  individual  atom.  The 
existence  of  such  free  affinity  accounts  for  the  formation  of  so-called 
molecular  compounds  by  the  interaction  of  substances  which  are 
apparently  already  fully  saturated.  Kekule's  assumption  that  all 
chemical  reactions  are  primarily  addition-reactions  which  are  followed 
by  secondary  rearrangements  has  thus  been  applied  by  Fischer  to 
cases  of  substitution.  He  supposes  that  one  group  does  not  directly 
replace  another  but  that  every  type  of  reaction  is  preceded  by  the 
formation  of  an  addition-product,  and  that  in  the  course  of  the  sub- 
sequent decomposition  of  this  primary  product  new  arrangements 
of  the  substituents  with  reference  to  the  carbon  atom  become  possible. 
Accordingly  the  new  group  may  either  occupy  the  same  position  as  that 
of  the  atom  or  group  which  it  replaces,  or  it  may  assume  a  different 
position.  If  the  latter  happens  in  the  case  of  an  asymmetric  carbon 
atom  the  result  will  be  a  Walden  rearrangement.  If  both  types  of 
reaction  take  place,  racemic  mixtures  will  result  in  which  racemization 
may  be  partial  or  complete  depending  upon  the  chemical  nature  of  the 
substituents  in  the  molecule. 

In  order  to  visualize  clearly  this  process  Fischer  made  use  of  the 


188  THEORIES  OF  ORGANIC  CHEMISTRY 

following  model:  the  central  carbon  atom  is  represented  by  a  small 
wooden  sphere,  the  surface  of  which  is  covered  with  wire  brush. 
Colored  celluloid  balls  are  used  to  represent  the  substituent  atoms, 
and  are  fastened  by  means  of  small  wooden  pegs  to  flat  pieces  of  cork. 
The  under  surfaces  of  the  cork  are  also  covered  with  wire  brush, 
thus  making  possible  a  junction  between  the  various  substituents  and 
the  carbon  atom  at  any  point  on  the  surface  of  the  latter.  Fig.  I  shows 
a  central  atom,  7,  which  is  holding  the  four  substituents,  1,  2,  3  and  4, 
and  which  in  addition  is  holding  a  system  compounded  of  two  separate 
units  5  and  6.  The  parts  of  this  system  are  represented  separately 
in  Fig.  II,  which  shows  two  balls  5  and  6,  similar  in  construction  to 
1,  2,  3  and  4,  and  the  mechanism  by  which  their  joint  union  with  the 
central  atom  is  effected. 


II 


This  model  serves  to  demonstrate  the  formation  of  addition-products 
as  preliminary  to  all  processes  of  substitution.  Thus,  for  example, 
in  the  addition  of  ammonia  to  a-brompropionic  acid,  the  balls  1,  2,  3 
and  4  represent  respectively  H,  Br,  CHa,  and  COONEU,  while 
5  and  6  represent  H  and  NH2,  the  latter  being  secured  by  means  of  a 
partial  valency  to  the  central  carbon  atom.  This  compound  sub- 
sequently breaks  down  with  loss  of  HBr.  The  position  left  vacant 
by  the  bromine  may  then  be  filled  by  NH2  or  by  any  one  of  the  three 
remaining  substituents,  supposing  that  these  are  free  to  move.  In 
the  latter  case  NH2  would  come  to  occupy  a  position  different  from 
that  previously  held  by  bromine  and  a  Walden  rearrangement  would 
have  occurred.  In  cases  where  both  processes  take  place  side  by 
side  partial  or  complete  racemization  results. 

While  this  conception  in  regard  to  the  mechanism  of  substitution 
reactions  has  been  developed  in  connection  with  the  study  of  sub- 
stances which  possess  an  asymmetric  carbon  atom,  it  may  be  extended 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         189 

to  include  all  known  cases  of  substitution  and  the  perfectly  general 
assumption  may  then  be  made  that  in  the  case  of  so-called  substitution 
processes  the  entering  group  need  not  necessarily  take  the  place  of 
the  group  which  it  eventually  comes  to  replace,  since  a  variety  of 
other  positions  are  equally  well  open  to  it. 

As  Fischer  points  out  the  same  general  conception  may  also  be 
applied  to  the  transformation  of  an  unsaturated  into  a  saturated 
compound.  It  has  been  observed,  for  example,  that  when  bromine 
adds  to  fumaric  acid  changes  in  the  configuration  of  the  molecule 
may  occur.1  As  in  the  case  of  the  preceding  illustrations  such 
changes  may  be  readily  followed  because  the  resulting  compound 
contains  an  asymmetric  carbon  atom.  They  are,  however,  in  no 
sense  to  be  regarded  as  due  to  the  special  action  of  the  halogen  but 
are  of  a  perfectly  general  and  normal  character. 

In  the  course  of  the  development  of  his  theory  of  valency,  and 
almost  simultaneously  with  Fischer,  Werner  evolved  a  similar  con- 
ception in  regard  to  the  process  of  substitution.2  As  a  result  of  exhaust- 
ive investigations  in  regard  to  the  configuration  of  stereoisomeric 
cobalt  compounds,  Werner  found  that  changes  similar  in  nature  to 
the  Walden  rearrangement  frequently  occur  in  the  course  of  sub- 
stitution, addition,  and  displacement  reactions  and  that  in  such 
cases  the  new  groups  entering  into  the  complex  radical  do  not  assume 
the  same  relative  position  in  space  as  was  occupied  by  the  atoms  or 
groups  which  they  replace.  It  has  been  suggested  that  such  processes 
are  similar  to  the  rearrangements  of  labile  into  stable  forms  of  the  case 
of  stereoisomers,  but  this  is  not  in  actual  harmony  with  all  of  the 
facts. 

In  order  to  understand  Werner's  explanation  of  the  problem  certain 
facts  must  be  borne  in  mind:  (1)  spacial  considerations  necessarily 
limit  the  number  of  atoms  which  may  actually  be  imagined  as  in 
direct  union  with  a  central  atom,  and  the  number  of  such  atoms  must 
be  regarded  as  comparatively  small;  (2)  the  space  which  they  occupy 
is  spoken  of  as  the  first  sphere  of  the  central  atom,  but  atoms  in  this 
sphere  may  in  turn  be  bound  to  other  atoms,  which  latter  may  be 
located  in  a  second  sphere  with  reference  to  the  central  atom;  (3)  the 
term  " coordination  number"  is  used  to  describe  the  number  of  atoms 
which  form  the  first  sphere.  (In  the  case  of  carbon  this  number  is 
four.) 

1  Annalen  der  Chemie,  381,  123  (1911);  386,  376  (1912);  also  compare  Michael 
Jour,  prakt.  Chemie,  38,  6  (1888);  40,  29  (1889);  43,  587  (1891);  46,  209,  381  (1892), 
62,  289  (1895);  75,  105  (1907). 

2Ber.,  44,  873  (1911). 


190  THEORIES  OF  ORGANIC  CHEMISTRY 

The  following  rules  have  been  deduced  from  a  study  of  stereoiso- 
meric  compounds  of  cobalt: 

1.  The  central  atom  of  the  complex  radical  exercises  an  attractive 

power  over  groups  which  are  found  outside  the  first  sphere,  and 
tends  to  draw  these  groups  into  the  space  limits  of  the  first 
sphere. 

2.  The   strength   of    this   attraction    depends   upon   the   chemical 

nature  of  these  groups. 

3.  The  position  in  the  first  sphere  which  an  entering  group  may 

occupy  is  conditioned  by  the  direction  of  the  force  acting 
between  it  and  the  central  atom. 

4.  Entrance   into   a   coordinately   saturated   compound   can   take 

place  only  as  the  result  of  a  loss  of  some  atom  or  group 
already  present.  It  follows  naturally  that  the  group  which 
is  least  firmly  bound  will  be  most  readily  displaced,  and  that 
this  will  in  no  way  depend  upon  the  location  of  such  a  group 
with  relation  to  the  position  occupied  by  the  entering  group. 

Werner  made  furthermore  the  discovery  that  intermediate  addition- 
products  (AX -BY)  are  frequently  formed  during  the  course  of  a 
substitution  reaction  of  the  type: 

AX  +  BY  -»  AY  +  BX 

Such     additive-compounds    are    sometimes     quite    stable,    but    when 
decomposed  they  yield  the  products  represented  in  the  above  equation. 
In  brief  Werner  supposes  that  substitution    processes  take  place 
in  three  phases: 

1.  The  formation  of  addition-products    in  which  the  groups  that 

are  finally  split  off  become  slightly  detached  from  the  atoms 
to  which  they  are  bound.1 

2.  The  penetration  of  the  first  sphere  by  atoms  or  -groups  which 

are  drawn  away  from  their  original  positions  in  the  second 
sphere  by  the  attractive  force  of  the  central  atom. 

3.  The   displacement   of   an   atom   or   group   coordinately   bound 

in  the  first  sphere  as  the  result  of  a  weakening  of  the  attrac- 
tion by  which  it  is  bound  to  the  central  atom. 

Although    these    conceptions    were    developed    from    experimental 
data  in  the  field  of  inorganic  chemistry,  they  have  also  been  applied 

Compare,  W.  Manchot,  Annalen  der  Chemie,  387,  257  (1912). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         191 

by  Werner  to  organic  compounds.  Thus  the  substitution  of  hydroxyl 
by  halogen,  represented  by  the  equation: 

R3COH  +  H  •  Hal.     ~*    R3C  •  Hal.  +  H2O 
may  be  regarded  as  taking  place  in  the  following  stages : 
RaCOH  +  H-Hal.     -»     (R3COH2)Hal. 
(R3COH2)  Hal.          ->    R3C-Hal.  +  H2O 

The  position  which  the  entering  atom  or  group  will  occupy  in  the  mole- 
cule does  not  depend  upon  that  of  the  substituted  group  but  upon  the 
direction  of  the  attractive  force  emanating  from  the  central  atom. 
It  follows  that  under  favorable  circumstances  asymmetric  compounds 
which  are  dextro  rotatory  may  give  derivatives  which  are  laevo  rota- 
tory and  vice  versa.  In  short  on  the  basis  of  these  observations  as 
well  as  of  those  brought  forward  by  Fischer,  Walden  rearrangements 
may  be  regarded  as  phenomena  which  are  normal  rather  than  abnormal 
in  character.1 

In  all  discussions  of  substitution  in  the  benzene  ring  it  is  generally 
assumed  that  the  six  hydrogen  atoms  of  benzene  are  equivalent. 
On  this  assumption  it  follows  that  in  the  reaction  between  benzene 
and  nitric  acid,  each  of  the  six  hydrogen  atoms  may  be  replaced  under 
the  same  conditions  with  equal  ease  by  a  nitro  group  to  give  nitro- 
benzene. The  situation  is  quite  different,  however,  as  soon  as  the  sub- 
stitution of  one  hydrogen  atom  has  taken  place  since  the  remaining 
five  hydrogen  atoms  are  no  longer  equivalent,  and  as  is  well  known 
three  isomeric  products  result  from  the  replacement  of  a  second 
hydrogen  atom  in  a  mono-substituted  benzene  derivative.  These 
substances  are  usually  formed  not  in  the  same  but  in  different  amounts 
and  this  is  explained  as  due  to  the  fact  that  the  speeds  of  reaction 
are  different  in  the  case  of  the  ortho,  meta,  and  para  isomers  respectively. 
In  other  words,  certain  of  the  five  hydrogen  atoms  in  a  mono-substi- 
tuted benzene  are  more  reactive  than  others  and  the  three  isomers 
are  formed  in  different  proportions  depending  upon  the  character  of  the 
mono-substituted  compound. 

It  has  been  observed,  for  example,  that  the  substituted  group 
in  mono-substitution  products  of  benzene  exercises  a  directive  or 
orientating  influence  upon  a  second  entering  group,  so  that  in  certain 
instances  ortho  and  para,  and  in  other  instances  meta  derivatives 

Compare  Pfeiffer,  Annalen  der  Chcmie,  383,  123  (1911). 


192  THEORIES  OF  ORGANIC  CHEMISTRY 

result.  Even  in  1875,  Hiibner 1  and  a  year  later,  Nolting 2  were 
able  to  enumerate  specific  groups  which  were  capable  of  directing 
substitution  in  a  given  manner.  For  example,  strongly  negative  groups 
such  as  NO2,  SOsH,  COOH  and  others  were  known  to  induce  substi- 
tution in  the  meta  position  while  positive,  neutral,  or  weakly  acid 
groups,  such  as  NH2,  CHs,  OH,  Cl,  Br,  etc.,  favored  substitution 
in  the  ortho  and  para  positions.  According  to  Nolting  the  halo- 
gens belong  to  the  class  of  weakly  acid  groups.  Many  reactions  of 
this  type  have  been  studied  by  Brown  and  Gibson,3  by  Armstrong  4 
and  also  by  Vorlander  5  and  a  number  of  rules  have  been  formulated 
in  regard  to  them.  According  to  Vorlander,  for  example,  meta  sub- 
stitution products  are  formed  in  cases  where  the  element  of  the  entering 
group  is  unsaturated,  while  ortha  and  para  derivatives  are  formed  in 
cases  where  the  element  is  saturated.  These  rules,  however,  suffer 
too  many  exceptions  to  be  classed  as  laws. 

In  general,  it  may  be  said  that  the  presence  of  N02,  SOsH,  or 
COOH  favors  the  formation  of  meta  substitution  products.  This  is 
also  true  of  CHO,  COCHs  and  CN  although  here  relatively  few 
cases  of  substitution  have  been  systematically  studied.  All  other 
groups  have  a  more  or  less  pronounced  influence  in  the  direction  of 
ortho  and  para  substitution.  In  every  case  one  isomer  generally 
forms  the  main  product  of  the  reaction,  a  second  is  present  in  smaller 
but  still  considerable  quantities,  while  the  third  is  either  completely 
absent  or  present  in  such  small  quantities  as  to  be  negligible.  Thus 
in  the  case  of  substitution  in  the  mein  position,  for  example,  there  is 
always  formed  simultaneously  a  mixture  of  ortho  and  para  derivatives, 
but  in  this  mixture  either  the  ortho  or  the  para  derivative  predominates 
while  the  other  is  negligible.  In  the  case  of  ortho  and  para  substitu- 
tion, a  mixture  results  in  which  the  para  derivative  usually 
predominates  and  which  contains  none  or  almost  none  of  the  meta 
compound. 

These  rules  hold  under  normal  conditions  but  several  excep- 
tions have  been  observed.  Thus  aniline  usually  reacts  with  nitric 
acid  to  give  a  mixture  of  ortho  and  para  nitroaniline.  If,  however, 
nitric  acid  is  added  to  aniline  which  has  been  strongly  acidified  with 
sulphuric  acid,  meta  nitroaniline  is  formed.  In  the  substitution  of 
halogens,  catalytic  agents  such  as  the  metallic  halides,  iodine,  etc., 

iBer.,  8,  873  (1875). 
2Ber.,  9,  1797  (1876). 

3  Jour.  Chem.  Soc.,  61,  367  (1892). 

4  Ibid. 

5Annalen  der  Chemie,  320,  122,  (1902). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE          193 

have  been  observed  to  exercise  a  directive  influence  upon  the  position 
of  the  entering  atom  or  group.1 

It  has  been  generally  supposed  that  the  process  of  substitution 
in  the  benzene  ring  consists  simply  in  the  direct  replacement  of 
hydrogen  by  some  other  atom  or  group.  Thus,  for  example, 

C6H5NH2  +  HO-SO2OH  ->  NH2  •  C6H4  •  SO2OH  +  H20 

Any  one  who  has  prepared  sulphanilic  acid  knows,  however,  that  the 
primary  product  of  the  reaction  between  aniline  and  sulphuric  acid 
is  aniline  sulphate,  which  forms  as  a  precipitate  and  which  passes 
over  into  sulphanilic  acid  only  upon  heating.  Thus  the  primary 
action  of  the  acid  does  not  involve  the  benzene  ring  as  might  naturally 
be  supposed,  but  the  primary  amino  group  in  the  side  chain.  Bam- 
berger2  has  also  demonstrated  experimentally  that  sulphonic  acids 
may  be  formed  from  sulphates  of  aniline,  having  actually  succeeded 
in  preparing  phenyl  sulphamic  acid,  CeHsNH  •  S02OH,  from  aniline 
sulphate  by  elimination  of  water.  This  compound  is  very  unstable 
in  the  presence  of  free  acids.  With  dilute  acids  it  rearranges  to  give 
aniline-o-sulphonic  acid,  and  with  concentrated  acids  at  high  tem- 
peratures, sulphanilic  acid: 

C6H5-NH2-HOSO2OH    -»    C6H5  •  NH  -  SO2OH 

/NH2  /NH2 

->    C6H4  ->     C6H4< 

X 


S03H(o-) 

From  this  it  follows  that  in  the  sulphonation  of  aniline  the  replace- 
ment of  hydrogen  in  the  benzene  nucleus  is  not  direct,  but  indirect. 
Nitration  of  aniline  might  naturally  be  assumed  to  take  place  in  the 
same  way: 

C6H5-NH2-HON02    ->    C6H5-NH.N02 

Aniline  nitrate  Phenylnitramine 

/NH2  /NH2 

->    C6H4<  ->    C6H4< 

\N02(o-)  XN02(p-) 

o-Nitraniline  p-Nitnv.iiline 

Experimental  evidence  in  support  of  this  has,  indeed,  been  offered  by 
Bamberger3  who  has  obtained  the  same  mixture  of  ortho  and  para 
ni  tramline  from  phenylnitramine  as  is  obtained  by  nitrating  aniline. 

1  Holleman,  "  Die  direkte  Einfiihrung  von  Substituenten  in  den  Benzolkern," 
Leipzig,  1910,  Veit  and  Co. 

2Ber.,  26,  490  (1893);  27,  361  (1894);  28,  401  (1895);  30,  654,  1261  and  2274 
(1897). 

3Ber.,  30,  1252  (1897);  Stormer,  Her.,  31,  2528  (1898). 


194 


THEORIES  OF  ORGANIC  CHEMISTRY 


Such  a  conclusion  is,  nevertheless,  disputed  by  Holleman,1  who 
definitely  maintains  that  rearrangement  of  oriho-  into  para-nitraniline 
does  not  take  place  in  this  case,  and  that  the  relations  must  be 
regarded  as  much  more  complicated. 

In  general,  the  chemical  nature  of  the  substituent  in  the  side 
chain  seems  to  determine  whether  a  second  substituent  will  assume 
an  ortho  or  a  para  position  in  the  benzene  ring.  Chattaway  and 
Orton  2  have  found,  for  example,  that  N-chloracetanilide, 

/Cl 

C6H5-N< 

XCOCH3 

rearranges  to  give  p-chloracetanilide,  Cl-CeH^NB-COCHa,  and 
does  not  form  the  corresponding  ortho  derivative.  If,  however,  the 
hydrogen  in  the  para  position  is  substituted  by  another  group  rear- 
rangement results  in  the  formation  of  a  derivative  of  or^o-chloracet- 
anilide.  The  complete  chlorination  of  the  anilide  may  therefore  be 
assumed  to  take  place  according  to  the  scheme: 

COCH3     COCH3    COCH3    COCH3    COCH3    COCH3        COCH3 
NH  NCI          NH  NCI          NH  NCI  NCI 


Cl     Cl 


Cl 


Cl  Cl  Cl  Cl  Cl 

Other  cases  of  indirect  substitution  will  be  considered  in  a  later 
chapter  dealing  with  the  subject  of  intramolecular  rearrangements. 
It  may  be  added,  however,  that  there  are  certain  indications  of 
indirect  substitution  of  phenol  hydrogen  since  the  following  rearrange- 
ments have  been  observed : 

/OH  3 

C6H4< 

XSO3H  (o) 

•     Br4C6HOH  4 

/OH  5 

C6H4< 

XCOOK  (o) 

1  Ber.,  44,  704  and  following  (1911). 

2Ber.,  32,  3573,  3635  (1899);  Blanksma,  Rec.  trav.  chim.  des  Pays-Bas,  21,  327 
(1902);  Hantzsch,  Ber.,  33,  505  (1900). 

3  Ber.,  9,  55,  1715  (1876);  11,  1907,  (1878). 
4Annalen  der  Chemie,  199,  127  (1879). 
6  Jour,  prakt,  Chemie,  31,  407  (1885). 


C6H5OSO3H 
Br3C6H2OBr 
C6H50-COOK 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE          195 

A  consideration  of  the  phenomena  which  have  just  been  described 
suggests  the  possibility  that  substitutions  in  the  ortho  and  para  posi- 
tions are  always  due  to  molecular  rearrangements  from  the  side  chain, 
and  therefore  always  take  place  indirectly.  According  to  Holleman 
and  Euwes,1  however,  such  an  assumption  is  not  justified  by  the 
facts.  Thus,  since  benzyl  bromide  does  not  rearrange  to  give  o-  and 
p-bromtoluenes  under  conditions  favorable  for  the  formation  of  these  sub- 
stances, it  seems  probable  that  they  form  as  a  result  of  direct  substitu- 
tion. Many  other  instances  of  what  seems  to  be  direct  substitution  in 
the  benzene  ring  are  known.  Moreover,  while  o-  and  p-substitutions  may 
be  direct  or  indirect  depending  upon  a  variety  of  conditions,  very  few 
cases  of  indirect  substitution  in  the  meta  position  have  as  yet  been 
observed.2 

A  great  many  attempts  have  been  made  to  interpret  the  mechanism 
of  substitution  reactions.  Certain  of  these  have  already  been  referred 
to  but  none  have  been  found  which  are  satisfactory  in  all  respects. 
Quite  recently  B.  Fliirscheim3  has  applied  Werner's  theory  to  the 
phenomena  of  substitution  in  an  effort  to  explain  the  various  empirical 
rules  which  have  been  formulated.  His  views  have  been  developed 
in  part  under  the  influence  of  Knoevenagel  and  may  be  considered 
briefly  at  this  point. 

It  will  be  recalled  that  according  to  Werner's  conception  all  atoms 
are  assumed  to  possess  a  definite  amount  of  chemical  affinity  which 
is  the  same  for  atoms  of  the  same  kind,  but  different  for  different 
kinds  of  atoms  and  which  radiates  from  the  center  of  the  atom  uniformly 
in  all  directions.  Contrary  to  the  old  ideas  of  valency  this  attractive 
force  is  not  imagined  as  broken  up  into  individual  units  having  con- 
stant strength  and  definite  direction  in  space.  Thus  when  two  or  more 
atoms  combine  they  may  be  supposed  to  form  addition-products  of 
the  type  suggested  by  Kekule.  Subsequent  adjustments  in  the 
relative  distribution  of  affinity  among  the  constituent  atoms  will 
depend  upon  their  mutual  attractions  and  also  upon  their  spacial 
relationships  and  may  lead  ultimately  to  the  formation  of  new  com- 
pounds. If,  for  example,  a  given  atom  exercises  a  large  part  of  its  total 
affinity  toward  a  second  atom,  or  in  other  words  is  in  close  combination 
with  this  atom,  it  follows  that  it  has  correspondingly  less  affinity  avail- 
able for  holding  other  atoms  which  may  be  in  union  with  it,  and 
these  being  less  firmly  bound  are  capable  of  rearrangements  within 
the  molecule  or  even  of  being  completely  split  off  from  the  molecule. 

!Rec.  trav.  chim.  des  Pays-Bas,  27,  443  (1908). 

2  Jour,  prakt.  Chemie,  82,  470  (1910). 

3  Jour,  prakt.  Chemie,  66,  321  (1902);  71,  497  (1905);  Ber.,  39,  2015  (1906). 


196  THEORIES  OF  ORGANIC  CHEMISTRY 

If  the  six  carbon  atoms  in  benzene  are  symmetrically  arranged  with 
reference  to  each  other  and  to  the  six  hydrogen  atoms  in  union  with 
them,  it  follows  that  the  distribution  of  affinity  may  be  supposed  to 
be  alike  for  all.  If  now  one  of  the  hydrogen  atoms  is  replaced  by 
halogen  and  if  it  is  assumed  that  the  latter  atom  is  more  closely  bound 
to  carbon  than  is  the  former,  it  follows  according  to  Fllirscheim  that 
readjustments  in  the  partition  of  affinity  among  the  six  carbon  atoms 
of  the  ring  must  take  place.  For  example,  if  one  carbon  atom  is 
exercising  a  relatively  great  fraction  of  its  total  affinity  in  holding 
bromine  it  must  have  a  relatively  small  fraction  of  affinity  available 
for  holding  the  two  adjoining  carbon  atoms.  Atoms  in  the  ortho 
position  must  therefore  possess  a  relatively  greater  amount  of  free 
affinity  than  before  substitution  took  place.  This  may  in  part  be 
used  up  in  holding  the  two  carbon  atoms  in  the  meta  position  so  that 
these  in  turn  have  less  affinity  available  for  holding  the  carbon  atom 
in  the  para  position.  In  brief,  if  these  assumptions  are  correct,  sub- 
stitution will  result  in  an  increase  in  the  free  affinity  of  those  carbon 
atoms  of  the  benzene  ring  which  occupy  ortho  and  para  positions 
respectively  with  reference  to  the  substituting  atom  or  group.  Such 
positions  offer  therefore  points  of  attack  for  new  substituents.  These 
conclusions  are  limited,  of  course,  to  cases  where  the  entering  atom 
or  group  is  more  closely  linked  to  carbon  than  was  the  hydrogen  which 
it  replaces.  In  cases  where  the  reverse  is  true  and  the  substituent 
is  less  firmly  bound  to  carbon  than  was  the  hydrogen,  this  carbon 
atom  will  exercise  a  relatively  greater  attraction  for  the  adjacent 
atoms.  Thus  the  free  affinity  of  carbon  atoms  in  the  ortho  positions 
will  decrease  while  that  of  the  meta  carbon  atoms  will  simultaneously 
increase.  The  latter,  therefore,  become  the  points  of  attack  for  new 
substituents  and  meta  disubstitution  products  result.  For  example, 
if  it  is  assumed  that  such  groups  as  SOsH,  COOH,  N(>2  are  less 
firmly  bound  to  carbon  than  is  the  hydrogen  which  they  replace,  it  is 
easy  to  understand  in  terms  of  Flurscheim's  hypothesis  why  these 
groups  induce  substitution  in  the  me ta  positions.  The  relationships 
may  be  represented  diagrammatically  as  follows: 


Cl  S03H 

I  II 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE          197 

Heavy  lines  are  here  used  to  denote  relatively  strong  bonds  and  light 
lines  loose  bonds,  while  the  arrows  serve  to  show  the  position  and 
direction  of  the  affinity  which  may  be  neutralized  by  substituents. 

J.  Obermiller  1  and  Holleman 2  raised  objections  to  this  attempt 
to  explain  the  phenomena  of  substitution  on  the  following  grounds: 
in  the  first  place  there  is  at  present  no  accurate  means  for  determining 
whether  a  given  constituent  is  closely  or  loosely  bound  to  the  carbon 
atom  of  the  ring.  One  and  the  same  substituent  may  appear  at  one 
time  to  be  in  close  combination  and  at  another  time  in  loose  combina- 
tion with  carbon.  In  other  words,  one  and  the  same  compound 
behaves  differently  in  the  presence  of  different  reagents.  For  example, 
it  may  be  assumed  on  the  one  hand  that  halogen  is  closely  bound  to 
carbon  in  monobrombenzene  since  it  does  not  react  readily  with  KCN, 
AgOH,  KSH,  etc.,  but  on  the  other  hand  it  may  equally  well  be  assumed 
that  it  is  loosely  bound  since  it  reacts  as  readily  as  alkyl  halogen  with 
magnesium  in  ether  solution.  "  Close  combination "  and  "  loose 
combination  "  are  thus  found  to  be  in  no  sense  absolute  terms  but 
to  need  very  close  definition  in  every  case. 

Flurscheim  assumes,  moreover,  that  chemical  affinity  is  of  the  nature 
of  gravitation  and  this  is  not  in  accord  with  the  facts.  According  to 
the  established  laws  governing  attraction  between  masses,  for  example, 
the  force  which  two  bodies  exercise  toward  each  other  is  independent 
of  the  presence  of  a  third,  and  if  the  earth  had  two  moons  instead  of 
one,  each  would  be  attracted  by  the  earth  just  as  strongly  as  if  but 
one  were  present.  On  the  other  hand  all  observations  at  the  present 
time  seem  to  show  that  chemical  affinity  is  much  more  closely  com- 
parable to  electricity  than  to  gravitation,  and  while  these  relationships 
do  not  lend  themselves  to  any  simple  summary  it  may  still  be  said 
that  relatively  little  affinity  remains  if  much  is  used  up. 

Even  the  diagrams  suggested  by  Flurscheim  are  misleading,  accord- 
ing to  Holleman,  since  a  heavy  and  a  light  line  intersect  in  both  ortho 
and  meta  positions,  thus  apparently  indicating  the  same  distribution 
of  affinity  in  the  two  cases.  It  follows,  that  compounds  representing 
both  types  (I  and  II)  should  give  either  para  derivatives  as  principal 
products  along  with  a  mixture  of  almost  equal  quantities  of  ortho  and 
meta,  or  else  they  should  give  almost  equal  quantities  of  ortho  and  meta 
derivatives.  Both  of  these  deductions  are  contrary  to  the  facts  of 
experiment. 

While  Flurscheim  assumes  that  substitution  is  always  primarily 

1  Jour,  prakt.  Chemie,  77,  78  (1908). 

2  Ibid.,    74,   157    (1906);    "Die    direkte  Einfiihrung  von  Substituenten  in    den 
Benzolkern,"  1910,  p.  211  and  following. 


198  THEORIES  OF  ORGANIC  CHEMISTRY 

an  additive-process  J.  Obermiller  1  believes  that  it  consists  in  the  direct 
exchange  of  atoms,  and  that  in  the  formation  of  nitrobenzene,  for 
example,  the  NO2  group  in  nitric  acid  HO-NO2,  changes  place  with 
the  hydrogen  atom  of  the  benzene  ring  and  vice  versa.  In  such  cases 
the  reactivity  of  the  hydrogen  atoms  is  supposed  to  be  due  to  their 
relatively  detached  positions.2  Obermiller  explains  the  formation 
of  ortho-,  para-,  and  meto-disubstitution  products  respectively  by 
assuming  that  groups  like  OH,  NH2,  etc.,  increase  the  reactivity 
of  hydrogen  in  the  ortho-para  positions  and  thus  tend  to  orientate 
substituents  in  these  directions  while  such  groups  as  NO2,  SOsH, 
etc.,  decrease  the  reactivity  of  hydrogen  in  the  ortho  and  para  positions 
with  the  result  that  the  hydrogen  in  the  meta  positions  becomes  relatively 
more  reactive  and  is  therefore  more  readily  replaced.  Decrease  in 
the  mobility  of  hydrogen  in  the  ortho  and  para  positions  is  explained 
as  due  to  steric  hindrance.  In  order  to  account  for  the  effect  of  steric 
hindrance  upon  a  hydrogen  atom  in  the  para  position  to  the  orientating 
group  Obermiller  proposed  the  use  of  a  new  formula  for  benzene,  viz. : 


This  stands  midway  between  the  diagonal  formula  proposed  by  Glaus 
and  the  centric  formula  of  Baeyer  and  Armstrong,  and  represents 
the  carbon  atoms  in  both  ortho  and  para  positions  as  being  in  direct 
union  with  the  carbon  atom  which  is  holding  the  orientating  atom 
or  group.  The  peripheral  ortho  bonds  represented  by  solid  lines  are 
supposed  to  be  stronger  than  the  diagonal  para  bonds  represented 
by  the  waving  lines. 

Obermiller  also  attempts  to  explain  the  effect  which  raeta-orientating 
groups  such  as  NO2,  SOsH,  etc.,  have  upon  hydrogen  atoms  in 
the  ortho  and  para  positions  as  due  to  steric  hindrance,  but  his  reasoning 
and  the  evidence  which  he  brings  forward  in  support  of  his  reasoning 
while  plausible  is  sometimes  far  from  convincing. 

In  order  to  account  for  the  increased  chemical  activity  of  hydro- 
gen in  the  ortho  and  para  positions  Obermiller,  like  Fliirscheim,  supposes 
that  groups  such  as  NH2,  OH,  etc.,  which  favor  substitution  in 

1  Jour,  prakt.  Chem.,  76,  1  (1907);  77,  65  (1908);  82,  462  (1910);  84,449  (1911); 
J.  Obeimiller:    "  Die  orieritierenden  Einfliisse  und  der  Benzolkern,"  Leipzig,  J.  A. 
Earth  (1909). 

2  Ibid.,  p.  40. 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         199 

this  sense,  demand  a  relatively  great  amount  of  affinity  from  the 
carbon  atom  with  which  they  are  combined,  and  that  as  a  result  the 
linkages  of  other  atoms  in  the  molecule  are  weakened  in  the  sense 
previously  described.  This  weakening  effect  is  most  felt  by  hydrogen 
atoms  in  the  ortho  and  para  positions. 

Obermiller  represents  a  relative  increase  or  decrease  in  the  chemical 
activity  of  ring  carbon  atoms  by  means  of  the  signs  +  and  — .  Thus 
for  example : 

OH(NH2  etc.)  N02(SO3H,  etc.) 


If  two  like  groups,  whether  of  the  meta,  or  ortho-para  orientating  type, 
are  present  in  the  molecule  thay  may  be  imagined  either  as  reenforcing 
or  as  neutralizing  each  other.  Instances  of  reenforcement  may  be 
found  in  the  case  of  resorcinol,  w-phenylene-diamine,  ra-cresol, 
m-xylene,  etc.  The  effect  of  two  ortho-para  orientating  groups  is 
invariably  the  same  and  finds  expression  in  the  following  formula: 

OH 


+f   M- 

OH 


In  the    case  of  m-orientating  groups,   however,   the    effect    is  quite 
different  and  may  be  represented  thus: 

NO2 


NO2 


Here  the  tendency  to  further  substitution  has  been  greatly  decreased, 
and  while  the  single  hydrogen  atom  designated  by  the  plus  sign  is  still 
capable  of  being  replaced  it  reacts  much  less  readily  than  in  the  case 
of  mono-nitrobenzene. 

Instances  of  the  complete  or  partial  neutralization  of  one  group 
by  another  are  to  be  found  among  the  ortho  and  para  disubstitution 
products  of  benzene.  In  such  cases  the  relative  influence  of  a  given 


200 


THEORIES  OF  ORGANIC  CHEMISTRY 


group  may  be  roughly  determined,  supposing  that  the  two  substituents 
are  different.  In  the  sulphonation  of  ortho  and  para-amidophenol, 
for  example, 

OH  OH 


—  NH2 


I 


(I) 


the  sulphonic  acid  group  will  assume  a  para  position  to  that  group  which 
has  the  stronger  power  of  orientation.  In  the  case  of  I,  for  example, 
it  has  been  found  that  the  hydroxyl  group  is  the  more  effective  since 
1,  2,  4-hydroxyamidobenzene  sulphonic  acid  is  formed.  This  con- 
clusion is  confirmed  by  the  fact  that  in  the  case  of  II  the  sulphonic 
acid  group  enters  the  molecule  in  the  ortho  position  with  reference  to 
the  hydroxyl. 

As  a  result  of  observations  based  upon  reasoning  of  this  sort  Ober- 
miller  has  arranged  the  following  series  as  representing  the  relative 
strength  of  different  groups: 

OH>NH2>C1>CH3>H 

He  also  concludes  that  N02  possesses  greater  power  of  orientation  than 
SO3H  and  also  than  OH.1 

If  several  substituents  are  present  in  benzene  it  must  be  assumed 
that  each  individually  exercises  an  influence  upon  the  remaining  hydro- 
gen atoms,  and  that  this  influence  tends  either  to  increase  or  decrease 
the  mobility  of  these  atoms.  The  resultant  of  these  various  influences 
will  determine  which  hydrogen  will  be  most  readily  replaced,  but  the 
theory  of  Obermiller  stops  here  and  possesses  no  advantage  over  any 
other  theory  in  predicting  the  position  of  such  a  reactive  hydrogen 
atom  in  the  case  of  a  given  derivative  of  benzene.  The  theory  involves, 
moreover,  many  supplementary  hypotheses  in  order  to  harmonize 
it  with  the  observed  facts,  and  because  of  this  offers  nothing  that  is 
really  fundamental  or  illuminating.  Nevertheless,  representing  as 
it  does  the  results  of  investigation  and  of  long  experience,  it  is  frequently 
suggestive  and  valuable. 

Still  other  interpretations  of  the  mechanism  of  substitution  are 
possible  on  the  basis  of  the  valence-electron  hypothesis  of  J.  Stark 

1  Jour,  prakt.  Chemie,  89,  70  (1914);  Zeitschr.,  angew.  Chemie,  27,  37  (1914). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE          201 

and  of  the  closely  related  theory  of  Vorlander.  Before  attempting 
a  detailed  study  of  these  conceptions,  however,  it  seems  advisable 
to  pause  and  consider  the  so-called  laws  of  substitution  in  the  benzene 
ring.  These  rules  have  been  formulated  by  Holleman  on  the  basis 
of  a  large  amount  of  experimental  evidence  which  has  been  tested  and 
amplified  as  the  result  of  special  investigations  by  Holleman  and 
his  students.  This  evidence,  although  far  from  complete  on  the  quanti- 
tative side,  is  fairly  comprehensive  and  is  rapidly  receiving  additional 
confirmation  as  new  methods  for  the  quantitative  study  of  organic 
processes  are  developed.  In  brief,  Holleman  l  has  been  able  to  demon- 
strate that  it  is  possible  to  predict  with  comparative  certainty  the 
position  which  will  be  occupied  by  the  substituting  group  in  the  case 
of  reactions  of  the  type, 

C6H4AB     ->     C6H3-A-B-C 

Attention  has  already  been  called  to  the  fact  that  in  the  case  of 
the  disubstitution  products  of  benzene  the  position  of  the  second 
group  depends  largely  upon  the  chemical  character  of  the  group 
already  present  in  the  molecule  and  only  slightly  upon  the  character 
of  the  entering  group.  According  to  Holleman  the  same  general  rule 
holds  in  the  case  of  trisubstitution.  Three  disubstitution  products 
are  formed  as  the  result  of  introducing  a  second  substituent  C  into 
the  monosubstituted  derivative, 

C6H5A 

These  isomers  are  always  present  in  the  reaction  mixture  although  at 
times  in  markedly  unequal  quantities,  due  to  the  fact  that  their  respect- 
ive rates  of  formation  are  different.  Thus  the  rate  with  which  the 
reaction  as  a  whole  proceeds  is  divided  among  the  three  isomers  in 
the  proportion  in  which  they  are  respectively  formed.  If  the  sub- 
stituent C  is  introduced  into  a  second  compound 

C6H5B 

the  relative  amounts  of  the  ortho,  meta  and  para  derivatives,  CeHjBC, 
which  are  formed  will  be  different  from  those  observed  in  the  case 
of  CG^AC,  because  the  relative  rates  of  formation  are  necessarily 
different  in  the  two  cases.2 

If  the  substituent  C  is  introduced  into  the  disubstituted  benzene 
C6H4A-B,  the  position  which  C  will  assume  will  depend  either  not 

1  Die  direkte  Einfiihrung  von  Substituenten  in  den  Benzolkern,  p.  480  and  fol- 
lowing,   Leipzig,    Veit   and    Co.    (1910);    also   Ber.,  44,  725,  2504,   3556   (1911); 
Obermiller,  Jour,  prakt  Chemie,  82,  462  (1910). 

2  Compare  with  Holleman's  speculations  regarding  the  nitration  of  aniline  and 
its  sulphate.     Ber.,  44,  725  (1911). 


202  THEORIES  OF  ORGANIC  CHEMISTRY 

at  all  or  only  slightly  upon  the  chemical  character  of  C,  and  largely 
upon  the  nature  of  A  and  B.  It  will  depend,  in  other  words,  upon  the 
power  of  A  and  B  respectively  to  orientate  to  the  ortho-para  or  meta 
positions  or  both.  From  the  relative  amounts  of  the  various  isomers 
corresponding  to  CeHsA  •  B  •  C,  it  will  be  possible  to  gauge  their  relative 
rates  of  formation  and  so  to  observe  which  positions  in  the  molecule 
are  most  readily  substituted.  Up  to  the  present  time,  however,  the 
relative  amounts  of  the  various  products  formed  in  substitution 
reactions  have  not  been  determined  with  accuracy,  and  the  rates  of 
reaction  have  not  been  studied  with  sufficient  care.  The  selective 
powers  of  orientation  of  the  groups  A  and  B  are,  however,  known  so 
that  it  is  possible  to  say  whether  a  given  group  will  orientate  to  the 
ortho-para,  or  to  the  meta  positions.  In  certain  individual  cases  the 
relative  rates  of  formation  of  the  various  isomeric  products  are  also 
known.  On  the  basis  of  the  facts  at  his  disposal  Holleman  has  suc- 
ceeded in  formulating  the  following  general  and  special  rules  governing 
substitution  reactions: l 

1.  The  rate  of  ortho  and  para  substitution  is  much  greater  than  that 

of  meta  substitution. 

2.  In  the  case  of  ortho-para  orientating  groups  the  rate  of  substi- 

tution varies  according  to  the  scale 

OH>NH22>Cl>I3>Br>CH34 

3.  In  the  case  of  meta  orientating  groups  the  rate  of  substitution 

varies  according  to  the  scale 

CO2H>SO3H>NO2 

Assuming  the  accuracy  of  these  generalizations  Holleman  has 
attempted  to  predict  which  isomers  might  be  expected  to  form  as  the 
main  products  of  a  reaction  and  which  as  by-products  in  cases  where  a 
substitutent,  C,  takes  the  place  of  hydrogen  in  a  given  compound, 
CeH^A-B.5  The  results  of  these  prognostications  are  contained  in  the 
following  tables.6  The  theoretical  and  experimental  data  are  arranged 
in  parallel  vertical  columns  and  show  fundamental  agreement: 

1  Holleman:   "  Die  direkte  Einfiihrung  "  etc.,  pp.  469-70  (1910);  also  Obermiller: 
"  Die  orientierende  Einfliisse  und  der  Benzolkern,"  p.  50  (1909). 

2  The  rate  of  substitution  of  OH  is  greater  than  that  of  -NH-COCH3,  Ober- 
miller, 1.  c.,  p.  54. 

3  It  is  at  present  uncertain  whether  I  should  come  before  or  after  CH3  in  this 
series.     See  Holleman,  I.e.,  p.  466. 

4  It  may  be  noted  that  this  arrangement  corresponds  to  that  of  Obermiller. 

5  Holleman,  1.  c.,  p.  470. 

6  Ibid.,  pp.  470-74. 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         203 


pq 


CO  CO(N  (M  Illl 

<N  (N  <N~CO  CO  (M"CO*~CO  (N  CO  CO~CO  CO 


I  I  I  I 


_  ^  ^  ^ 

ooo  o  o««fflhHhH 


WWW  WWW 


COCO  CO^O  CO  CO  CO 

'  »O  TJH" 


COCO        COCOiO        CO  COCOCO 


PQ 


www 


wwww  w  ww 
o  oo 


w  w  w  w'w 


204 


THEORIES  OF  ORGANIC  CHEMISTRY 


c,         c, 


I    I    I 


I    I 


w      w 

8    8 

O         O 


i 


'  '  ' 


p? 


»o      10      10      10 


10*0*0*0*0 


ffl 


§  8  g  8  8    ' 


«      w 

O     O 


W     W     o 


PQ 


»O        CO        "t1        CO 


w      w 


" 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         205 


206 


THEORIES  OF  ORGANIC  CHEMISTRY 


With  these  and  similar  considerations  as  a  basis  Holleman  developed 
the  following  hypothesis  in  regard  to  the  cause  and  mechanism  of 
substitution : 

The  study  of  the  chemistry  of  aliphatic  compounds  has  demon- 
strated the  fact  that  the  chemical  character  of  both  individual  atoms 
and  groups  of  atoms  is  very  much  influenced  by  their  proximity  to 
doubly-bound  carbon  atoms.  Thus  for  example,  in  CH2=CC1CH3, 
the  properties  of  the  chlorine  are  very  different  from  those  of  the  chlorine 
in  CH2=CH  •  CH2C1,  since  double  decomposition  reactions  take  place 
much  less  readily  in  the  former  than  in  the  latter  case.  This  influence 
has  been  found  to  be  reciprocal,  the  substituents  also  affecting  the 
reactivity  of  the  double  bonds.  Now  assuming  that  Kekule's  formula 
for  benzene  is  correct,  it  is  obvious  that  each  substituent,  X,  is  in  union 
with  an  unsaturated  carbon  atom. 


If  the  same  general  relations  hold  in  the  case  of  derivatives  of  benzene 
as  in  the  case  of  aliphatic  compounds  the  presence  of  the  substituent 
X  should  tend  either  to  increase  or  decrease  the  reactivity  of  the  double 
bond,  1-6.  Moreover,  its  influence  upon  the  conjugated  system 
1-6-5-4  must  also  be  felt  in  the  sense  of  either  increasing  or  decreasing 
1-4  addition  reactions.  The  double  bond  2-3,  on  the  other  hand,  may 
be  assumed  to  be  only  slightly  affected  by  the  presence  of  a  substituent 
in  the  position  shown. 

According  to  Holleman  addition  reactions  may  be  assumed  to  pre- 
cede substitutions.  If  then  CoHsX  is  nitrated  three  primary  addition 
products  are  possible,  viz. : 


HO 
H 


H 


V 


X 


H 


HNO2 


This  addition  is  followed  immediately  by  the  loss  of  one  molecule  of 
water  and  the  formation  of  the  following  disubstituted  benzenes: 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         207 
X 


02N- 

ii         i:  :ii         ii 

-NO2 
^/ 

NO2 

If  the  substituent  X  is  such  as  to  favor  1-6  and  1^  additions,  ortho 
and  para  derivatives  of  benzene  will  be  formed  as  the  principal  products 
of  the  reaction,  while  2-3  additions  representing  meta  substitution 
products  will  be  negligible.  If,  however,  the  influence  of  the  substituent 
is  such  as  to  retard  this  type  of  addition,  2-3  addition  will  appear  to 
possess  the  greater  relative  velocity  and  meta  derivatives  of  benzene 
will  be  formed  as  the  principal  products  of  the  reaction.  The  nitra-' 
tion  of  phenol  and  toluene  respectively  may  be  considered  in  illustra- 
tion. Inspection  of  the  series  of  orienting  groups  already  referred  to  shows 
that  the  presence  of  hydroxyl  in  position  1  has  the  effect  of  strongly 
increasing  the  chemical  activity  of  the  adjacent  atoms.  This  condition 
favors  1—6  and  1-4  additions,  and  as  a  matter  of  fact  the  sole  products 
resulting  from  the  nitration  of  phenol  are  ortho-  and  para-nitrophenol. 
The  substitution  of  a  methoxy  for  an  hydroxyl  group  in  this  position 
decreases  the  rates  with  which  such  additions  take  place  with  the 
result  that  the  reaction  mixture  contains  a  small  quantity  of  meta- 
nitro  methoxy  benzene  in  addition  to  the  ortho-  and  para-derivatives. 
If  the  substituent  X  decreases  the  reactivity  of  the  conjugate  system 
1-6-4-5  still  more,  2-3  addition  may  come  to  predominate  and  meta 
substitution  products  of  benzene  may  become  the  principal  products 
of  the  reaction. 

It  is  possible  in  terms  of  this  theory  to  understand  why  meta  sub- 
stitution products  of  benzene  generally  form  slowly  since  2-3  addition 
represents  addition  to  a  single  unsaturated  double  bond  and  this 
type  of  reaction  is  generally  recognized  as  taking  place  with  less  ease 
than  the  so-called  1-2  and  1^  additions  to  conjugate  systems  of 
double  bonds. 

The  preceding  illustrations  serve  to  show  that  the  quantitative 
relationships  which  govern  the  relative  amounts  of  the  different  isomers 
which  are  formed  as  the  result  of  substitution  vary  considerably  even 
in  the  case  of  a  particular  type  of  reaction.  In  general,  it  may  be 
said  that  the  influences  which  may  be  regarded  as  capable  of  producing 
variations  are  (1)  the  nature  of  the  substituents  which  are  already 
present  in  the  molecule,  (2)  the  nature  of  the  entering  group,  (3)  external 
conditions  such  as  temperature,  catalyzers,  etc. 


208  THEORIES  OF  ORGANIC  CHEMISTRY 

The  fact  must  again  be  emphasized  that  all  rules  in  regard  to  sub- 
stitution reactions  are  purely  empirical  in  character  and  that  their  chief 
importance  is  to  be  found  in  the  fact  that  they  serve  as  a  means  by 
which,  reasoning  strictly  from  analogy,  it  is  possible  to  predict  the 
position  which  a  substituting  group  will  assume  in  a  given  derivative 
of  benzene.  These  empirical  rules  are  of  especial  interest  at  the  present 
time  because  they  are  now  capable  of  a  rational  interpretation  in  terms 
of  the  electro-chemical  conceptions  of  Stark  and  Vorlander.  Attention 
has  recently  been  directed  to  this  fact  by  H.  Pauly  l  and  his  exposition 
of  the  matter  must  now  be  considered. 

Pauly  points  out  that  a  comprehensive  survey  of  all  of  the  facts 
in  regard  to  the  physical  and  chemical  relationships  of  benzene  and  its 
derivatives  which  have  been  developed  up  to  the  present  time  as  the 
result  of  the  most  careful  investigation  of  these  substances,  leads 
to  the  following  conceptions  in  regard  to  the  structure  of  the  benzene 
molecule : 

1.  The  six  carbon  atoms  must  all  lie  in  the  same  plane. 

2.  They  must  be  bound  together  in  exactly  the  same  way  to  form 

a  ring  which  is  absolutely  symmetrical  in  all  of  its  arrangements. 

3.  The  ring  cannot  be  assumed  to  contain .  either  the  centric  or 

ethylene  type  of  carbon  linkages  since  all  of  the  bonds  between 
the  several  carbon  atoms  must  be  the  same  and  each  must 
represent  a  degree  of  saturation  which  is  more  than  that  of 
a  single  and  less  than  that  of  a  double  bond  between  carbon 
atoms. 

Of  the  existing  formulas  for  benzene  the  Kekule-Thiele  formula 
is  the  only  one  which  embodies  these  fundamental  conceptions  in 
regard  to  the  relationships  which  exist  between  the  six  carbon  atoms. 
Pauly  points  out,  however,  that  in  accepting  it  as  the  final  expression 
for  these  conceptions  the  assumption  must  be  made  that  the  symbol  *=* 
actually  represents  a  degree  of  saturation  which  is  more  than  that 
of  a  single  and  less  than  that  of  a  double  bond.  According  to  this 
formulation  of  the  nrntter,  the  rearrangement  from  the  olefine  to  the 
so-called  "  inactive  "  condition  may  be  supposed  to  take  place  according 
to  the  scheme: 


1  Jour,  prakt.  Chemie,  98,  118  (1918). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         209 

The  Kekule-Thiele  formula  for  benzene  when  interpreted  in  this 
way  has  the  further  advantage  of  being  the  only  formula  by  means 
of  which  it  is  possible  to  explain  the  behavior  of  benzene  and  its 
derivatives  in  terms  of  the  electron  theory.  In  order  to  embody 
the  conceptions  set  forth  in  the  electron  theory  of  J.  Stark  this  formula 
must,  however,  be  subjected  to  further  slight  modifications  and  when 
so  modified  may  for  convenience  be  called  the  Pauly-Stark  formula 
for  benzene.  In  order  to  understand  fully  the  significance  of  this  name 
a  brief  review  of  certain  fundamental  facts  is  necessary. 

It  will  be  recalled  that  a  simple  union  between  two  atoms  is  rer  i  e- 
sented  in  terms  of  the  valence-electron  theory  in  the  following  way : 


A2 


where  the  symbol  o  is  used  to  signify  a  valence-electron.  On  the 
basis  of  this  conception  Hugo  Kauffmann  l  reasoned  that  the  union 
of  six  carbon  atoms  in  a  ring  might  be  assumed  to  take  place  according 
to  the  scheme: 


Since,  however,  a  condition  such  as  that  which  is  represented  by  the 
above  formula  must  be  assumed  to  result  in  an  immediate  redistri- 
bution of  forces  within  the  molecule  and  since  in  terms  of  Kauffmann's 
particular  conceptions  this  ultimately  leads  to  a  condition  of  general 
dispersion  of  affinity,  it  seems  probable  that  six  electrons  become 
partially  dissociated  from  their  respective  atoms.  This  final  con- 
dition of  the  benzene  molecule  may  be  represented  by  the  formula  below : 


1  Die  Valenzlehre,  p.  539. 


210  THEORIES  OF  ORGANIC  CHEMISTRY 

which  not  only  serves  to  express  the  existence  of  saturated  linkages 
between  the  six  carbon  atoms,  i.e., 


C3,  etc., 


by  means  of  heavy  lines  and  the  presence  of  partially  dissociated 
electrons  by  means  of  the  symbol  —  o  —  ,  but  also  has  the  additional 
advantage  of  resembling  the  Kekule-Thiele  formula  in  its  main 
features. 

The  earlier  conceptions  of-  J.  Stark  in  regard  to  the  constitution 
of  benzene  found  expression  in  a  somewhat  similar  formula,  viz., 

H 


H 

This  was,  however,  superseded  by  an  expression  which  aimed  to  give 
a  much  more  intimate  and  complete  picture  of  the  spacial  relationships 
between  the  six  carbon  atoms  of  the  ring,  but  which  at  the  same  time 
was  not  sufficiently  simple  to  allow  of  very  general  application  to  the 
problems  involved  in  the  chemistry  of  benzene.  Since  this  later 
formula  has  been  embodied  to  some  extent  in  the  conceptions  which 
constitute  what  has  been  called  the  Stark-Pauly  formula,  it  must  be 
considered  briefly  at  this  point.  It  supposes  that  the  six  carbon 
atoms  are  arranged  with  their  centers  of  mass  at  the  centers  of  six 
regular  tetrahedra  and  with  their  four  respective  electrons  at  each  of 
the  four  corners.  Each  atom  is  so  placed  with  reference  to  the  others 
that  a  plane  drawn  through  three  of  its  four  electrons  is  at  right  angles 
to  a  plane  of  six  corners  in  which  lie  six  electrons  belonging  respectively 
to  each  of  the  six  carbon  atoms.  Under  these  circumstances  two  other 
electrons,  also  belonging  respectively  to  each  of  the  six  carbon  atoms, 
may  obviously  be  made  to  fall  within  two  other  planes  lying  above 
and  below  the  first  plane  and  equidistant  from  it,  while  the  fourth 
electron  of  each  of  the  six  carbon  atoms  will  thus  come  to  lie  within 
the  first  plane  but  in  positions  outside  the  space  bounded  by  the  first 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         211 

six  electrons.  Fig.  1  attempts  to  show  in  perspective  the  relative 
positions  of  three  electrons  belonging  respectively  to  each  of  six  carbon 
atoms,  the  electrons  which  belong  to  one  and  the  same  carbon  atom 
are  joined  together  by  straight  lines.  The  six  electrons  which  lie  in 
the  plane  of  six  corners, — which  may  for  convenience  be  assumed 
to  correspond  to  the  plane  of  the  paper, — are  dotted,  those  lying  in  a 
common  plane  above  this  are  solid  black,  and  those  lying  in  a  common 
plane  below  the  plane  of  the  paper  are  striped.  The  bent  arrows  serve 
to  show  that  in  any  given  case  a  dotted  electron,  as  for  example,  EI3, 
is  bound  to  the  succeeding  carbon  atom  by  means  of  lines  of  force 
which  terminate  in  positive  zones  located  midway  between  the  solid 
black  (Eni)  and  the  striped  (EII2)  electrons  of  the  second  atom 


112 


FIG.  1. 


Enl  and  EII2  are  in  turn  bound  to  the  first  carbon  atom  by  means  of 
lines  of  force  which  terminate  in  positive  zones  located  respectively 
on  either  side  of  the  dotted  electron  (EI3)  or,  in  other  words,  between 
EI3  and  Eji,  and  EI8  and  EI2  respectively. 

According  to  this  conception,  ring  formation  gives  rise  to  two 
distinct  types  of  valence  fields.  Thus  EI3 — Cn,  EII3 — Cm,  EIIIS — CIV, 
EIV — Cv,  EV3 — CVI,  and  EVI3 — Cr  represent  practically  the  same 
arrangement  as  has  been  assumed  in  the  case  of  single  bonds  (C — C), 
while  Eni — Cr  and  EII2 — CI?  Emi — Cm  and  EIII2 — Cn,  etc.,  corre- 
spond to  the  arrangement  assumed  in  the  case  of  triple  bonds  (C=C). 
In  the  latter  case,  however,  the  similarity  is  much  less  marked  and 
suffices  merely  to  suggest  that  the  condition  which  is  thus  represent  cd 
as  present  in  benzene  results  directly  from  the  condition  C=C  through 
the  closing  of  the  ring.  Thus  the  two  types  of  union  which  may  be 
regarded  as  present  in  aromatic  compounds  and  in  acetylene  deriva- 


212  THEORIES  OF  ORGANIC  CHEMISTRY 

tives  respectively,  while  similar  in  certain  aspects,  must  be  regarded 
as  different  in  others.  This  is  shown  in  the  relative  ease  with  which 
the  two  kinds  of  union  may  be  ruptured  as  well  as  by  differences  in  their 
energies  of  formation. 

The  complexity  of  the  relationship  which  exists  between  the  six 
carbon  atoms  of  the  ring  is  apparent  when  the  following  facts  are 
considered.  It  has  been  calculated  that  the  work  required  to  break 
the  union  between  EI3  and  Cn  is  less  than  that  required  to  break  the 
single  bond  between  Ez  and  Cn.  On  the  other  hand  the  energy  of 
the  18  electrons  which  are  engaged  in  ring  formation  is  less  than  the 
energy  of  18  electrons  engaged  in  maintaining  3  triple  bonds  plus  that 
of  6  electrons  engaged  in  maintaining  3  single  bonds.  It  is  also 
greater  than  the  sum  of  the  energies  required  by  18  electrons  in  main- 
taining 6  triple  bonds.  A  consideration  of  these  facts  led  Stark  to 
conclude  that  the  symbols  which  are  used  in  Kekule's  formula  for 
benzene  to  represent  the  nature  of  the  union  between  the  carbon 
atoms,  do  not  correctly  express  the  facts  of  the  case;  he  therefore 
substituted  the  symbol: 

C  >—  C 

in  an  effort  to  show  that  the  union  between  any  two  carbon  atoms  is 
always  the  same,  and  that  it  corresponds  neither  to  the  type  of  single 
nor  double  bonds  but  is  peculiar  to  ring  formation: 


H-C          C-H 


H 

A  comparison  of  the  above  formulas  shows  that  Stark's  expression 
does  not  presuppose  the  existence  of  isomeric  diortho-  substitution 
products  and,  therefore,  escapes  one  of  the  objections  raised  against 
Kekule's  formula.  It  has  the  added  advantage  of  being  able  to  account 
for  the  observed  low  heat  of  combustion  of  benzene  and  for  the 
observed  difference  between  the  band  spectra  of  benzene  and  the 
spectra  of  substances  known  to  possess  ethylene  linkages.1 

Since  the  stability  of  the  forked  single  bond,  Ci  >-  €2,  depends  upon 
the  presence  of  identical  structures  similarly  arranged  on  either  side, 
1  Compare  Stark,  "  Die  Elektrizitat  im  Chemischen  Atom,"  p.  215. 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         213 

it  obviously  cannot  exist  except  in  ring  compounds.  Thus  the  effect 
of  opening  the  ring  or  even  of  adding  two  hydrogen  atoms  to  one  of 
the  C  >-  C  groups,  is  to  render  the  other  forked  single  bonds  unstable 
and  so  to  bring  about  their  ultimate  rearrangement  into  alternate 
double  and  single  bonds.  For  example,  when  benzene  is  reduced  to 
cyclohexadiene  a  ring  is  formed  which  contains  a  system  of  two 
conjugate  double  linkages.  This  and  other  related  phenomena  tend 
to  prove  that  Stark's  theory  is  in  complete  harmony  with  current  views 
in  regard  to  the  structural  formulas  of  dihydro-  and  tetrahydro- 
benzene  and  their  derivatives. 

The  relatively  great  stability  of  the  combinations  C  >-  C,  as 
observed  in  the  properties  of  benzene  and  its  derivatives,  is  explained 
by  supposing  that  since  the  axes  of  two  adjoining  carbon  atoms  always 
form  exactly  the  same  angle  with  each  other,  the  valence  field,  C  >-  C, 
will  tend  to  develop  attractive  forces  which  are  directed  toward  the 
center  of  the  ring  and  which,  in  the  case  of  rings  containing  an  even 
number  of  atoms,  are  arranged  in  pairs  the  members  of  which  are 
equal  in  size  but  opposite  in  direction.  The  fact  that  the  resultants 
of  all  centripetal  forces  in  such  a  molecule  act  along  axes  which 
correspond  to  the  diagonals  of  a  hexagon,  tends  greatly  to  increase  the 
stability  of  this  form  of  combination.  It  must  be  emphasized,  however, 
that  this  reasoning  applies  only  to  rings  which  contain  an  even  number 
of  atoms  and  that  in  any  given  case  the  relative  stability  of  the  ring 
will  depend  to  a  very  great  degree  upon  the  total  number  of  atoms 
present  in  the  compound.  This  follows  from  the  fact  that  one  char- 
acteristic angle  between  the  axes  of  two  adjacent  carbon  atoms  is 
obviously  determined  by  the  number  of  such  atoms  which  are  present 
in  the  ring  and  this  in  turn  affects  the  character  of  the  partial  valence 
field  of  the  forked  single  bonds.  Taking  the  extreme  case  of  a  ring 
consisting  of  two  carbon  atoms, 


it  must  be  asumed  that  the  common  axes  of  these  atoms  form  an  angle 
of  180°  with  each  other,  and  that  under  these  circumstances  the 
relationship  corresponds  to  the  triple  bond  of  the  acetylene  linkage. 
Such  a  picture  serves  to  explain  why  acetylene  at  red  heat  polymerizes 
to  benzene  and  why  benzene  under  the  same  conditions  dissociates 
to  give  acetylene. 

The  fact  that  a  six-membered  ring  possesses  much  greater  relative 
stability  than  a  four-membered  ring  is  explained  by  Stark  as  largely 
due  to  the  position  of  the  four  electrons  at  the  four  corners  of  a 


214 


THEORIES  OF  ORGANIC  CHEMISTRY 


tetrahedron  whose  center  corresponds  to  the  center  of  mass  of  the 
atom. 

H.  Pauly  has  recently  returned  to  Stark's  original  formula 
for  benzene  and  has  made  it  the  basis  for  further  speculation 
along  stereochemical  lines.  He  has  thus  succeeded  in  developing 
a  conception  which  affords  a  satisfactory  interpretation  from  the  stand- 
point of  the  chemist  of  all  of  the  most  complicated  relationships  of  ben- 
zene and  its  derivatives.  Pauly  assumes  in  the  first  place  that  the 
four  valence-electrons  which  belong  to  a  given  carbon  atom  are  located 
at  the  four  corners  of  a  regular  tetrahedron  which  may  be  imagined 
to  superinscribe  the  sphere  which  bounds  the  carbon  atom  in  such  a 
way  that  the  surface  of  the  sphere  touches  four  middle  points  on  the 
four  surfaces  of  the  tetrahedron.  He  further  assumes  that  the  six 
tetrahedra,  which  represent  respectively  the  six  carbon  atoms  of  the 
benzene  ring,  are  so  disposed  that  one  edge  (k)  of  each  is  vertical  to 
an  imaginary  plane  bounded  by  a  regular  hexagon  and  that  in  every 
case  A;  is  incident  to  this  plane  at  points  which  represent  its  six  corners. 
If  now  each  tetrahedron  is  imagined  to  rotate  around  its  vertical  edge 
until  the  horizontal  edge  (&'),  which  is  parallel  to  the  plane  of  the 
hexagon,  comes  to  a  position  directly  in  front  of  the  vertical  edge  (&) 
of  the  succeeding  tetrahedron,  a  completely  symmetrical  ring  structure 
will  result: 


The  six  horizontal  edges  (&')  of  the  six  tetrahedra  will  then  all  lie  in  a 
common  plane  and  this  plane  will  also  contain  the  centers  of  the 
spheres  which  represent  the  six  carbon  atoms.  Those  surfaces  of  the 
tetrahedra  which  thus  come  to  face  the  center  of  the  space  bounded 
by  the  hexagon,  may  be  pictured  as  resembling  the  paddles  of  a  water- 
wheel.  The  electrons  which  are  located  at  the  three  corners  of  the 
above  mentioned  inner  surfaces  of  the  six  tetrahedra,  in  each  case 
come  to  lie  in  relatively  close  proximity  to  the  middle  points  (positive 
zones)  on  the  surfaces  of  adjacent  tetrahedra.  Fields  of  force  are  thus 
established  which  operate  between  the  negative  electrons  and  the 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         215 

positive  zones  of  the  carbon  atoms  in  the  manner  indicated  by  arrows 
in  the  following  diagram: 


In  comparing  these  different  fields  of  force  it  may  be  assumed 
that  the  innermost  are  the  shortest  and  are,  therefore,  the  strongest. 
While  a  "  centric  grouping  "  of  12  valence  electrons  might  conceivably 
result  from  a  slight  rotation  of  the  tetrahedra  around  the  vertical 
axes  (k),  this  would  entail  the  close  proximity  of  the  negative  charges 
which  are  located  at  the  ends  of  the  horizontal  edges  of  adjacent 
tetrahedra  and  does  not,  therefore,  recommend  itself  as  a  probable 
arrangement.  It  would  seem  more  reasonable  to  suppose  that  those 
forces  which  operate  at  the  center  of  the  ring  act  in  a  centrifugal  direc- 
tion while  those  on  the  periphery  act  in  a  centripetal  direction.  Such 
opposing  tendencies,  which  would  correspond  to  the  lengthening  or 
shortening  of  the  fields  of  force,  would  be  expected  to  equalize  ulti- 
mately each  other  when  a  condition  of  equilibrium  would  result. 
Under  these  circumstances  changes  in  the  distribution  of  affinity  on 
the  different  atoms,  due  to  any  cause  whatsoever,  should  be  accom- 
panied by  corresponding  changes  in  the  condition  of  equilibrium  within 
the  molecule.  This  latter  conception  is  obviously  of  fundamental 
importance  in  interpreting  the  changes  which  have  been  observed 
in  the  character  of  the  benzene  ring  itself  as  a  result  of  substitution 
reactions  and  it  places  the  Stark-Pauly  electron  theory  in  an  exceptional 
position  among  the  various  theories  which  have  been  advanced  to  explain 
the  constitution  of  benzene. 

The  logical  assumption  that  a  connection  exists  between  changes 
in  the  equilibrium  of  the  fields  of  force  within  the  molecule  and 
changes  in  the  properties  of  benzene  and  its  derivatives,  makes  it 
possible  to  arrive  at  a  satisfactory  interpretation  of  innumerable 


216  THEORIES  OF  ORGANIC  CHEMISTRY 

reactions.  For  example,  von  Baeyer  observed  that  the  ease  with 
which  the  dihydrophthalic  acids  lose  hydrogen  to  give  the  correspond- 
ing derivatives  of  benzene  differs  very  considerably.  Thus: 


V^O-l. 

CH     CH- 


CH  CH-COOH 

COOH  •  CH     CH 

I  and  ||        || 

CH    CH-COOH  CH     CH 

\/  \/ 

CH  CH-COOH 

A3~5  dihydro-o-phthalic  acid  A2~5  dihydro-terephthalic  acid 

lose    their   hydrogen   very    readily   while    the    corresponding    change 
takes  place  only  with  difficulty  in  the  case  of 


/ 


CH2  CH 

\  /  \ 

CH-COOH  CH2    C-COOH 

|  and            |         | 

H    C-COOH  CH2    C-COOH 


CH  CH 

A2-4  dihydro-o-phthalic  acid  A2~6  dihydro-o-phthalic  acid 

Finally,  no  reaction  whatever  occurs  in  the  case  of 

CH2  C-COOH 

/\  /\ 

CH    C-COOH  CH2   CH 

and 

COOH  CH     CH2 


C-COOH 

dihydro-o-phthalic  acid  Al~4  dihydro-terephthalic  acid 

If  the  above  formulas  are  correct  it  must  be  assumed  that  none 
of  these  acids  contains  a  true  benzene  nucleus.  It,  therefore,  follows 
that  since  the  benzene  condition  represents  the  most  stable  arrange- 
ment of  the  six  carbon  atoms  of  the  ring,  each  acid  must  in  varying 
degrees  possess  the  tendency  to  lose  hydrogen  and  pass  over  into 
this  condition.  The  change  from  a  diethylene  to  a  true  benzene 
structure  must,  however,  be  imagined  as  taking  place  with  greater 
ease  in  cases  where  the  particular  carbon  atoms  of  the  ring  to  which 
carboxyl  groups  are  attached  are  so  placed  as  to  be  able  to  maintain 
a  state  of  comparative  rest  during  the  course  of  the  intramolecular 
rearrangements,  To  understand  the  matter  better  it  is  necessary 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         217 

to  pause  and  consider  just  what  readjustments  of  forces  would  be 
involved  in  such  a  change.  It  is  apparent  in  the  first  place  that  the 
loss  of  hydrogen  would  be  attended  by  the  sudden  appearance  of  free 
affinity  on  those  carbon  atoms  to  which  the  hydrogen  had  been  attached 
and  that  this  free  energy  would  act  as  a  powerful  force  upon  the 
mass  of  the  neighboring  atoms.  This  would  in  every  case  have  the 
effect  of  drawing  these  atoms  into  a  closer  ring  structure  by  shortening 
the  lines  of  force  already  in  operation  between  them.  That  such  a 
contraction  actually  occurs  as  a  result  of  the  change  has  been  demon- 
strated by  means  of  physical  measurements.  If  now  under  certain 
conditions  this  pull  upon  a  given  atom  is  very  powerful  and  is  exerted 
veiy  suddenly,  it  may  act  to  disrupt  the  union  between  this  atom 
and  some  other  atom  or  group  which  it  is  holding,  and  if,  for  example, 
the  group  should  happen  to  be  carboxyl,  carbon  dioxide  might  be 
split  off.  In  such  a  case  the  ease  with  which  carbon  dioxide  would  be 
eliminated  might  be  supposed  to  depend  among  other  things  upon  the 
relative  position  of  the  carboxylated  carbon  atom  with  reference  to  the 
center  of  the  ring.  If  the  atom  is  far  from  the  center  and  at  the  same 
time  is  attached  to  the  adjacent  carbon  atoms  by  means  of  single  bonds, 
the  change  which  it  suffers  will  be  relatively  great  and  will  be  accom- 
panied by  a  loss  of  CO2  from  its  substituent  group.  If,  on  the  other 
hand,  it  is  so  placed  as  to  be  near  the  center  of  the  ring  and  in  union 
with  one  of  the  adjacent  carbon  atoms  by  means  of  a  double  bond, 
the  change  in  position  will  be  relatively  slight  and  the  carboxyl  group 
will  retain  its  integrity  during  the  process.  These  considerations 
find  expression  in  the  fact  that  both  of  the  first  pair  of  acids  whose 
formulas  are  represented  above,  tend  to  lose  CO2  readily.  The  fact 
that  A2"6  dihydrophthalic  acid  also  loses  CO2  although  both  of  its 
carboxylated  carbon  atoms  are  in  union  with  an  adjacent  carbon 
atom  by  means  of  a  double  bond,  may  be  understood  if  the  reaction 
is  interpreted  by  means  of  the  models  shown  below  : 


i          .  A. 

v 

and  in  which  the  two  carboxylated  carbon  atoms  are  represented  by 
the  symbol  A-  A  study  of  these  models  shows  that  the  transforma- 
tion from  I  to  II  can  take  place  only  as  the  result  of  a  change  in  the 
position  of  at  least  one  of  the  carboxylated  carbon  atoms  with  reference 
to  the  other  in  the  sense  indicated  in  the  diagram  by  means  of  the 
small  arrow. 


218 


THEORIES  OF  ORGANIC  CHEMISTRY 


If  similar  models  are  employed  to  interpret  the  behavior  of  the 
two  A1'4  acids,  it  is  apparent  that  in  neither  case  is  any  influence 
at  work  which  could  effect  a  change  in  the  position  of  either  of  the 
carboxylated  carbon  atoms.  A  rearrangement  to  the  benzene  con- 
dition might,  therefore,  be  expected  to  take  place  as  the  result  of 
readjustments  among  the  other  carbon  atoms, — the  carboxyl  groups 
remaining  intact  during  the  process. 

The  Stark-Pauly  electrochemical  formula  for  benzene  also 
possesses  the  advantage  of  affording  a  satisfactory  interpretation  of 
the  rules  which  have  already  been  described  as  governing  substitutions 
in  the  ring.  If  Pauly's  conception  is  correct  a  condition  of  equilibrium 
exists  among  the  forces  operating  between  the  atoms  of  the  benzene 
nucleus  which  corresponds  to  a  very  definite  and  characteristic  dis- 
tribution of  affinity.  This  being  the  case  it  is  easy  to  understand  why 
the  introduction  of  a  strongly  polar  atom  such  as  chlorine  or  oxygen 
in  place  of  an  indifferent  atom  such  as  hydrogen  should  have  the  power 
to  produce  fundamental  disturbances.  Under  these  circumstances 
the  force  fields  within  the  molecule  may  be  supposed  to  be  affected  in 
either  of  two  ways:  (a)  by  the  lengthening  or  shortening  of  the  fields 
of  force  acting  between  the  different  atoms,  (b)  by  the  lengthening 
or  shortening  of  the  lines  of  force  acting  between  a  given  atom  and  its 
unsaturated  valence  electron.  The  difference  in  the  distribution  of 
benzene  and  phenol,  for  example,  finds  expression  in  the  following 
formulas: 


II 


II 


H 


II 


Phenol 


where  the  heavy  lines  serve  to  indicate  condensed  force-fields  and  where 
the  valence-electrons  are  represented  by  the  small  circles.  In  inter- 
preting the  mechanism  by  which  such  a  change  is  effected,  it  is  assumed 
that  the  affinity  expended  by  a  carbon  atom  in  holding  oxygen  is 
relatively  much  greater  than  that  required  for  holding  hydrogen.  The 
particular  carbon  atom  which  is  engaged  in  this  way  has,  therefore, 
relatively  less  affinity  available  for  holding  the  carbon  atoms  adjacent 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         219 

to  it.  This  results  in  a  complete  readjustment  in  the  affinity  relation- 
ships of  all  of  the  other  carbon  atoms  of  the  ring,  as  indicated  in  the 
above  formulas  by  light  and  heavy  lines.  The  presence  of  partially 
dissociated  (gelockert)  valence  electrons  on  the  ortho  and  para  carbon 
atoms  serves  to  indicate  that  these  atoms  are  characterized  by  increased 
chemical  reactivity,  since  according  to  Stark's  theory  electrons  of  this 
type  are  supposed  to  exercise  strong  attractions  for  other  atoms. 

Since  it  may  be  assumed  that  the  action  of  other  non-metallic  atoms 
is  similar  to  that  of  oxygen  and  results  in  the  increased  chemical 
reactivity  of  ortho  and  para  carbon  atoms,  this  conception  affords 
a  satisfactory  basis  for  interpreting  the  sequence  in  which  one  non- 
metallic  element  follows  another  in  Holleman's  series  which  is  given 
below  and  which  has  been  arranged  to  show  the  relative  strength  of 
different  groups  in  orientating  to  the  ortho  and  para  positions  of  the 
benzene  ring: 

OH  >  NH2  >  Cl  >  I  >  Br  >  CH3 

This  arrangement  is  purely  empirical  in  character,  but  when  it  is  con- 
sidered that  the  strongly  polar  oxygen  atom  has  the  most  pronounced 
influence  of  any  element  upon  the  carbon  atoms  of  the  ring,  it  is  easy 
to  understand  in  terms  of  Stark's  theory  why  it  should  be  represented 
in  the  first  group.  It  is  equally  easy  to  see  why  carbon  should  be 
found  in  the  last  group  since  this  element,  as  its  position  in  the  periodic 
system  shows,  is  electro-chemically  neutral  in  character. 

The  presence  of  hydrogen  in  the  above  groups  has  no  appreciable 
influence  upon  the  action  of  the  non-metallic  atom  since  hydrogen, 
like  carbon,  is  what  Stark  calls  an  electro-dual  element.  If,  on  the 
other  hand,  the  non-metallic  atom,  as  for  example  nitrogen,  is  in  com- 
bination with  one  or  more  strongly  polar  atoms,  as  in  N(>2,  the  strength 
of  its  union  with  carbon  will  obviously  be  weakened,  and  this  will 
affect  the  affinity  relationships  among  the  carbon  atoms  in  the  ring 
in  such  a  way  as  to  produce  a  relative  increase  in  the  chemical  reac- 
tivity of  carbon  atoms  in  the  raeta-positions.  That  the  influence  of 
different  atoms  or  radicals  which  may  replace  hydrogen  in  any  of  the 
above  groups,  is  relative  in  character  may  be  seen  from  a  study  of  the 
effect  of  substitution  on  the  orientating  power  of  the  methyl  group  in 
toluene.  As  the  original  hydrogen  atoms  present  in  this  group  are 
replaced  one  by  one  by  chlorine,  it  gradually  changes  from  an  ortho- 
para  to  a  meta  orientating  group.  This  is  demonstrated  by  a  considera- 
tion of  the  nitration  products  which  result  from  the  action  of 
upon  toluene,  mono-,  di-,  and  tri-chlortoluene  respectively: 


220 


THEORIES  OF  ORGANIC  CHEMISTRY 


Substituent  present  in 
the  benzene  ring 

Position  of  the  entering 
nitro  group 

1222 

—  CHHH 

2,4,3 

1222 

—  CHHC1 

4,  2,  3 

1222 

—  CHC1C1 

3 

1222 

—  CC1C1C1 

3 

Other  illustrations  of  the  relative  effect  of  atoms  of  marked  polarity 
upon  the  orientating  power  of  non-metallic  atoms  in  direct  union 
with  ring  carbon  atoms,  are  given  in  the  following  table: 


Substituent  present  in  the 


POSITIONS  OF  THE  DIFFERENT  ENTERING 
GROUPS 


benzene  ring 

Cl 

Br 

I 

N02 

SO3H 

1          22 

—  C  :OH 

2,  3 

? 

? 

3,2 

? 

22333 

—  C  :  OC  HHH 

? 

T 

? 

3,2,4 

| 

1         223 

—  C;OOH 

2,3 

3 

3 

3,2,4 

3,4 

1         2    2 

—  N;OO 

3 

3 

T 

3,2,4 

3,2,4 

1       2223 

--S  i  000  H 

? 

3 

? 

3,4,2 

3,4 

1         2 

—  c  ;  N 

* 

» 

» 

3 

» 

It  may  be  noted  in  this  connection  that  certain  groups  such  as 

12  1212  123 

— N=N—    — C=N,  — CH=CH—      and     —  CH=CH-COOH 

do  not  orientate  to  the  me ta  position.  This  apparent  anomaly  is 
explained  by  supposing  that  the  influence  of  nitrogen  and  carbon  in  the 
2-position  is  relatively  much  weaker  than  that  of  oxygen.  In  general, 
it  may  be  concluded  that  the  orientating  power  of  any  group  substi- 
tuting in  the  benzene  ring  depends  definitely  upon  the  character  of  the 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         221 


atoms  which  occupy  the  2-positions  in  the  group.  The  fact  that 
phenyl  nitromethane 

1         23 

C6H5-CH2N02 

orientates  to  the  meta  and  not  to  the  ortho-para  positions  might 
seem  at  first  sight  to  be  in  open  contradiction  to  this  conclusion,  but 
that  this  is  not  actually  the  case  is  apparent  when  it  is  considered  that 
the  nitrogen  atom  present  in  the  2-position  must  here  be  assumed  to 
be  strongly  non-metallic  in  character  since  it  is  represented  as  in  union 
with  two  oxygen  atoms. 

Still  another  contradiction  to  this  rule  is  to  be  found  in  the  fact 

123 

that  the  group  — HgO(Cl,  etc.)  orientates  to  the  ortho-para  and  not 
to  the  meta  position  despite  the  fact  that  a  strongly  polar  oxygen  atom 
is  present  in  the  2-position.  The  seeming  anomaly  is,  however, 
readily  accounted  for  if  the  metallic  character  of  the  mercury  atom 
is  considered,  since  it  then  becomes  apparent  that  the  polarity  of  the 
oxygen  might  here  be  insufficient  to  overbalance  the  opposing  prop- 
erties of  the  metal. 

The  substitution  of  carbonyl  for  ring  hydrogen  results,  as  might 
be  expected,  in  an  increase  in  the  reactivity  of  the  ortho-para  positions. 
In  the  case  of  derivatives  of  those  compounds  which  contain  hydroxyl 
a  number  of  different  configurations  are  possible.  If  the  hydroxyl 
and  carbonyl  groups  are  in  union  with  adjacent  carbon  atoms,  as  in 
salicylaldehyde  for  example,  any  one  of  the  following  arrangements 
may  result: 


V 


H 


II 


III 


In  the  case  of  I  the  union  between  the  carbonyl  carbon  atom  and  the 
carbon  atom  of  the  ring  may  be  assumed  to  be  unsaturated.  Reac- 
tivity at  this  point  in  the  molecule  is  indicated  by  the  presence  of  a 
partially  dissociated  valence-electron  on  the  ring  carbon  atom  and 
corresponds  to  the  tendency  of  the  substance  (a)  to  lose  CO  when 


222 


THEORIES  OF  ORGANIC  CHEMISTRY 


heated  with  sulphuric  acid,  (6)  to  form  2,  4,  6-tribromphenol  when 
treated  with  an  excess  of  bromine  and  finally  (c)  to  form  catechol 
when  treated  with  hydrogen  peroxide.  In  the  case  of  the  other  two 
formulas  (II  and  III)  the  reactivity  of  ring  carbon  atoms  in  the  3-  and 
5-positions  with  reference  to  the  carbonyl  carbon  atom  is  indicated 
by  the  presence  of  a  partially  dissociated  valence-electron  at  these 
points  in  the  molecule  and  corresponds  to  the  tendency  of  the  sub- 
stance, when  in  either  of  these  two  conditions,  to  form  substitution 
products  in  which  Br,  NO2,  etc.,  occupy  respectively  the  3-  or  the 
5-positions. 

Meta  hydroxy-aldehydes  may  be  assumed  to  possess  any  of  three 
possible  configurations : 


Since  none  of  these  formulas  indicates  the  presence  of  free  affinity  on 
the  carbonyl  carbon  atom,  such  as  has  been  described  in  the  case  of 
one  of  the  possible  or^o-hydroxy-aldehydes  (I),  the  substance  shows 
no  tendency  to  lose  CO  under  the  action  of  H2SO4,  H2O2  or  Br2.  The 
above  formulas  indicate,  on  the  other  hand,  that  the  substance  is 
distinctly  aldehydic  in  character  and,  therefore,  has  a  tendency  to 
oxidize  readily  to  form  the  corresponding  acid. 

Para   hydroxy-aldehydes   may   be   represented    by   either    of   two 
possible  configurations: 


H 


H 


H- 


In  cases  where  the  symmetrical  arrangement  may  be  assumed,  the 
substance  will  show  a  marked  similarity  to  the  corresponding  ortho- 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE          223 

compound  (I)  and  will  readily  lose  CO  under  the  action  of  H2S04,  H2O2 
and  Br2.     Substitution  takes  place  in  the  positions  indicated. 

The  question  as  to  why  the  carbonyl  group  is  very  reactive  in 
dimethyl  «,  7-diketohydrindene  (I)  and  other  homologues  is  one  which 
it  is  difficult  to  answer  satisfactorily  in  terms  of  the  older  structural 
theories  of  organic  chemistry  but  which  becomes  intelligible  from  a 
study  of  the  following  formulas: 


Interpreted  in  terms  of  the  Stark-Pauly  electroatomic  theory,  it  is 
obvious  that  in  the  case  of  dimethylindandione  the  carbon  atoms  of  the 
methyl  groups  (C*)  are  strongly  bound  to  the  carbon  atom  of  the  ring 
while  in  the  case  of  diethyl  a,  7-diketohydrindene  (II)  and  other  homo- 
logues this  union  is  weakened  because  of  the  carbon  atoms  of  the  original 
methyl  groups  (C*)  are  now  each  engaged  in  holding  a  hydrocarbon 
residue  in  place  of  one  of  the  three  hydrogen  atoms.  A  comparison  of 
the  two  formulas  shows  this  substitution  of  hydrogen  is  necessarily 
accompanied  by  a  redistribution  of  affinity  within  the  molecule  and 
that  one  result  of  this  is  a  marked  decrease  in  the  reactivity  of  the 
carbonyl  group.  This  is  indicated  by  a  difference  in  the  relative  positions 
of  the  partially  dissociated  valence  electrons  which  are  represented  as 
being  present  on  each  of  the  carbonyl  carbon  atoms. 

These  illustrations  suffice  to  show  the  manner  in  which  various 
phenomena  connected  with  the  chemistry  of  benzene  and  its  deriva- 
tives may  be  explained  by  means  of  the  valence-electron  theory.  If, 
as  now  seems  probable,  this  theory  may  be  successfully  applied 
to  the  solution  of  all  problems  in  the  fields  of  aromatic  chemistry, 
a  way  will  obviously  have  been  opened  to  a  much  more  intimate 
understanding  of  atomic  relationships  than  has  ever  been  possible  in 
the  past. 

Stark's  formulas  for  naphthalene  and  anthracene  suppose  a  periph- 
eral arrangement  of  successive  forked  bonds  (C  >-  C  >•  C,  etc.) 


224  THEORIES  OF  ORGANIC  CHEMISTRY 


Anthracene 

In  the  case  of  naphthalene  the  effect  of  two  condensed  benzene  rings 
is  produced  by  means  of  a  single  bond  (C  —  C)  joining  the  two  central 
atoms,  while  in  anthracene  the  presence  of  three  benzene  rings  is  indi- 
cated by  the  presence  of  two  single  bonds.  In  both  naphthalene  and 
anthracene  the  stability  of  the  ring  may  be  assumed  to  have  been 
decreased  as  a  result  of  the  formation  of  these  single  bonds. 

These  formulas  serve  to  explain  differences  in  the  chemical  proper- 
ties of  the  various  derivatives  of  benzene,  naphthalene  and  anthracene 
as  well  as  differences  in  the  general  stability  of  these  three  classes  of 
compounds.  Thus,  for  example,  it  is  easy  to  understand  why  carbon 
atoms  in  the  naphthalene  and  anthracene  rings  which  are  common  to 
two  so-called  benzene  nuclei  and  which  are  represented  as  in  union 
with  other  carbon  atoms  in  similar  positions  by  means  of  simple  link- 
ages, should  possess  markedly  different  properties  from  carbon  atoms 
which  are  in  union  with  hydrogen. 

Pyridine,  pyrazine  and  pyrimidine  may  be  represented  by  means  of 
formulas  which  are  similar  to  that  of  benzene  ; 


•  V 

H-<             C-H               H-           C-H                   A  I 

^  ^ 

c 


In  all  of  these  systems,  however,  the  continuity  of  the  ring  is  interfered 
with  by  the  presence  of  a  nitrogen  atom.  This  follows  from  the  fact 
that  the  valence  fields  of  nitrogen  must  be  assumed  to  differ  from  those 
of  carbon,  but,  while  the  resulting  structure  is  less  homogeneous  than  in 
the  case  of  benzene,  it  is,  nevertheless,  capable  of  considerable  stability. 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         225 

The  valence-electron  theory  is  of  particular  importance  to  the 
chemist  because  it  affords  a  means  for  establishing  a  relationship  between 
the  optical  properties  of  a  given  substance  and  the  number  and  nature 
of  the  valence  fields  which  are  present  in  its  molecule.  This  connection 
is  made  by  assuming  that  the  oscillation  of  valence  fields  around  given 
positions  of  equilibrium  on  the  atoms  is  electromagnetic  in  character 
and  is  accompanied  by  the  emission  and  absorption  of  light.  Since, 
moreover,  the  waves  of  light  which  have  their  origin  in  the  valence 
fields  of  the  atoms  are  assumed  to  possess  the  same  frequencies  as  the 
valence  fields,  it  follows  that  optical  measurements  may  be  used  to 
define  the  character  of  the  valence  fields.  In  other  words,  by  determin- 
ing the  frequencies  and  the  intensities  of  the  different  waves  of  light 
which  are  emitted  or  absorbed  by  a  given  chemical  compound,  it  is 
possible  to  draw  certain  definite  conclusions  in  regard  to  the  nature, 
strength,  and  stability  of  the  various  forms  of  union  which  exist  between 
the  valence  electrons  and  the  atoms  which  are  present  in  that  particular 
kind  of  molecule.  It  follows,  moreover,  that  the  chemical  constitution 
of  different  molecules  may  be  theoretically  deduced  from  considerations 
based  upon  the  optical  properties  of  the  corresponding  compounds. 

Stark's  theory  has  been  described  only  in  the  barest  outline,  but 
even  so  it  is  apparent  that  it  agrees  fundamentally  with  the  electro- 
chemical conceptions  of  Berzelius.  The  new  theory  possesses  the 
advantage,  however,  of  correlating  these  conceptions  with  the  results 
of  the  most  recent  investigations  of  physicists  in  the  field  of  atomic- 
structure  and  it,  therefore,  affords  a  much  more  intimate  understanding 
of  the  interatomic  relationships  than  was  possible  in  Berzelius'  time. 

Another  theory  which  seeks  to  interpret  atomic  relationships  has 
been  advanced  by  D.  Vorlander  1  and  must  now  be  considered  briefly. 
Vorlander  uses  the  conception  of  the  positive  and  negative  character 
of  atoms  and  radicals  as  the  vehicle  by  means  of  which  he  ultimately 
arrives  at  conclusions  which  are  closely  analogous  to  those  which  have 
just  been  described  as  deduced  by  means  of  the  valence-electron  theory. 
It  may  be  recalled  that  Vorlander  2  was  led  to  assume,  as  the  result  of  a 
careful  study  of  a  long  series  of  reactions,  that  NC>2,  SOaH,  and  COOH 
function  as  positive  groups  in  benzene,  while  NH2  and  OH  function  as 
negative  groups.  He  also  made  the  further  observation  that  a  benzene 
carbon  atom  appears  to  be  negative  in  character  when  in  union  with  a 
positive  group  and  positive  in  character  when  in  union  with  a  negative 
group.  The  relation  of  these  various  atomic  changes  is  expressed  by 
means  of  the  following  formulas: 

1  Ber.,  52,  263  and  274  (1919). 
*Ber.,  62,  263  (1919). 


226 


THEORIES  OF  ORGANIC  CHEMISTRY 


Bz.  C.  N  O2 
Bz.  C.  S  63  H 
Bz.  C.  C  62  H 


Bz.  C.  N  H2 
Bz.  C.  6  H 
Bz.  C.  C  H3 


The  distribution  of  the  electro-positive  and  electro-negative  charges 
among  the  six  carbon  atoms  of  the  benzene  ring  is  supposed  to  be  differ- 
ent in  the  case  of  these  two  general  classes  of  derivatives  and  is  repre- 
sented diagrammatically  as 


and 


In  brief  Vorlander  assumes  that  the  bound  and  free  charges  on  the 
carbon  atoms  of  the  ring  are  alternately  positive  and  negative  in  char- 
acter: 


or 


In  cases  where  the  relative  difference  in  potential  needs  to  be  expressed 
still  other  diagrams  are  used 


In  the  first  formula  (I)  it  will  be  noted  that  the  positive  hydrogen  atom 
of  the  ring  has  been  replaced  by  an  atom  of  the  same  sign  and  it  may, 
therefore,  be  assumed  that  the  resulting  compound  will  be  relatively 
stable  and  that  in  general  it  will  resemble  the  parent  substance.  In  the 
second  formula  (II),  on  the  other  hand,  the  substituting  atom  possesses 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE         227 

an  opposite  charge  and  the  resulting  derivative  will,  therefore,  show  a 
marked  increase  in  the  potential  difference  between  certain  of  the 
carbon  atoms  of  the  ring  and  the  atoms  which  are  in  union  with  them. 
Such  a  condition  may  be  assumed  to  be  accompanied  by  an  increase  in 
the  chemical  reactivity  of  the  substance  and  in  cases  where  the  potential 
difference  is  very  great  this  may  even  result  in  the  complete  disruption 
of  the  ring. 

In  the  formation  of  disubstitution  products  as  the  result  of  nitration, 
sulphonation,  halogenation,  etc.,  the  position  taken  by  the  entering 
group  will  be  one  where  the  ring  carbon  atom  is  assumed  to  possess  a 
negative  charge.  A  comparison  of  the  formulas  of  nitro-  and  amino- 
benzene  shows  that  this  will  be  the  raefa-position  in  I  and  the  ortho-para- 
positions  in  II.  Thus  the  rules  governing  substitution  in  the  benzene 
ring  may  be  expressed  in  terms  of  Vorlander's  conceptions  in  the  follow- 
ing way:  "  in  the  formation  of  disubstitution  products  of  benzene  by 
processes  of  nitration,  sulphonation,  halogenation,  etc.,  the  entering 
group  will  be  orientated  to  the  meta  position  by  the  presence  of  positive 


elements  in  the  side  chain,  as  for  example  in  the  case  of  CeH^E,  and  to 
the  ortho-para-positions  by  the  presence   of  negative  elements,  as  for 


example  in  the  case  of  CeHsE.  The  following  groups  are  assumed  to 
have  the  same  effect  upon  the  carbon  atoms  of  the  ring  as  has  been 
described  in  the  case  of  a  positive  element  : 

SO3H,  NO2,  CHO,  CH=NO2H  (in  phenylnitromethane), 

COOH,  C02  alkyl,  CONH2,  CO  alkyl,  CO-COOH, 

v 
=COH  (in  triphenyl  carbinol),  CN,  CC13,  NH3X, 

NH2  alkyl  X,  NH(alkyl)2X,  NH2  acyl  X 

while  the  following  atoms  and  groups  are  assumed  to  exert  a  negative 
influence 

in       in  in 

F,  Cl,  Br,  I,  OH,  0-alkyl,  0-acyl,  NH2,  NH-alkyl,  N(alkyl)2, 

in  in     in 

NH  acyl,  N=N,  CH3,  CH2  alkyl,  CH(alkyl)2,  C(CH3)3, 

CH2C1,  CH2ON02,  CH2S03H,  CH2NH2,  CH2CN,  CH2COOH, 
CH2CH2COOH,  CH=CHCOOH,  CH=CHN02, 
C=C-COOH,  C6H5." 


228 


THEORIES  OF  ORGANIC  CHEMISTRY 


If  the  three  nitro-brom  derivatives  of  benzene  are  examined  it  will 


III 

be  observed  that  the  first  two  compounds  in  which  the  negative  bromine 
atom  is  represented  as  in  union  with  a  positive  carbon  atom  (I  and  II), 
contain  reactive  bromine  while  the  third  (III),  in  which  the  negative 
bromine  is  in  union  with  a  carbon  atom  possessing  a  like  charge,  con- 
tains indifferent  bromine.  In  other  words,  ortho  and  para-substituted 
bromine  is  more  reactive  than  raeta-substituted  bromine,  and  the  same 
general  rule  holds  for  other  atoms  and  groups,  as  for  example,  the 
alkoxyl  and  alkylamino  radicals.  In  the  light  of  these  considerations 
it  is  easy  to  understand  why  only  three  of  the  chlorine  atoms  present  in 
1,  2,  4,  6-tetrachlor-3,  5-dinitrobenzene 


are  chemically  reactive  while  the  fourth,  namely  that  in  the  1-position, 
is  unreactive. 

Vorlander's  conception  lends  itself  to  the  satisfactory  interpretation 
of  a  great  many  other  facts,1  which  are  included  in  Holleman's  rules  in 
regard  to  substitution.  This  theory  may  also  be  applied  to  explain  the 
close  analogy  which  exists  between  pyridine  and  mono-substitution 
products  of  benzene  as  is  apparent  from  a  study  of  the  following  formula: 

(-) 

/N\ 

+  /  -  \  + 
HC+  +CH 

+  I          1  + 
HC_   _CH 


H 


Ber.,  52,  279  (1919);  also  Meisenheimer,  Ber.,  53,  361  (1920). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE        229 

The  only  difference  is  that  in  the  case  of  pyridine  the  negative  charge 
on  the  nitrogen  is  represented  as  free  while  in  the  case  of  the  corre- 
sponding carbon  atom  in  benzene  the  negative  charge  is  neutralized  by 
the  positive  charge  on  a  hydrogen  atom.  The  addition  of  acids  and  of 
alkyl  halides  to  pyridine  acts  to  change  the  charge  on  the  nitrogen 
from  negative  to  positive.  It  is  easy  to  understand  in  terms  of  this 
conception  why  the  hydrogen  atoms  of  the  methyl  group  are  so  reactive 
in  quaternary  alkyl  derivatives  of  picoline: 


yv\   4.      sjCLU^yi 
/N\ 

HC+    +CH-CH3 
HC_    _CH 


H 

The  reader  is  referred  to  the  writings  of  A.  von  Weinberg  1  for  a  con- 
sideration of  the  kinetic  aspects  of  the  subject. 

Some  very  important  discoveries  have  been  made  recently  in  regard 
to  the  coupling  of  diazo-compounds  with  substances  which  contain  the 
atomic  grouping 


In  the  study  of  this  reaction  a  variety  of  opinions  has  been  expressed  in 
regard  to  the  mechanism  involved  in  the  process.  For  example,  some 
chemists  have  regarded  the  reaction  as  one  of  double  decomposition  in 
the  sense  (I) : 

-c=o  -c=o 

->    H20+ 

-C  |  H+HO  |  N2R  -C-N=NR 

H~  H 

I 

Others  have  assumed  that  rearrangements  of  the  keto  into  the  enol  modi- 
fications were  always  preliminary  to  the  coupling  of  these  substances 
with  diazo-compounds.  In  this  case  any  one  of  three  other  equations 
is  possible:  II.  The  hydrogen  which  is  in  union  with  the  carbon  atom 
holding  the  enol  group  may  be  directly  substituted 

— C-OH  — C— OH 

||     ->     H20+ 

-C-  |  H+HO  |  -N2R  -C— N=N-R 

II 

ifier.,  62,  928  and  1501  (1919). 


230  THEORIES  OF  ORGANIC  CHEMISTRY 

III.  The  hydrogen  of  the  enol  group  may  itself  react  in  which  case  an 
ether  of  diazobenzene  would  be  formed  as  a  primary  product,  but 
would  probably  rearrange  to  give  a  true  azo-compound : 


— C— 0  |  H+HO  |  N2R                       -C— ON2R  -COH 

II                                  -  H20+          ||  || 

— CH                                                      -CH  -C-N=NR 

III                                           (a)  (6) 

IV.  Addition  of  the  diazo-compound  may  take  place  as  the  primary 
reaction  and  this  may  then  be  followed  by  the  splitting  off  of  water: 


/OH 


— C— OH     HO 


—a 


\j Vy-L-L  J.J.V7  \      /-\TT  V> \Jii 

I  +1  -  H          -»    H2°   +      II 

C— H  N2R  n/-  -C— N= 


IV  N=NR 


— C— OH 


=NR 


In  an  effort  to  reach  a  decision  in  this  matter  O.  Dimroth  1  investi- 
gated the  action  of  diazo-compounds  upon  desmotropes  such  as  acetyl- 
dibenzoylmethane,  tribenzoylmethane,  mesityloxidoxalic  ester,  a  and  /3 
diacetosuccinic  ester,  etc.  His  results  showed  that  the  enol  form 
reacted  in  every  case  while  the  keto  modifications  were  entirely 
inert.  This  eliminates  the  first  equation  (I)  as  affording  a  possible 
interpretation  of  the  mechanism  of  this  reaction.  The  second  equation 
(II)  can  also  be  discarded  since  it  has  been  observed  that  substances 
which  contain  the  combination 

I       I 
RC=COH 

couple  with  diazo-compounds  as  readily  as  substances  which  contain 
the  combination 

I       I 
HC=COH 

This  narrows  the  number  of  possibilities  to  the  last  two  equations, 
and  of  these  IV  was  rejected  and  III  accepted  as  a  result  of  the  following 
considerations:  It  was  observed  that  the  enol  modifications  of  tri- 
benzoylmethane reacted  with  diazo-compounds  to  give  a  yellow  product 
and  that  this  upon  heating  was  transformed  first  into  a  red  and  then  into 
a  white  substance.  -The  yellow  addition  product  behaves  like  a  true 
diazo-compound  in  that  it  decomposes  with  the  evolution  of  nitrogen 
when  boiled  with  alcohol.  It  also  resembles  a  true  diazo-compound 
in  that  it  couples  with  /3-naphthol,  and  on  reduction  gives  a  mixture  of 
iBer.,  40,  2404,  4460  (1907);  41,  4012  (1908). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE        231 

phenylhydrazine  and  tribenzoylmethane.  The  change  from  a  yellow 
to  a  red  compound  on  heating  may  be  interpreted  as  due  to  the  forma- 
tion of  an  azo-compound  (Ilia  ->  III6) .  The  change  from  the  red  to  the 
colorless  isomer  is  somewhat  more  difficult  to  formulate,  but  has  been  ex- 
plained by  Dimroth  as  due  to  the  formation  of  the  benzoylphenylhydra- 
zone  of  diphenyltriketone. 

/C6H* 
(C6H5CO)2C=N-N< 

XX)-C6H5 

The  mechanism  of  the  complete  reaction  may  therefore  be  formu- 
lated as  follows: 


C6H5C.O|H+HO|.N2C6H5    _>         C6H5CO  -  N2C6H5    -> 
(C6H5CO)2C  (C6H5CO)2C 

Yellow  diazo-ether  (a) 

C6H5C=0  C6H5C=0 

I  I  /C6H5 

(C6H5CO)2C-N=NC6H5    ->    C6H5CO-C=N-N< 

XCOC6H5 

Red  azo-compound  (6)  Colorless  hydrazone 

Dimroth  and  Hartmann  have  succeeded  in  isolating  a  number  of 
ether  like  bodies  which  form  as  intermediate  products  in  the  process  of 
coupling  diazo-compounds  with  phenols.  These  also  rearrange  upon 
heating  to  give  what  seem  to  be  the  corresponding  azo-compounds. 
For  example  p-brombenzene-diazonium  chloride  couples  with  p-nitro- 
phenol  to  give  the  oxygen  derivative: 

' V      -N02 
and  this  then  rearranges  on  heating  to  give 


Br— < 


N02 

p-brombenzene-azo-p-nitrophenol 


These  investigations  of  Dimroth  and  his  students  have  helped  to 
simplify  the  problem  by  limiting  the  number  of  interpretations  which 
are  possible  in  explaining  the  mechanism  of  these  reactions.  They 


232  THEORIES  OF  ORGANIC  CHEMISTRY 

have  also  served  to  bring  out  the  analogy  between  the  action  of  diazo- 
compounds  upon  phenols  and  upon  amines. 

Further  important  contributions  to  the  same  general  problem  have 
been  made  by  Kurt  Hans  Meyer.  These  developed  naturally  as  a 
result  of  Meyer's  investigations  in  the  field  of  enol-keto  tautomerism,1 
in  the  course  of  which  he  discovered  that  the  enol  group  is  one  of  the 
most  reactive  complexes  known  in  organic  chemistry.  For  example, 
if  compounds  of  the  general  formulas 

H    H  H     OH 

II  II 

R.C=C-R'     and     RC=C— R/ 

are  compared  it  is  found  that  those  belonging  to  the  second  class  react 
much  more  readily  with  bromine,  nitrous  acid,  aldehydes,  etc.,  than 
those  of  the  first  class.  This  increased  activity  cannot  be  due  to  the 
mobility  of  the  enol  hydrogen  since  this  may  be  replaced  by  alkyl 
without  affecting  the  reaction.  Thus  K.  H.  Meyer  and  Lenhardt  have 
found  that  enol  ethers  of  the  general  formula 

H     O  Alkyl 
R— C=C— R' 

are  just  as  reactive  as  the  free  enol  combination.  They  have  also 
demonstrated  that  in  neither  case  is  the  reactivity  due  to  the  presence 
of  negative  groups  in  the  molecule  nor  to  any  tendency  of  the  substance 
to  ionize.  They  conclude,  therefore,  that  it  must  be  a  function  of  the 
ethylene  linkage  and  assume  that  the  reactivity  of  this  part  of  the 
combination 

H\ 
>C=C 

w 

is  enormously  increased  by  the  substitution  of  OH  or  of  OR.  This 
type  of  ethylene  linkage  is  referred  to  by  Meyer  as  "  an  active  double 
bond."  2 

In  the  course  of  these  investigations  Meyer  made  the  remarkable 
discovery  that  diazo-compounds  couple  as  readily  with  phenol-ethers 
as  with  the  free  phenols,  and  that  in  the  former  case  addition  takes 
place  without  the  splitting  off  of  alkyl  groups.  Meyer  succeeded  in 

^nnalen  der  Chemie,  380,  212  (1911);  398,  49  (1913);  Ber.,  44,  2718  (1911); 
45,  2843  (1912)  Ber.,  47,  1741  (1914). 

2  Annalen  der  Chemie,  398,  71  and  80  (1913). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE        233 


preparing  well-defined  addition  products  in  the  case  of  a  large  number 
of  phenol-ethers  so  that  the  reaction  came  to  possess  general  signifi- 
cance. It  was,  moreover,  attested  to  by  other  investigators.1 

These  discoveries  make  it  impossible  to  explain  the  mechanism  of 
the  reaction  by  supposing  that  it  takes  place  with  the  formation  of 
diazo-ethers  as  intermediate  products,  since  this  would  involve  the 
hydrolysis  of  the  phenol  ether  and  it  has  been  observed  that  alkyl 
groups  are  not  split  off  during  the  process  of  coupling.  It  would,  there- 
fore, seem  to  follow  that  coupling  in  such  cases  must  represent  the 
addition  of  the  diazo-body  to  a  so-called  active  double  bond. 


OCH3 


H20 


JN.C.H. 


The  process  of  coupling  thus  appears  to  be  exactly  analogous  to  bro- 
mination : 


OCH2 


H3CO 


+    Br. 


H-     BHr 


Under  certain  circumstances  the  addition  product  may  lose  alcohol 
instead  of  water  in  which  case  oxazo-compounds  are  formed.  Indeed 
both  reactions  may  take  place  simultaneously  as  is  expressed  by  the 
following  equations: 


N-NH 


-N=NP 


P.  Karrer2  has  recently  observed  that  alkyl  groups  are  sometimes 
split  off  during  the  process  of  coupling  diazonium  salts  with  tertiary 

1  K.  von  Auwers  and  Michaelis,  Ber.,  47,  1275  (1914). 
2Ber,  48,  1398(1915). 


234  THEORIES  OF  ORGANIC  CHEMISTRY 

amines.  Thus,  for  example,  the  amyl  group  is  eliminated  when 
N-diisoamyl  aniline  is  used  in  the  reaction.  In  order  to  explain  this 
phenomenon  Karrer  assumed  that  the  reaction  took  place  in  two  stages, 
the  first  of  which  involved  direct  addition  to  the  nitrogen.  The  addition 
product  which  was  formed  in  this  way  was  then  supposed  to  undergo 
rearrangement  to  an  azo-compound.  Under  normal  conditions  rear- 
rangement was  accompanied  by  the  elimination  of  one  molecule  of 
halogen  acid  but  under  other  conditions  the  elimination  of  a  molecule 
of  alkylhalide  was  assumed  to  be  possible. 


-N(R)2+XN2Aryl 


->    Ar-N=N— < 

This  phenomenon  is  obviously  analogous  to  that  which  has  been 
described  as  taking  place  in  the  case  of  certain  phenols  and  phenol 
ethers.  The  reaction  in  the  latter  case  was,  therefore,  explained  by 
assuming  that  the  product  which  is  formed  by  the  primary  addition  of 
the  reacting  molecules  belongs  to  the  class  of  quaternary  oxonium  com- 
pounds. Since  addition  products  which  are  formed  in  this  way  are  in 
their  nature  difficult  to  isolate,  the  reaction  has  recently  been  studied 
from  the  standpoint  of  kinetics  by  A.  Skrabal l  and  O.  Dimroth.2  The 
data  which  were  obtained  as  a  result  of  measuring  the  rates  of  reaction 
in  the  case  of  a  number  of  different  phenols,  showed  that  the  velocity 
with  which  coupling  takes  place  varies  greatly  and  that  it  seems  to 
depend  both  upon  the  nature  and  the  position  of  the  group  substituting 
in  the  phenol.3  Thus,  for  example,  weakly  acidic  phenols  with  sub- 
stituents  in  the  ortho-  and  para-positions  react  with  great  ease  to  form 
diazo-ethers  but  do  not  give  azo-compounds.  The  fact  that  the  forma- 
tion of  azo-compounds  thus  seems  to  depend  upon  the  presence  of  ring 
hydrogen  in  the  ortho-  and  para-positions  tends  to  support  Karrer's 
view  that  these  substances  result  from  a  process  of  intramolecular 
rearrangement. 

These  facts  have,  however,  been  interpreted  in  quite  a  different 
manner  by  K.  H.  Meyer,  who  assumes  that  in  both  types  of  coupling 
which  have  been  described,  addition  depends  upon  the  presence  of 
unsaturated  ethylene  linkages  and  not  upon  the  presence  of  hydroxy  or 
amido  groups.  According  to  this  conception  it  should  be  possible  to 

1  Monatsh.,  Chemie,  37,  137  (1916);  38,  29  and  following  (1917). 

2Ber.,  60,  1534  (1917). 

3  Compare  Auwers  and  Borsche,  Ber.,  48,  1716  (1915). 


QUESTION  AS  TO  THE  CONSTITUTION  OF  BENZENE        235 

couple  unsaturated  hydrocarbons  with  diazo-compounds.  Acting  upon 
this  hypothesis  Meyer  has  in  fact  made  the  remarkable  discovery  l 
that  hydrocarbons  such  as  butadiene,  CH2==CH-CH==CH2;  a-methyl- 
butadiene,  CH3CH=CH  •  CH=CH2 ;  and  /3-7-dimethylbutadiene, 
CH2=C(CH3)-C(CH3)==CH2,  are  able  to  couple  with  nitrobenzene 
diazonium  hydrate  in  acetic  acid,  in  alcohol,  and  even  in  aqueous 
solution,  to  give  true  azo-compounds.  A  very  careful  investigation  of 
the  product  derived  from  0-7-dimethylbutadiene  showed  that  it  cor- 
responded to  the  formula 

CH3    CH3 
02N  •  C6H4N=N  •  CH=C C=CH2 

and  that  it  closely  resembled  the  aromatic  azo-compounds  in  its  chem- 
ical properties.  Since  butadiene  also  reacts  readily  with  diazotized 
dinitroaniline  to  give  a  true  azo-compound,  namely, 

(NO2)2C6H3N=N  •  CH=CH  •  CH=CH2 

the  reaction  cannot  be  explained  on  the  assumption  that  it  is  due  to  the 
reactivity  of  a  methyl  group  which  is  in  union  with  ethylene  carbon. 
The  explanation  brought  forward  by  Meyer  supposes  that  the  transfor- 
mation takes  place  according  to  the  following  scheme: 

CH3CH...  HO 

CH2...  N=N-R 

CH3-CHOH  CH3-CH 

->  I  ->    .  I  +  H2° 

CH2-N=N-R  CH-N=NR 

I  II 

although  it  was  never  actually  possible  in  any  given  case  to  effect  a 
separation  of  the  intermediate  product  represented  by  the  first  (I)  of 
the  above  formulas. 

Meyer  is  strongly  of  the  opinion  that  while  it  is  desirable  to  be  able 
to  use  a  single  type  of  transformation  for  the  interpretation  of  all 
coupling  reactions,  it  is  not  possible  to  do  this  at  the  present  time  since 
at  least  two  general  types  of  reaction  occur: 

I.  Coupling  by  means  of  oxygen  and  nitrogen,  with  the  formation 
of  an  ammonium  or  oxonium  salt. 

II.  Coupling  by  means  of  unsaturated  carbon  linkages. 

iBer.,  62,  1468(1919). 


236 


THEORIES  OF  ORGANIC  CHEMISTRY 


The  particular  type  of  reaction  which  occurs  in  any  given  case  can 
be  determined  only  as  the  result  of  experiment. 

The  coupling  of  tertiary  amines  with  diazonium  salts  is  interpreted 
by  Meyer  as  belonging  to  the  second,  and  not,  as  Karrer  assumes,  to 
the  first  type  of  reaction  and  is  represented  as  taking  place  according 
to  the  scheme; 


R   R 


Rearranges 
to  give  ^      /^ 


H    N=N.R 
NR2 


=N.R 


The  question  as  to  the  possibility  of  the  existence  of  meta-qumoid 
derivatives  of  benzene  has  been  reopened  in  recent  years  as  a  result  of 
the  investigations  of  O.  Stark  1  who  has  obtained  a  hydrocarbon  by  the 
reduction  of  tetraphenylmethoxylene  dichloride,  to  which  he  has  assigned 
the  following  formula: 


The  correctness  of  this  expression  has  been  challenged  by  W.  Schlenk 
and  M.  Brauns.2  who  maintain  that  this  hydrocarbon  reacts  in  a  manner 
which  is  characteristic  of  triphenylmethyl  and  its  derivatives  and  that 
it  therefore  corresponds  to  the  formula 


<: 


C(C6H5)2 


(Cells)  2 


This  discussion  fails,  however,  to  bring  the  question  of  the  existence 
of  weta-quinoids  to  a  satisfactory  settlement.3 

^er.,  46,  659,  2252,  2542  (1913);  47,  125  (1914). 

2Ber.,  48,  661,  716(1915). 

3  Compare  E.  Bamberger,  Ber,,  48,  1354  (1915). 


CHAPTER  XI 
TAUTOMERISM1   AND   DESMOTROPISM 

IN  the  course  of  the  development  of  structural  organic  chemistry 
the  rule  was  formulated  that  every  pure  organic  compound  possesses 
a  fixed  and  definite  configuration,  and  that  this  particular  arrangement 
of  atoms  within  the  molecule  can  be  expressed  by  one  and  only  one 
structural  formula,  assuming  of  course  that  no  stereoisomerism  is 
possible.  Such  a  constitutional  formula  was  supposed  to  express  the 
collective  reactions  of  the  substance.  In  1880,  however,  individual 
exceptions  to  this  rule  came  to  be  recognized. 

In  1882  A.  von  Baeyer  and  Oekonomides2  discovered  that  isatin, 
CgHsC^N,  when  treated  with  acetic  anhydride  gives  an  acetyl  deriva- 
tive in  which  the  acetyl  group  is  in  union  with  nitrogen  and  which,  on 
the  basis  of  its  synthesis  and  properties,  possesses  the  formula: 

CO 


N-COCH3 

Reasoning  from  this  fact,  isatin  itself  should  have  the  formula  I. 

CO 


NH 
I 

Now  free  isatin  gives  a  mono  silver  salt  which  when  treated  with  methyl 
iodide  passes  into  an  imido  ester  having  the  formula: 

CO 


N 

iSchaum,  Annalen  der  Chemie,  300,  205  (1898). 
2fier.,  15,  2093  (1882);    16,  2193  (1883). 
237 


238  THEORIES  OF  ORGANIC  CHEMISTRY 

from  which  it  follows  that  isatin  itself  should  possess  the  isomeric 
structure  containing  an  hydroxyl  group  as  represented  in  formula  1  II, 

CO 


. 

N 


II 

According  to  all  previous  experience  these  two  formulas,  I  and  II, 
should  correspond  to  two  separate  and  distinct  chemical  substances 
which  form  as  the  result  of  replacing  the  acetyl  and  methyl  respectively 
by  hydrogen.  Observation  has  proved,  however,  that  one  and  the 
same  chemical  individual,  namely  isatin,  is  obtained  in  both  cases.  It 
follows  therefore,  that  this  cyclic  compound  is  capable  of  reacting  in 
the  sense  of  two  different  but  isomeric  formulas,  laciam  and  lactim,  or 
in  other  words  it  possesses  a  dual  nature. 

Baeyer  and  his  students  further  made  the  discovery  that  this 
property  of  isatin  is  due  to  the  activity  of  a  hydrogen  atom  not  in  the 
benzene  ring,  and  that  such  a  phenomenon  ceased  to  be  observed  after 
this  particular  hydrogen  had  been  replaced  by  other  substituents. 

As  a  result  of  his  experiments  Baeyer  became  convinced  that  isatin 
itself  possesses  the  lactim  formula 

CO 


N 

but  that,  in  the  course  of  certain  reactions,  it  is  transformed  into  an 
isomeric  compound  corresponding  to  the  laciam  formula,  and  that  this 
then  reacts  to  give  derivatives  of  the  latter  type.  Baeyer  called  this 
supposedly  unstable  isomer  the  pseudo  form  of  isatin. 

Similar  phenomena  were  observed  by  Baeyer  and  Comstock2  in 
the  case  of  oxindole,  by  von  Pechmann  in  the  case  of  hydroxynicotinic 
acid,3  by  Hantzsch  in  the  case  of  methyl  pseudo-lutidosiyril,4  by  Knorr 
and  Antrick  in  the  case  of  hydroxyquinaldine,5  and  by  Friedlander  and 
Weinberg  in  the  case  of  a-hydroxyquinoline  (carbostyril).6  In  general, 
Baeyer's  idea  of  a  pseudo  form  was  accepted  and  abnormal  products 
were  regarded  as  derived  from  it.  This  view  involved  the  assumption 

1  Heller  has  recently  discovered  another  isomer  of  isatin.     See  Ber.,  49,  2757 
(1916);  60,511  (1917). 

2  Ber .,16,  1704,2188(1883). 

3  Ber.,  17,  2387  (1884);  18,  317  (1885). 

4  Ber.,  17,  1026,  2903  (1884). 
6  Ber.,  17,  2873  (1884). 

6  Ber .,18,  1528(1885). 


TAUTOMERISM  AND  DESMOTROPISM 


239 


that  such  pseudo-compounds  actually  exist  momentarily  but  rearrange 
almost  instantly  into  normal  or  stable  forais: 

CO  CO  CO 

C6H4/\CO     -»      C6H4/\CO  -»  C6H4/\COH 
N-COCH3  NH  N 

Acetylpseudo-isatin  Pseudo-isatin  Isatin 

/V\  /\/\  /\/\ 


/\OH 


N 

carbostyril 


N-CHs 

Methyl-pseudo-carbostyril 


X 

NH 

Pseudo-carbostyril 


Conrad  Laar1  immediately  advanced  a  quite  different  interpreta- 
tion based  on  the  assumption  that  one  and  the  same  chemical  individual 
may  possess  two  or  more  different  structural  formulas.  This  explana- 
tion is  plausible  since  all  observed  phenomena  of  this  type  may  be 
accounted  for  on  the  assumption  that  a  hydrogen  atom  is  continually 
moving  from  one  position  to  another  within  the  molecule  and  that  this 
change  involves  a  rearrangement  in  the  affinity  relations  existing  between 
the  other  atoms.  The  hypothesis  is  moreover  supported  by  the  follow- 
ing considerations :  according  to  the  kinetic  theory  of  gases  the  atoms 
within  the  molecules  as  well  as  the  molecules  themselves  are  supposed  to 
be  in  a  state  of  constant  motion.  The  hydrogen  atom  in  particular  is 
regarded  as  possessing  a  relatively  great  mean  velocity  and  as  being  ex- 
tremely active.  There  will  be  a  characteristic  oscillation  of  the  atom  only 
in  the  case  of  gaseous  molecules,  while  in  all  other  states  of  aggregation  a 
more  or  less  irregular  movement  forward  and  back  may  be  imagined  as 
resulting  from  constant  encounters  with  other  atoms.2  Even  in  solid 
substances,  however,  the  motion  of  the  hydrogen  atom  may,  under 
certain  conditions,  become  regular  and  in  a  sense  like  the  motion  of  a 
pendulum.  The  two  atomic  groupings  within  the  molecule,  which  may 
be  imagined  as  representing  the  extremes  of  atomic  movements  of  this 
type,  would  thus  appear  and  disappear  periodically,  and  in  the  case  of 
isatin,  might  be  represented  by  the  scheme: 


C6H4 


CO 

/ 


CO 


0 


H 


•OH 


.,  18,  648  ((1885);    Ber.,  19,  730  (1886);     Kekulc,  Annalen  der  Chemie, 
162,  77  (1872);  Butlerow,  Annalen   der  Chemie,  189,77(1877);  Erlenmeyer,  Ber., 
13,  309  (1880);  14,  320  (1881);  Knorr,  Annalen  der  Chemie,  279,  212  (1894). 
2  Ber.,  19,  732(1886). 


240  THEORIES  OF  ORGANIC  CHEMISTRY 

Thus  the  substance  isatin  could  be  regarded  as  possessing  both  of  these 
formulas,  or  possibly  one  representing  a  mean  of  these  two  conditions, 
viz., 

CO 


Moreover  since  the  constitution  of  a  substance  cf  this  type  changes 
periodically,  it  readily  passes  into  derivatives  having  one  or  the  other 
formula. 

Different  structural  formulas  in  such  cases  indicate  not  isomeric 
substances  but  a  single  substance  having  a  dual  nature,  and  in  order  to 
distinguish  this  particular  phenomenon  Laar  called  such  hypothetical 
isomers,  tautomers.  All  observed  cases  of  tautomerism  were  systemat- 
ically arranged  under  classes  of  the  so-called  diads  and  triads.  In 
general,  the  atomic  groupings  which  exhibit  most  frequently  this 
behavior  are  the  following: 

-C=O          — COH  —  C=O      — COH 


.if         JL       and       ' 


— NH  — N  >CH          >C 

Lactam  Lactim  Keto  Enol 

While  the  word  tautomerism  has  become  well  intrenched  in  organic 
chemistry  Laar's  particular  interpretation  of  the  phenomena  has  not, 
however,  found  universal  acceptance  among  chemists.  -On  the  con- 
trary it  has  been  generally  assumed  that  tautomeric  substances  fre- 
quently possess  definite  constitutions,  but  that  these  groupings  undergo 
change  during  the  course  of  certain  reactions.  P.  Jacobson  1  has,  there- 
fore, proposed  the  term  desmotropism  as  offering  a  more  suitable 
expression  than  that  suggested  by  Laar.2 

A.  Hantzsch  and  F.  Herrmann3  are  of  the  opinion  that  the  terms 
tautomerism  and  demotropism  may  both  be  used  to  advantage,  the 
former  to  denote  the  fact  that  a  given  chemical  compound  exhibits  a 
dual  nature,  and  the  latter  to  signify  that  a  complete  and  definite 
rearrangement  of  affinities  within  the  molecule  has  actually  taken  place. 
According  to  this  conception  the  formulas  representing  different  desmo- 
tropic  conditions  bear  the  same  relation  to  each  other  as  do  the  formulas 
of  isomeric  substances.  Desmotropism  may  be  distinguished  readily 
from  isomerism  by  the  characteristic  that  only  one  desmotropic  modi- 

1  Ber.,  20, 1732  (1887);  21,  2628  (1888). 

2  Compare  Michael,  Annalen  der  Chemie,  363,  20  (1908). 

3  Ber.,  20,  2802(1887). 


TAUTOMERISM  AND  DESMOTROPISM  241 

fication  is  stable  under  a  given  set  of  physical  conditions,  so  that  a 
change  in  the  physical  conditions  is  necessary  in  order  to  insure  the 
stable  existence  of  the  second  isomer.  This  is  true  at  least  of  desmo- 
tropic  substances  in  the  solid  state. 

In  1896  L.  Claisen,1  W.  Wislicenus,2  and  others  succeeded  in  isolat- 
ing pairs  of  isomers  corresponding  to  the  atomic  groupings 

—0=0  -COH 

and 
>CH  >C 

These  substances,  which  in  many  cases  had  previously  been  regarded  as 
desmotropic  in  character,  were  now  found  capable  of  existing  in  two 
forms  and  both  modifications  were  found  to  be  stable  under  ordinary 
conditions.  In  fact,  in  many  instances  they  were  readily  transformed 
one  into  the  other. 

For  example,  Claisen  obtained  a  substance  by  the  action  of  benzoyl 
chloride  upon  the  sodium  salt  of  benzoylacetone,  which  corresponded 
in  chemical  properties  to  the  formula: 

CH3— C=C(COC6H5)2 
OH 

This  compound  melted  at  101-102°,  decomposed  alkali  carbonates  in 
the  cold,  and  when  dissolved  in  alcohol  and  treated  with  ferric  chloride 
gave  a  red  coloration  indicating  the  presence  of  a  free  hydroxyl  group. 
When  this  strongly  acidic  body  was  heated  at  80°-90°,  or  when  crystal- 
lized from  hot  dilute  alcohol,  it  immediately  rearranged  to  give  another 
substance  which  melted  at  107-110°.  This  new  compound  was  found  to 
have  the  same  percentage  composition  and  the  same  molecular  weight  as 
the  original,  but  corresponded  in  chemical  properties  to  the  true  ketone 
combination  represented  by  formula,  CHsCO-CHXCOCeH^.  Thus 
while  the  product  melting  at  101-102°  was  strongly  acid  in  character, 
dissolving  in  alkali  carbonates  in  the  cold,  the  compound  melting  at 
107-110°  was  neutral  and  not  only  did  not  react  with  alkali  carbonates 
but  did  not  even  react  with  potassium  hydroxide.  Upon  long  standing 
with  alkalies  the  new  substance,  it  is  true,  slowly  passed  into  solution, 
but  when  this  solution  was  acidified,  the  isomeric  acid  melting  at  101- 
102°  was  precipitated.  It  was  thus  possible  by  simply  heating,  or  by 
crystallization  from  hot  alcohol,  or  by  solution  in  alkali  and  subsequent 
precipitation,  to  pass  at  will  from  one  modification  to  the  other. 

1  Annalen  der  Chemie,  291,  25  (1896). 

2  Annalen  der  Chemie,  291,  147  (1896). 


242  THEORIES  OF  ORGANIC  CHEMISTRY 

Thus  it  was  demonstrated  that  substances  having  the  desmotropic 
formulas 

CHaC-OH  CH3C  =  O 

||  and  | 

C  -  (COC6H5)2  CH(COC6H5)2 

could  be  prepared,  could  exist  side  by"  side  under  ordinary  conditions, 
and  finally  could  be  transformed  one  into  the  other. 

Claisen  succeeded  in  isolating  similar  pairs  of  isomers  in  the  case  of 
many  other  desmotropic  substances  and  observed  characteristic  differ- 
ences in  the  chemical  properties  of  the  two  modifications.  Substances 
containing  hydroxyl  groups  were  referred  to  as  the  enol,  and  those 
containing  carbonyl  as  the  keto  forms  respectively.  Structurally  the 
two  modifications  of  this  series  of  ketones  may  be  expressed  as  follows: 

OH  O 


C-R 


C-COR'  and  CH—  COR' 

\X)R'  \COR' 

Enol-form  (a-form)  Keto-form  (/S-form) 

In  general,  the  enol  modifications  were  found  by  Claisen  to  be  strongly 
acidic  and  soluble  in  carbonate  solution  with  effervescence.  They  also 
gave  a  red  coloration  when  allowed  to  interact  with  ferric  chloride  and 
reacted  directly  with  diazobenzene.  The  keto  modifications,  on  the 
other  hand,  are  neutral  compounds  and  insoluble  in  alkali  carbonate 
solution  but  soluble  in  cold  solutions  of  strong  alkalies.  On  long  stand- 
ing in  alkaline  solution  at  ordinary  temperature  the  keto  forms 
undergo  intramolecular  transformation  into  the  enol  modifications. 
In  other  words  there  is  a  tendency  for  such  molecules  to  become  acidic. 
This  arrangement  is  accelerated  by  the  application  of  heat.  The 
keto  forms  give  no  coloration  with  ferric  chloride  solution  and  fail 
to  react  with  diazobenzene. 

Tautomeric  compounds  having  the  above  general  formulas  show 
great  variation  in  stability  and  this  seems  to  depend  upon  the  nature 
of  the  substituents  represented  by  R  and  R'.  Thus  both  isomers,  in 
which  R  represents  the  methyl  group  and  R'  the  phenyl  radical, 

OH  O 

^C-CH3  /C.CHs 

C-COC6H5  and  CH—  COC6H5 


TAUTOMERISM  AND  DESMOTROPISM  243 

are  capable  of  existing  side  by  side  for  a  long  time  in  the  solid  state, 
the  enol  passing  into  the  keto  form  at  room  temperature  only  after  the 
lapse  of  weeks.  In  the  case  of  the  corresponding  phenyl  derivatives 
(R  and  R'=phenyl), 

OH  O 

x,C  •  CfiH5  xC  • 

C-COC6H5  and  CH— COC6H5 

\COCGH5 


the  enol  modification  is  so  unstable  that  it  is  exceedingly  difficult  to 
isolate.  Even  in  the  solid  state  and  at  room  temperature  it  changes 
into  the  keto  form  in  the  course  of  two  days. 

Other  combinations  corresponding  to  the  formulas 

OH  O 

y^-C '  CH3  sC '  CH3 

C-COCH3  and  CH— COCH3 

OH  0 

/,&  •  CH3  xC  •  CH3 

C-COCH3  and  CH— COCH3 


exist  only  in  the  enol  form,  the  isomeric  keto  modification  being 
usually  unstable  even  at  ordinary  temperatures.  In  other  words 
unsaturation  or  increase  in  negative  character  of  the  radicals  R  and  R7 
seems  to  favor  the  existence  of  tautomeric  isomers. 

Comparisons  of  this  kind  have  led  to  the  formulation  of  more  or  less 
definite  conclusions  in  regard  to  the  influence  exercised  by  various 
radicals  upon  the  relative  stability  of  the  atomic  groupings.  Claisen 
has  shown  conclusively  that  in  the  series 

CHAa,  CHA2B,  CHAB2,  CHB3 

where  A  represents  an  acetyl  and  B  a  benzoyl  group,  the  first  two  are 
stable  only  in  the  enol  form,  the  third  exists  in  both  modifications,  while 
the  fourth  possesses  a  stable  keto  but  an  unstable  enol  form.  Assuming 
the  acetyl  group  to  be  more  strongly  acid  in  character  than  benzoyl,  it 
may  be  said  in  the  words  of  Claisen  that  the  formation  of  the  enol  form 


244  THEORIES  OF  ORGANIC  CHEMISTRY 

is  favored  both  by  the  negative  character  of  the  acetyl  groups  and  by 
the  number  of  such  groups  in  union  with  the  methane  carbon  atom.1 

While  these  assumptions  of  Claisen  have  received  strong  experi- 
mental support  they  have,  nevertheless,  been  challenged  by  able  critics. 
Among  the  latter  is  Michael,2  to  whom  the  assumption  that  acetyl  is 
more  negative  than  benzoyl  has  from  the  beginning  seemed  very  improb- 
able. Michael  holds  that  the  stability  of  an  enol  form  does  not  neces- 
sarily increase  with  increase  in  the  number  of  negative  radicals,  and 
has  suggested  another  explanation  of  the  phenomena  of  desmotropism 
and  mesotropism  which  is  based  entirely  upon  considerations  of  the  law 
of  entropy  or  neutralization  law.  Michael 3  has  recently  shown  that 
the  keto  combinations  examined  by  Claisen  are  capable  of  existing 
in  more  than  two  isomeric  modifications.  For  example,  acetyldi- 
benzoyl  methane 4  and  the  corresponding  propionyl  compound  5  exist 
in  three  different  modifications  namely:  the  enol,  a  liquid  form,  and  two 
keto  forms  (  ft  and  7).  The  keio  form  (ft)  in  the  case  of  the  acetyl  com- 
pound 6  melts  at  107-110°  while  the  7-isomer  is  also  solid  and  melts  at 
146-149°.  The  ft  form  of  the  propionyl  derivative  melts  at  122.5° 
while  the  7-isomer  melts  at  152-3°.  The  latter  is  slowly  transformed 
into  its  /3-isomer  at  100°  and  is  much  less  easily  enolized  than  the  ft  form. 
In  the  case  of  the  acetyl  compound  the  two  isomeric  keto  forms  undergo 
a  reversible  reaction  without  enolization  in  acetic  acid,  acetic  anhydride, 
and  methyl  iodide.  The  isomerization  of  these  types  of  compounds  is 
probably  connected  with  the  spacial  arrangement  of  the  atoms  in  the 
molecule  and  is  due,  according  to  Michael,  to  hindered  rotation  of  the 
benzoyl  group  on  the  asymmetric  carbon  — CH —  as  expressed  by  the 
structural  formulas  below: 

C6H5COCH  COCH3  C6H5CO  •  CH  •  COCH3 

I  I 

0  =  C-C6H5  C6H5C  =  0 

(/3-form)  (-y-form) 

Claisen  7  also  observed  in  his  later  researches  that  the  condensation 
product  formed  by  interaction  of  diethyl  oxalate  with  mesityloxide 
exists  in  isomeric  modifications.  Each  is  capable  of  passing  into  the 

1  Annalen  der  Chemie,  291,  37  (1896). 

2  Annalen  der  Chemie,  363,  24  (1908);  compare  K.  Meyer,  Ber.,  45,  2849  (1912). 

3  Annalen  der  Chemie,  390,  30  (1912). 

4  Annalen  der  Chemie,  390,  46  (1912). 
6  Annalen  der  Chemie,  390,  68  (1912). 

6  Claisen,  Ber.,  27,  3182  (1894) ;  Knoevenagel,  Annalen  der  Chemie,  281,  62  (1894). 

7  Annalen  der  Chemie,  291,  39  (1896). 


TAUTOMERISM  AND  DESMOTROPISM  245 

other  and  their  relationships  may  therefore  be  expressed  structurally 
by  the  formulas: 


=  CHCOCH3     +     (COOC2H5)2 

OH 

^  \ 

\C=CH—  CO  CH=C-COOC2H5 
/ 

Enol-form  (a-form) 

and 


. 

>C==CHCOCH2CO  -  COOC2H5 
/ 


Keto-form  (/3-form) 

The  enol  modification  is  stable  at  temperatures  above  100°  and  inter- 
acts with  ferric  chloride  giving  a  red  coloration.  The  keto  form,  on  the 
other  hand,  is  stable  only  below  100°  and  does  not  interact  with  ferric 
chloride. 

Wislicenus1  discovered  as  early  as  1887  that  ethyl  formylphenyl- 
acetate,  resulting  from  the  condensation  of  ethyl  formate  with  ethyl 
phenylacetate,  exists  in  two  isomeric  forms  which  readily  rearrange 
each  into  the  other.  These  isomers  have  received  much  attention  and 
later  investigation  seems  to  show  that  they  also  are  structural  isomers 
of  the  enol-keto  type:2 

H  H 

C6H5  -  C=C  •  OH  C6H5CH  •  C=O 

I  I 

COOC2H5  COOC2H5 

In  the  case  of  these  substances  the  relationships  are  not  quite  as 
simple  as  in  the  case  of  the  triacetyl  derivatives  of  methane  and  as  a 
result  certain  abnormalities  appear.  Thus  the  ,8-form  dissolves  more 
readily  than  the  a-form  in  sodium  carbonate  solution  even  though  the 
former  is  neutral  while  the  latter  is  acid  in  character.  Moreover  since 
both  modifications  show  acid  properties  the  phenomenon  of  desmo- 
tropism  extends  also  to  their  salts.  L,  Knorr3  closely  followed  the 

1  Ber.,  20,  2933  (1887). 

2Ber,  28,  767  (1895);  Annalen  der  Chemie,  291,  147  (1896);  312,  34  (1900) 

3  Annalen  der  Chemie,  293,  70  (1896);  303,  133  (1898);  306,  332  (1899). 


246  THEORIES  OF  ORGANIC  CHEMISTRY 

publications   of   Claisen   and   Wislicenus   with   several   papers  which 
helped  to  clear  up  mooted  questions  in  the  matter  of  nomenclature. 

Recent  work  by  Michael '  leads  to  the  conclusion  that  we  have  to  deal 
with  a  more  complicated  type  of  isomerism,  in  the  case  of  ethyl  formyl- 
phenylacetate, than  is  expressed  structurally  by  the  enol  and  keto 
forms  alone.  He  finds  that  the  /8-ester  described  by  Wislicenus  as 
melting  at  70°  is  not  a  homogeneous  substance  and  is  a  mixture  consisting 
of  25  per  cent  of  the  ^-modification  melting  at  40°,  and  75  per  cent  of  a 
third  7-form  to  which  is  assigned  a  melting  point  of  100°.  According 
to  Michael  we  are  dealing  here  with  a- (liquid),  j8-  and  7-modifications  of 
ethyl  formylphenylacetate  and  all  these  are  monomeric  modifications  of 
the  enoZ-form.  Fuller2  has  made  an  examination  of  the  behavior  of 
these  three  forms  of  this  ester  and  studied  their  behavior  in  twenty-four 
different  solvents.  His  work  shows  that  there  is  no  relation  between  the 
dielectric  constants  of  solvents  and  their  isomerizing  power.  That  the 
a-modification  of  ethyl  formylphenylacetate  is  to  be  assigned  a  ketone 
structure  is  supported  by  the  fact  that  it  interacts  with  phosphorus 
pentachloride  to  form  a  /3-dichlor  derivative  as  is  expressed  in  the  fol- 
lowing equation ; 3 

OCH.CH(C6H5)COOC2H5+PC15  -» C12CH.CH(C6H5)COOC2H5. 

Very  complicated  isomeric  relationships  have  been  discovered  as 
the  result  of  the  investigation  of  dibenzoyl  and  diacetyl  succinic  esters, 
since  in  addition  to  stereoisomers,  structural  isomers  are  formed  which 
are  convertible  into  each  other: 

OH 


C6H5C=C  COOC2H5  C6H5CO  •  CH  COOC2H5 

and 
C6H5C=C  COOC2H5  C6H5CO  •  CH  COOC2H5 

OH 
OH 

CH3C=C-COOC2H5  CH3CO-CH  COOC2H5 

and 
CH3C=C  •  COOC2H5  CH3CO  •  CH  •  COOC2H5 

OH 

1  Annalen  der  Chemie,  391,  235  (1912). 
2Annalen  der  Chemie,  391,  275  (1912). 
3  Wislicenus,  Ber.,  61,  1366  (1918). 


TAUTOMERISM  AND  DESMOTROPISM  247 

Two  somewhat  similar  acetylangelica  lactones, 
OH 

CH3C=CH  •  C=C  •  CH3      and      CH3C=CH  •  CH— COCH3 

I  I  I 

0 CO  O—     —CO 

have  also  been  discovered  and  show  the  characteristic  properties  of 
enol  and  keto  isomers.  Examples  of  this  type  of  isomerism  continue 
to  be  discovered,  but  it  will  be  impossible  to  refer  to  more  than  a  few 
representative  types  in  this  text. 

Until  quite  recently  desmotropic  forms  had  not  been  isolated  in  the 
isatin  series.  R.  Pummerer,1  however,  claims  to  have  succeeded  in 
separating  two  isomeric  modifications  of  the  anil  of  isatin,  first  pre- 
pared by  Sandmeyer,  which  he  regards  as  the  lactam  and  lactim  modifi- 
cations respectively.  These  desmotropic  substances  are  convertible 
one  into  the  other  and  are  represented  structurally  by  the  following 
formulas  : 


NH 

and 


CO  CO 

Isatin-2-anil  Isatin-2-anilide 

We  are  dealing  here  with  a  rare  case  of  amidine  isomerism  rather  than 
one  involving  true  lactim  and  lactam  modifications  (  —  N=C-OH  and 
NH-CO)  and  it  still  remains  to  be  established  whether  the  correct 
explanation  of  their  desmotropic  properties  has  been  given.2  Desmo- 
tropic forms  of  phenylnitromethane  have  also  been  discovered  by 
Hantzsch  and  Schultze.3  These  substances  correspond  to  the  formulas 


and 

and  will  be  referred  to  in  detail  later. 

Although  a  great  variety  of  structural  isomers  containing  the  atomic 
grouping  —  CO-CH  —  have  been  prepared,   it  has  been  possible  in 

'Ber.,  44,  338  (1911). 

2  Wheeler  and  Johnson,  Am.  Chem.  Jour.,  31,  577  (1904).     See  also  Meldola, 
Eyre,  and  Lane,  Jour.  Chem.  Soc.,  83,  1185  (1903);  86,  1592  (1904). 
3Ber.,  29,  699,  2251  (1896). 


248  THEORIES  OF  ORGANIC  CHEMISTRY 

almost  every  instance  to  convert  one  modification  into  the  other.     It 
thus  happens  that  although  the  atomic  expressions 

—C-OH  -C=O  -C-OH  —  C=0 

+±  and  also  +± 

>C  >CH  — N  — NH 


were  originally  used  to  represent  only  the  end  phases  of  atomic  move- 
ments operating  within  one  and  the  same  substance,  they  are  now 
employed  to  represent  different  chemical  individuals,  albeit  these  must 
be  assumed  to  be  so  closely  related  as  to  pass  readily  from  one  into  the 
other.  Baeyer's  labile  and  so-called  "  pseudoforms  "  are  in  terms  of 
this  conception  actually  capable  of  existence.  In  order  to  honor 
Baeyer,  Claisen  proposed  the  substitution  of  the  term  "  pseudomerism  " 
for  tautomerism.  Pseudomerism  might,  he  suggested,  be  either 
absolute  or  relative,  the  former  covering  those  cases  where  only  one 
substance  is  known  and  the  latter,  cases  where  two  distinct  modifications 
have  been  isolated.1  It  has  also  been  suggested  by  von  Pechmann2 
that  the  expression  tautomerism  might  itself  very  well  be  qualified  by 
the  adjectives  "  functional  "  and  "  virtual."  Functional  tautomerism 
may  be  used  to  designate  structural  differences  between  two  isomeric 
modifications  where  these  are  characterized  by  marked  chemical 
differences,  such  as  acid  and  neutral  properties,  etc.  Examples  of  this 
are  to  be  found  among  the  various  enol-keto  and  lactim-lactam  modi- 
fications which  have  just  been  considered.  Virtual  tautomerism,  on 
the  other  hand,  may  be  applied  to  cases  where  both  isomeric  forms 
possess  the  same  constitution,  as  for  example,  among  the  amidine  and 
diazoamido  compounds. 

Accepted  nomenclature  at  the  present  time  follows  neither  of  the 
above  suggestions  but  is  founded  upon  proposals  made  by  Knorr  to 
which  reference  has  already  been  made.  The  terms  desmotropism 
and  tautomerism  are  both  retained,  the  former  being  used  to  designate 
actual  cases  of  structural  isomerism,  where  one  modification  has  been 
observed  to  pass  readily  into  the  other  as  the  result  of  the  transposition 
of  the  hydrogen  atom  and  a  redistribution  of  affinities  within  the  mole- 
cule. For  example,  the  two  isomeric  modifications  of  acetyldibenzoyl 
methane  (a  and  |8  forms)  are  desmotropic  substances.  The  term  tauto- 
merism, on  the  other  hand,  is  used  to  designate  cases  where  the  separa- 
tion of  the  desmotropic  varieties  has  not  actually  been  realized  and 
where  one  and  the  same  substance  reacts  in  the  sense  of  two  or  more 
structural  formulas  to  give  derivatives  which  are  isomeric.3  Even 

1  Annalen  der  Chemie,  291,  46  (1896). 

2Ber.,  28,  876  (1895). 

3  Tafel,  Zeitschr.  Elektrochemie,  23,  60  (1917). 


TAUTOMERISM  AND  DESMOTROPISM  249 

these  two  terms  do  not,  however,  cover  all  varieties  of  this  class  of 
phenomena. 

The  relationship  represented  by  the  atomic  groupings 

—0=0    * 
>CH 

is  frequently  very  complicated  and  the  conditions  which  govern  the 
transformation  of  one  isomer  into  the  other  are  in  many  cases  not 
thoroughly  understood.  Usually  one  form  is  much  less  stable  than  the 
other  and  tends  to  rearrange  even  at  ordinary  temperatures.  Thus  in 
the  case  of  the  two  substances 


CH3C-OH  C6H5C-OH 

II  and  || 

C(COC6H5)2  C(COC6H5)2 


the  former  rearranges  into  the  corresponding  keto  modification  only 
after  weeks,  while  the  latter  undergoes  a  corresponding  change  in  the 
course  of  a  few  days.  The  nature  of  the  substance  seems  to  be  a 
decided  factor  in  transformations  of  this  type.  Temperature  repre- 
sents another  important  factor.  Thus: 

CH3COH 

|| 
C-(COC6H5)2 

rearranges  into  the  corresponding  keto  form  in  a  few  minutes  at  90° 
instead  of  requiring  several  weeks  as  is  the  case  at  ordinary  tempera- 
tures. 

Since  these  and  similar  rearrangements  take  place  in  organic  solvents 
an  effort  has  been  made  by  Claisen,  W.  Wislicenus,  Kurt  H.  Meyer, 
L.  Knorr  and  others  to  follow  them  quantitatively.  This  has  been 
possible  because  of  the  well-known  fact  that  enol  modifications  of 
desmotropic  substances  give  color  reactions  with  ferric  chloride  while 
the  corresponding  keto  forms  do  not.  In  the  case  of  ethyl  formyl- 
phenylacetate  Wislicenus  made  the  observation  that  when  solu- 
tions of  equal  volume  but  of  different  concentrations  were  treated 
with  equal  small  quantities  of  ferric  chloride  the  intensity  of  the  colora- 
tion varied  with  the  concentration  of  the  enol  form.  He  also  noted 
that  when  equal  quantities  (a)  of  the  pure  enol  and  (b)  of  the  pure  keto 
modification  were  dissolved  in  equal  volumes  of  alcohol  and  treated 

1  Wislicenus,  "Uber  Tautomerie."     Ahrens'   Sammlung   chem.  u.  chem.-techn 
Vortrage,  2,  232  (1898). 


250  THEORIES  OF  ORGANIC  CHEMISTRY 

with  ferric  chloride,  the  intensity  of  coloration  became  ultimately  the 
same  in  both  cases.  This  happened,  of  course,  only  after  the  lapse  of  a 
period  of  time  since  the  immediate  effect  of  adding  ferric  chloride  was 
quite  different  for  the  two  solutions,  a  becoming  at  once  intensely 
colored  while  b  remained  colorless.  The  conclusion  seems  obvious 
that  in  alcohol  solution  both  phases  coexist  in  a  condition  of  dynamic 
equilibrium.  Equilibrium  in  such  a  system  depends  in  general  upon 
the  nature  of  the  solvent,  the  temperature,  and  the  concentration.  It 
has  been  observed,  for  example,  that  labile  modifications  of  desmotropic 
substances  may  be  crystallized  unchanged  from  certain  solvents  even 
when  the  solutions  have  been  warmed,  and  that  under  these  circum- 
stances and  within  definite  limits  of  temperature,  only  one  single  modi- 
fication exists.  In  other  solvents  both  forms  coexist  at  the  same  tem- 
perature and  in  such  cases  solution  must  involve  a  reversible  reaction, 
as  for  example, 


C6H5CO 

II  ^  I 

C  -  (COC6H5)2  CH(COC6H5)2 

This  finally  results  in  a  condition  of  equilibrium  which  varies  with  the 
temperature.  In  the  case  of  ethyl  formylphenylacetate,  increase  in 
concentration  favors  the  enol  modification. 

The  application  of  colorimetric  methods1  to  the  approximate 
determination  of  the  relative  amounts  of  the  enol  and  keto  present  in 
various  solvents  led  W.  Wislicenus  to  conclude  that  at  room  temperature 
solutions  in  methyl  alcohol  contain  only  keto,  in  chloroform  and  in 
benzene  only  enol,  while  solutions  in  ethyl  alcohol  and  ether  contain 
both  modifications  in  equilibrium.  These  facts  suggested  the  arrange- 
ment of  various  solvents  in  a  series  showing  the  respective  influence 
which  each  exercised  upon  the  condition  of  equilibrium  in  chemical 
systems  of  the  type  : 

Enol  <=»  Keto. 

Such  an  arrangement  was  found  to  coincide  in  general  with  that  repre- 
senting the  dissociation  constants  of  these  solvents.  Since,  moreover, 
dissociation  and  dielectric  constants  form  parallel  series,  the  relation 
of  equilibrium  constants  to  the  latter  is  obvious.  Wislicenus  2  has 

1  Annalen  der  Chemie,  291,  179  (1896);  K.  Meyer,  Ber.,  44,  2725;  Knorr,  ibid., 
2772  (1911). 

2  Annalen  der  Chemie,  379,  249  (1911). 


TAUTOMERISM  AND  DESMOTROPISM  251 

claimed  quite  recently  that  similar  relationships  exist  in  the  case  of 
such  solvents  as  the  indifferent  hydrocarbons,  chloroform,  and  alcohols, 
but  no  general  statement  covering  the  case  of  all  solvents  can  be  made. 
It  has  indeed  been  demonstrated  as  a  result  of  the  investigation  of 
Michael  and  Hibbert,1  Dimroth,2  Kurt  H.  Meyer  3  and  others,  that,  at 
least,  in  certain  instances  it  is  quite  impossible  to  establish  such  rela- 
tionships. 

Michael  and  Hibbert  studied  the  behavior  of  dibenzoylmethane  and 
ethyl  diacetylsuccinate  in  solution  in  a  greater  number  of  solvents  than 
had  previously  been  compared.  These  substances  are  particularly 
adapted  to  this  sort  of  investigation  since  in  both  cases  the  presence  of 
the  enol  and  keto  modifications  in  the  reaction  mixture  can  be  readily 
and  almost  quantitatively  determined.  The  results  obtained  by  them 
show  that: 

1.  No  simple  relationship  exists  between  the  dielectric  constant, 
power   of   dissociation,    association   factor,    heat   of   vaporization,    or 
tautomerizing  energy  4  of  organic  solvents  and  their  respective  rates  of 
isomerization. 

2.  The  rate  and  the  limit  of  isomerization  in  organic  solvents  are 
independent  of  each  other  and  the  latter  does  not  stand  in  any  simple 
relationship  to  the  above  physical  constants. 

3.  The  power  of  isomerization  of  an  organic  solvent  is  not  a  con- 
stant property  but  varies  with  the  character  of  the  desmotropic  sub- 
stance.    Thus  the  same  solvent  may  at  times  enolize  and  at  times 
ketoize  in  very  different  degrees. 

Michael  and  Hibbert  not  only  concluded  that  no  relationship  exists 
between  the  power  of  isomerization  and  the  physical  constants  of 
organic  solvents,  but  they  also  regarded  it  as  highly  improbable  that 
electrolytic  dissociation  plays  any  part  in  the  replacement  of  hydrogen 
which  is  bound  to  carbon  by  other  radicals.  According  to  Michael, 
chemical  forces  are  at  work,  along  with  other  influences,  in  the  process 
of  desmotropic  rearrangements  in  solution.  He  assumes  that  complex 
and  unstable  addition  products  are  formed,  as  the  result  of  interaction 
between  the  molecules  of  the  solvent  and  of  the  solute,  and  that  this 
involves  a  constant  interchange  between  the  free  and  bound  molecules 

iBer.,  41,  1080  (1908). 

2  Annalen  der  Chemie,  377,  131  (1910). 

3  Annalen  der  Chemie,  380,  226  (1911). 

4Briihl  designates  this  as  "Medialenergie,"  Zeitschr.  physikal.  Chemie,  30,  42 
(1899). 


252  THEORIES  OF  ORGANIC  CHEMISTRY 

of  the  solvent.  If  now  the  alteration  of  this  complex,  as  for  example, 
the  transformation  of 

[Keto+ sol  vent]  <=^  [Enol-f-  solvent], 

is  accompanied  by  an  increase  in  entropy  it  follows  that  solution  in  this 
particular  solvent  will  result  in  isomerization,  the  degree  of  isomeriza- 
tion  depending  upon  the  relative  increase  in  entropy  in  any  given  case. 
This  in  turn  depends  collectively  upon  the  total  change  suffered  by  the 
various  physical  and  chemical  forces  taking  a  part  in  the  transformation. 
It  seems  .very  questionable  under  these  circumstances  whether  a  simple 
relationship  between  any  given  physical  constant  of  a  solvent  and  its 
power  of  isomerization  can  ever  be  established.1 

O.  Dimroth  and  Kurt  H.  Meyer  have  brought  forward  even  more 
exact  evidence  to  show  that  the  rates  of  isomerization  of  organic  sol- 
vents are  in  no  way  paralleled  by  their  respective  dielectric  constants.2 
While  according  to  these  investigations  the  rate  of  isomerization  does 
undoubtedly  depend  upon  the  nature  of  the  solvent,  the  relation  is  far 
from  being  as  simple  as  was  originally  supposed.  Not  one,  but  many 
factors  determine  the  influence  which  a  given  solvent  will  exercise  upon 
the  rate  of  isomerization  of  desmotropic  substances  dissolved  in  it. 
Among  these  solubility  seems  to  play  an  important  part. 

O.  Dimroth  3  made  a  number  of  quantitative  determinations  in  differ- 
ent solvents  of  the  rate  of  the  rearrangement  expressed  below :  By  com- 

C6H5  CO-NHC6H5 


A/} 

H°-9     N  i\4 


CHsOOC  •  C N  COOCHs 

Methyl  phenyltriazolone  carboxylate  Anilide  of  methyl  diazomalonate         * 

paring  the  solubility  number  found  in  each  case  with  the  numbers  repre- 
senting the  solubility  of  the  acid  and  neutral  forms  respectively  in  the 
given  solvent  he  discovered  that  the  rate  of  change  of  the  acid  into  the 
neutral  ester  was  influenced  by  the  solubility  of  the  former,  viz.,  the 
greater  the  solubility  the  slower  became  the  rate  of  isomerization. 
Solubility  and  rate  of  isomerization  are,  however,  only  approximately 
inversely  proportional,  since  other  influences  which  are  not  as  yet 
understood  make  themselves  felt  in  this  process. 

'Ber.,  41,  1091  (1908). 

2  Annalen  der  Chemie,  399,  91  (1913). 

3  Annalen  der  Chemie,  377,  131  (1910). 


TAUTOMERISM  AND  DESMOTROPISM  253 

The  effect  of  the  solvent  upon  the  condition  of  equilibrium  must 
now  be  considered  in  detail  since  in  one  and  the  same  solvent  two 
reciprocal  reactions  have  frequently  been  observed.  This  effect  has 
been  analyzed  by  van't  Hoff,1  who  is  of  the  opinion  that  it  is  dependent 
upon  two  factors.  Each  of  the  reciprocal  reactions  may  be  supposed 
to  be  influenced  equally  by  what  may  be  called  the  contact  action 
of  solvent  and  solute.  The  second  factor  which  influences  the  course 
of  the  reaction  is,  however,  specific  in  character,  and  depends  in  some 
way  upon  the  relative  affinity  of  the  solvent  for  each  of  the  two  modifica- 
tions. The  effect  of  this  factor  is  therefore  different  in  the  case  of  each 
of  the  desmotropic  forms,  and,  according  to  Dimroth,  depends  upon  the 
relative  solubility  of  the  isomers  in  the  given  solvent.  This  relation 
may  be  expressed  by  a  modification  of  van't  Hoff  's  equation  : 


where  CA  and  CB  represent  the  respective  concentrations  of  the  desmo- 
tropic modifications  A  and  B,  and  where  LA  and  LB  represent  the 
solubilities  of  these  substances.  G  is  a  constant  which  depends  upon 
the  nature  of  the  solvent  and  which  Dimroth  calls  the  absolute  equilib- 
rium constant.  Two  desmotropic  forms  are  therefore  in  equilibrium 
with  each  other  in  a  given  solvent  if  the  ratio  of  their  concentrations 
equals  the  ratio  of  their  solubilities  multiplied  by  the  constant  G.2 
The  value  of  this  constant  may  be  calculated  if  the  solubilities  of  the 
two  substances  and  the  concentration  of  their  equilibrium  mixture  is 
known  : 

I/ 


Since  this  value  is  different  for  different  substances  it  may  be  regarded 
as  one  of  the  distinguishing  characteristics  of  any  given  tautomeric 
compound. 

Dimroth  has  determined  the  value  of  G  for  the  desmotropic  modi- 
fications of  benzoyl  camphor 

OH 

I 

/CH  •  COC6H5  /C=C  •  C6H5 

C8H14<  |  <=*      C8H14<  | 

\CO  XCO 

1  "  Vorlesungen  liber  theoret.  und  physikal.  Chemie,"  2d  edition,  Vol.  I,  pp.  217 
and  following. 

2  Annalen  der  Chemie,  377,  134  (1910). 


254 


THEORIES  OF  ORGANIC  CHEMISTRY 


in  solution  in  a  number  of  different  solvents.  Both  the  enol  and  keto 
forms  of  this  substance  may  be  readily  obtained  in  a  pure  crystalline  con- 
dition and  isomerize  slowly  enough  to  make  solubility  determinations 
possible.  The  results  of  these  experiments  are  given  in  the  following 
table: 


Solvent 

CE 
CK 

LE 
LK 

r     CE  LK 
G-^K'TE 

Ether 

87.2 
=  6  81 

12'86     639 

1  06 

12.8 

66.5 
1    Q8 

2.012 
12  79-l  81 

1   OQ 

Ethyl  alcohol  

33.5     IW 
^-6  =  1.67 

7.05 
^-1.57 

1.06 

IVlethyl  alcohol 

37.4 
465-0869 

1.31 
^-0748 

1  15 

53.5 
460    0852 

1.99 
11.69 

0  £0 

1    OR 

54.0 

14.53 

It  will  be  observed  that  the  values  calculated  for  G  agree  closely  and 
fall  within  the  limits  of  experimental  error.  The  law  has,  of  course, 
been  found  to  hold  in  other  similar  cases.1 

This  work  was  followed  by  a  systematic  investigation  of  the  influ- 
ence which  the  substitution  of  Ri,  R2  and  Ra  has  upon  equilibrium  in 
the  system 


RiCOCH 


R2 
R3 


Ra 


In  developing  the  problem  Kurt  H.  Meyer2  studied  the  condition  of 
equilibrium  in  the  case  of  ethyl  acetoacetate,  methyl  benzoylacetate  and 
acetylacetone.  Fusion  mixtures  of  the  desmotropic  modifications  of 
these  substances  were  examined  and  also  their  solutions  in  different 
solvents,  and  in  each  instance  the  percentage  of  enol  present  in  the 
equilibrium  mixture  was  determined.  These  results  are  given  in  the 
following  table : 


1  Annalen  der  Chemie,  377,  134  (1910);  399,  93  (1913). 
2Ber.,  45,  2846  (1912);  47,  826,  832,  837  (1914). 


TAUTOMERISM  AND  DESMOTROPISM 


255 


Solvent 

Temp. 
°C. 

Ethyl 
Acetoacetate 

Methyl 
Benzoylacetate 

Acetyl- 
acetone 

Water  

0 

0  4 

0  8 

10 

Formic  acid  

20 

1  l 

2  g 

4Q 

Acetic  acid        .    .  . 

20 

5  7 

14  0 

74. 

Methyl  alcohol 

o 

6  9 

13  4 

70 

Liquid  mixture  

20 

7  4 

16  7 

7fi 

Chloroform     

20 

8  2 

15  3 

7Q 

Ethyl  alcohol  
Benzene 

0 
20 

12.7 

18  0 

26.0 
31  0 

84 

QK 

Hexane  

20 

48.0 

69.0 

92 

The  figures  are  significant  because  they  show  the  same  general  variation 
in  the  case  of  all  three  substances.  In  other  words,  the  different  solvents 
appear  to  have  much  the  same  influence  upon  the  condition  of  equi- 
librium in  the  case  of  ethyl  acetoacetate,  methyl  benzoylacetate,  and 
acetylacetone,  respectively.  This  is  even  more  apparent  when  figures 
representing  the  equilibrium  constants  are  compared.  These  values 
may  be  readily  calculated  from  those  which  have  just  been  given  and 
will  be  found  in  the  following  table: 


Solvent 

Ethyl 
Acetoacetate 

Methyl 
Benzoylacetate 

Acetyl- 
acetone 

Water.  . 

0  004 

0  008 

0  24 

Formic  acid  

0  Oil 

0  028 

0  9 

Acetic  acid  

0  061 

0  16 

2  8 

Methyl  alcohol 

0  074 

0  16 

2  6 

Liquid  mixture  

0  079 

0  20 

3  2 

Chloroform  

0  089 

0  19 

3  8 

Ethyl  alcohol 

0  15 

0  35 

5  3 

Benzene 

0  22 

0  45 

5  7 

Hexane  

0.9 

2.2 

12.0 

Very  interesting  relationships  are  at  once  apparent.  If  the  equilibrium 
constants  for  the  three  substances  in  first  one  and  then  another  of  the 
given  solvents  are  compared,  it  will  be  observed  that  a  rough  ratio  is 
maintained.  For  example,  the  value  for  methyl  benzoylacetate  is 
approximately  2.2  times  as  great  as  that  of  ethyl  acetoacetate  in  the 
same  solvent,  while  that  of  acetylacetone  is  30-40  times  as  great.  If, 
therefore,  the  equilibrium  constant  of  ethyl  acetoacetate  in  a  given 
solvent  is  known,  that  of  methyl  benzoylacetate  in  the  same  solvent 


256 


THEORIES  OF  ORGANIC  CHEMISTRY 


may  be  roughly  calculated  by  multiplying  by  2.2.  And,  in  general,  it 
may  be  said  that  if  the  equilibrium  constant  of  a  given  desmotropic 
substance  in  a  given  solvent  is  known,  its  condition  when  dissolved  in 
other  solvents  may  be  predicted  with  a  certain  degree  of  accuracy.1 

As  a  result  of  his  investigation  of  the  equilibrium  constants  of  ben- 
zoylcamphor  in  methyl  and  ethyl  alcohol,  0.  Dimroth  formulated  the 
following  rule  in  regard  to  equilibrium  relationships.2  If  a  represents 
the  equilibrium  constant  for  ethyl  acetoacetate  in  a  given  solvent  (I), 
n  •  a  the  equilibrium  constant  in  a  second  solvent  (II)  and  ra  •  a,  that  in 
a  third  (III)  and  if,  further,  b  and  c  represent  respectively  the  equilibrium 
constants  of  methyl  benzoylacetate  and  acetylacetone  in  the  first  sol- 
vent, it  follows  that  the  equilibrium  constants  of  the  last  two  substances 
in  the  second  and  third  solvents  will  be  n-b,  m-b  and  n-c,  m-c, 
respectively. 

In  other  words  it  is  possible  to  express  the  general  relationships 
noted  in  the  preceding  table  in  the  following  way : 


Solvent 

Ethyl 
Acetoacetate 

Methyl 
Benzoylacetate 

Acetylacetone 

I 

a 

b 

c 

II 

n-a 

n-b 

n-c 

III 

m-a 

m-b 

m-c 

where  a,  6,  and  c  represent  the  ratios 
Concentration  of  Enol 


Concentration  of  Keto 


=   Equilibrium  constant 


in  the  case  of  the  three  substances,  respectively.     From  this  it  follows 
that 


and  that,  therefore,  the  value  of  n  depends  not  upon  the  character  of 
Hi,  R£,  or  Rs  in  ethyl  acetoacetate,  methyl  benzoylacetate,  and  acetyl- 
acetone, respectively,  but  upon  the  nature  of  the  solvent.  This  rela- 
tionship is,  of  course,  not  quantitatively  exact,  but  is,  nevertheless, 
approximately  correct,  and,  according  to  Kurt  H.  Meyer,  demonstrates 
that  the  influence  of  constitution  upon  the  condition  of  equilibrium  in 
such  systems  is  negligible. 

iBer.,  45,  2847(1912). 

2  Annalen  der  Chemie,  399,  96  (1913). 


TAUTOMERISM  AND  DE8MOTROPISM  257 

Meyer  has  found  that  G  =  0.09  in  the  case  of  ethyl  acetoacetate.  He 
also  discovered  that  equilibrium  in  solutions  of  ethyl  acetoacetate 
depends  upon  the  concentration.  These  observations  will  be  referred 
to  again  later  in  the  chapter. 

L.  Knorr  has  also  investigated  the  effect  of  heating  and  of  solution 
upon  equilibrium  in  the  case  of  ethyl  diacetylsuccinate.  He  found  that 
while  the  anti  modification,  melting  at  68°, 

CHsCO-CH-COOCaHs 
CHsCO-CH-COOCsHs 

gives  no  coloration  with  ferric  chloride  at  ordinary  temperatures  nor 
after  heating  for  five  minutes  at  65°,  it  nevertheless  gives  an  immediate 
coloration  at  its  melting  point,  thus  showing  that  rearrangement  of  the 
keto  to  the  enol  modification  takes  place  at  this  temperature.  Similar 
observations  have  been  made  in  the  case  of  other  desmotropic  sub- 
stances, so  that  in  general  it  may  be  said  that  fusion  is  frequently  accom- 
panied by  intramolecular  rearrangement.  This  has  suggested  to  Knorr 
the  possibility  that  in  such  cases  the  phenomena  of  fusion  (and  there- 
fore the  melting  point)  are  actually  rearrangement  phenomena  and 
involve  chemical  as  well  as  physical  changes. 

In  still  other  instances  rearrangement  has  been  observed  to  begin 
at  temperatures  above  the  melting  point.  This  would  seem  to  indicate 
that  in  such  cases  one  of  the  desmotropic  modifications  is  stable  at  low 
temperatures  and  that  increase  in  temperature  leads  to  a  point  at 
which  it  ceases  to  be  stable  and  tends  to  rearrange  wholly  or  in  part 
into  its  corresponding  isomer.  The  temperatures  at  which  desmotropic 
substances  cease  to  be  stable  are  referred  to  by  Knorr  as  the  "  limits 
of  stability."1  These  limits  may  or  may  not  correspond  to  the  melting 
points.  Indeed  observation  shows  that  they  vary  considerably.  Thus 
in  the  case  of  the  0-  and  7-diacetyl  succinic  esters,  for  example,  the 
limits  of  stability  correspond  to  the  melting  points  of  the  substances, 
while  in  the  case  of  the  corresponding  /5-dibenzoyl  succinic  ester, 

H 

C6H5OOC— C— COOC2H5 
H5C2OOC— C— COC6H5 


Annalen  der  Chemie,  293,  88  (1896). 


258  THEORIES  OF  ORGANIC  CHEMISTRY 

there  is  no  coloration  with  ferric  chloride  at  128°- 130°  which  represents 
the  melting  point  of  the  substance,  although  brief  heating  at  170° 
produces  enolization,  On  the  other  hand  7-ethyl  dibenzoyl  succinate 

H 

C6H5CO— C— COOC2H5 
C6H5CO— C— COOC2H5 

H 

which  melts  at  75°,  shows  a  tendency  to  enolize  after  short  heating  at 
150°- 160°.  The  ketone  ester  obtained  by  condensation  of  mesityloxide 
with  diethyl  oxalate 

(CH3)2C  =  CH  •  COCH2COCOOC2H5 

which  melts  at  59°-60°,  isomerizes  only  at  130°. 

Knorr  assumes  that  in  these  and  similar  cases  enolization  begins  at 
the  melting  point  of  the  substances,  although  this  cannot  be  demon- 
strated experimentally.  He  explained  the  equilibrium  relationships 
which  have  been  observed  in  the  case  of  Claisen's  desmotropes  by 
supposing  that  the  limits  of  stability  of  the  enol  form, 


CHs-COH 

C-(COC6H5)2 

which  melts  at  101°-102°,  are  80°-90°,  while  those  of  the  keto  form, 

CH3CO 

:H(COC6H5)2 


>  v 

i, 


which  melts  at  107°-110°,  are  about  110°.  On  these  assumptions  it 
follows  that  below  90°  both  desmotropic  modifications  may  be  regarded 
as  stable  and  capable  of  existing  side  by  side;  between  90°  and  110° 
the  enol  form  is  labile,  the  keto  stable;  and  above  110°  both  forms  are 
labile.  In  this  last  case  neither  form  is  capable  of  maintaining  an 
independent  existence  since  each  tends  to  pass  into  the  other  and  a 
condition  of  equilibrium  between  the  two  is  established  which  is  anal- 
ogous in  character  to  that  which  is  observed  in  connection  with  solution 
phenomena. 


TAUTOMERISM  AND  DESMOTROPISM  259 

Equilibrium  mixtures,  the  composition  of  which  varies  with  changes 
in  temperature,  are  referred  to  by  Knorr  as  "  allelotropic  mixtures." 
In  other  words,  an  allelotropic  mixture  is  a  homogeneous  mixture  of  two 
desmotropic  substances  which  mutually  rearrange  each  into  the  other. 
From  a  consideration  of  energy  relations  it  is  obvious  that  allelotropism 
is  possible  only  in  the  case  of  liquids  or  of  dissolved  substances.1 

The  phenomena  of  allelotropism  may  be  readily  interpreted  in  terms 
of  C.  Laar's  hypothesis.  Assuming  that  both  enol  and  keto  modifica- 
tions are  continually  changing  one  into  the  other  and  that  in  a  given 
interval  of  time  the  amounts  of  each  which  undergo  change  are  equal, 
it  follows  that  a  condition  of  equilibrium  must  result  from  the  fusion 
as  well  as  from  the  solution  of  the  substance.  Changes  in  temperature 
will  bring  about  conditions  which  favor  one  form  at  the  expense  of  the 
other  so  that  ultimately  a  temperature  must  be  reached  at  which  one 
form  will  so  predominate  in  the  mixture  as  to  make  the  presence  of  the 
other  negligible  and  even  incapable  of  experimental  demonstration. 
Baeyer's  term  "  pseudo-form  "  is  employed  by  Knorr  to  denote  this 
almost  completely  eclipsed  modification.  According  to  this  conception 
the  pseudo-form  represents  the  limit  of  allelotropism. 

Acetylangelica  lactone  exists,  for  example,  in  two  distinct  forms, 

OH 


CH3C=CH  •  C=C  •  CH3  CH3C=CH  •  CH  -  COCH3 

II  and  |  | 

0—     -CO  O—     —CO 

Enol  (melting  at  63°)  Keto  (melting  at  177-180°) 

These  two  forms  readily  pass  into  each  other  and  therefore  represent  a 
typical  case  of  desmotropism.  Each  is  partially  isomerized  on  solution 
in  hot  alcohol,  benzene,  acetone,  and  ethyl  acetate,  and  the  resulting 
mixture  is  found  to  contain  both  modifications  in  proportions  which 
are  constant  for  any  given  temperature,  but  which  vary  for  different 
temperatures.  These  solutions  obviously  contain  allelotropic  mixtures 
of  the  two  acetylangelica  lactones.  Such  mixtures  may  also  be  obtained 
by  heating  the  A;e£o-modification  at  temperatures  above  its  melting 
point.  If,  however,  the  ketone  is  distilled  under  diminished  pressure  it 
practically  disappears  and  must,  therefore,  be  regarded  as  having  passed 
into  the  pseudoform  of  acetylangelica-lactone.  To  recapitulate: 

1.  Desmotropism  may  be  regarded  as  a  special  form  of  structural 
isomerism  where  the  difference  in  the  properties  of  any  two  substances 
depend  upon  a  difference  in  the  relative  position  of  a  hydrogen  atom  in 

1  Compare  Schaum,  Ber.,  31,  1964  (1898);  also  "Die  Arten  der  Isomerie,"  Habili- 
tationsschrift  Marburg. 


260  THEORIES  OF  ORGANIC  CHEMISTRY 

the  molecule.  It  is  observed  in  the  case  of  substances  in  the  solid  state. 
Thus  the  isomeric  triacylme thanes, 

CHaC-OH  CH3CO 

||  and 

C  •  (COC0H«)2  CH  •  (COCV,HS)2 

may  exist  independently  of  each  other  below,  but  never  above  80°. 
The  fact  that  one  modification  does  in  time  pass  over  into  the  other  is 
explained  by  Knorr  as  due  to  the  presence  of  minute  traces  of  some 
solvent.1 

2.  Tautomerism  is  the  phenomenon  observed  when  one  and  the 
same  substance  reacts  in  two  senses — viz.,  as  enol  and  keto.  In 
such  cases,  however,  the  two  forms  have  never  been  separated.  The 
phenomenon  occurs  in  connection  with  substances  in  both  the  solid  and 
the  liquid  state.  If  the  substance  is  a  solid,  even  though  it  reacts  in 
two  senses,  it  itself  must  be  regarded  as  having  a  perfectly  definite 
structure  and  as  representing  either  the  enol  or  keto  modification. 
This  is  in  agreement  with  Baeyer's  conception,  but  in  contradiction  to 
that  of  C.  Laar.  If  the  tautomeric  substance  is  a  liquid  it  must  in 
general  be  regarded  as  representing  an  allelotropic  mixture.  Such  a 
mixture  may  be  of  two  types:  (a)  where  both  enol  and  keto  modifica- 
tions are  present  and  where  their  presence  may  be  experimentally 
demonstrated;  (6)  where  the  conditions  of  temperature,  etc.,  favor 
one  form  to  the  exclusion  of  the  other,  so  that  the  presence  of  only  one 
of  the  two  possible  modifications  can  be  experimentally  demonstrated. 
Such  a  condition  is  referred  to  as  pseudomerism  and  represents  the 
extreme  limit  of  allelotropism. 

Mixtures  of  solid  desmotropic  substances  must  not  be  confused  with 
allelotropic  mixtures.  Examples  of  the  latter  class  are  recorded  in  the 
following  table: 


Acetyl  benzoylmethane 
Mesityloxide  oxalic  ester 
Tribenzoylmethane 
p-Brombenzoyl-dibenzoylmethane 
Ethyl  diacetylsuccinate 
Ethyl  dibenzoylsuccinate 
Ethyl  acetoacetate,  etc. 


When  present  in 
fluid    condition 
and  in  solution. 


To  this  list  may  be  added  ethyl  formylphenylacetate,  if,  according 
to  Knorr,  it  may  be  regarded  as  a  mixture  consisting  of  large  quantities 
of  the  a  and  small  quantities  of  the  /3  form  in  equilibrium  with  each 
other.2 

^nnalen  der  Chemie,  313,  147  (1900). 

2  Annalen  der  Chemie,  396,  340  (1899);  313,  141  (1900). 


TAUTOMER1SM  AND  DESMOTROPISM  261 

Pseudomerism  is  limited  to  cases  of  tautomerism  where  only  one 
form,  whether  enol  or  keto,  may  be  experimentally  demonstrated. 
Illustrations  are  to  be  found  in  benzoyl  diacetyl  methane,  p-brom  ber> 
zoyl  acetone  and  the  anhydride  of  diacetyl  succinic  acid  and  its  ester. 
These  substances  are  known  only  in  the  enol  form.  The  correspond- 
ing ketones  have  not  been  isolated  in  solid  condition  and  do  not  appear 
to  be  formed  when  the  substances  are  either  fused  or  dissolved. 

The  quantitative  determination  of  the  relative  amounts  of  the  enol 
and  keto  modifications  which  are  present  in  allelotropic  mixtures  is 
naturally  of  the  greatest  importance.  This  task  is  the  more  complicated 
and  difficult  because  of  the  fact  that  the  equilibrium  relationships  in 
such  systems  are  extremely  sensitive  to  changes  in  temperature  and 
to  the  action  of  different  solvents,  catalytic  agents,  etc.  If,  for  example, 
the  enol  modification  reacts  with  ferric  chloride,  an  iron  enolate, 
FeCU-R  (where  R  represents  the  enol  residue)  is  formed  and  at  the 
same  time  a  molecule  of  hydrochloric  acid  is  set  free.  This  acid  in 
concentrated  form  can  materially  alter  the  condition  of  equilibrium 
even  in  the  very  short  space  of  time  required  for  the  color  change.  It 
has  also  been  discovered  that  ferric  chloride  itself  acts  directly  as  a 
catalyst  and  in  the  case  of  ethyl  acetoacetate,  for  example,  brings 
about  the  enolization  of  the  keto  form.  In  this  particular  case  the 
quantity  of  enol  which  forms  depends  upon  the  quantity  of  ferric  chloride 
which  is  used,  but  is  not,  however,  directly  proportional  to  it.1  These 
errors  in  experimentation  have  been  reduced  to  a  minimum  by  employ- 
ing various  artifices  and  in  particular  by  working  at  low  temperatures. 
L.  Knorr2,  in  co-operation  with  H.  Schubert,  has  recently  succeeded 
in  perfecting  a  colorimetric  method  for  the  quantitative  determination 
of  the  enol  modification  present  in  allelotropic  mixtures,  to  which 
reference  will  be  made  again  later  in  the  text. 

Kurt  H.  Meyer  3  has  developed  another  quantitative  method  which 
will  be  referred  to  again  in  connection  with  the  more  detailed  considera- 
tion of  ethyl  acetoacetate.  It  is  founded  upon  the  observation  that 
enol  forms  in  contrast  to  keto  react  readily  with  bromine.  In  its  typical 
form  this  reaction  involves  the  direct  addition  of  bromine  to  the  double 
bonds  of  the  carbon  atoms,  and  of  course  presupposes  that  bromine 
does  not  react  with  other  groups  present  in  the  molecule.  There  are 
naturally  many  sources  of  error  in  the  application  of  this  reaction, 
especially  since  bromine  on  mere  contact  acts  as  a  catalyst  and  tends 
to  bring  about  rearrangements.  In  spite  of  these  difficulties,  however, 

1K.  Meyer,  Ber.,  44,  2726  (1911). 

2Ber.,  44,  2772  (1911). 

'Annalen  der  Chemie,  380,  212  (1911). 


262  THEORIES  OF  ORGANIC  CHEMISTRY 

Meyer,  in  co-operation  with  P.  Kappelmeier,1  succeeded  by  the  use  of 
various  artifices  in  so  perfecting  the  method  as  to  have  it  compare  favor- 
ably with  the  ordinary  analytical  methods  in  accuracy. 

It  may  be  said  in  general  that  the  application  of  many  physical  meth- 
ods involves  fewer  possibilities  of  experimental  error  than  is  the  case  with 
chemical  methods.  This  is  especially  true  if  in  the  process  the  substances 
themselves  are  subjected  to  no  change.  The  results  which  are  obtained 
by  the  use  of  physical  methods  are,  however,  valuable  only  in  so  far  as 
they  may  serve  for  comparisons  between  the  substance  in  question 
and  other  very  closely  related  substances  whose  exact  chemical  character 
has  been  accurately  determined.  The  reliability  of  the  conclusions 
which  are  drawn  from  such  data  depends  naturally  upon  whether  suf- 
ficient material  is  available  for  comparison.  The  methods  of  molecular 
refraction  and  rotation,  Drude's  method  of  electrical  oscillations  of 
definite  period,  together  with  many  others,  have  been  of  great  service 
in  the  solution  of  the  problems  of  structural  chemistry.2 

As  has  already  been  stated,  solvents  are  instrumental,  along  with 
other  agencies,  in  inducing  rearrangements  in  the  case  of  tautomeric 
and  desmotropic  substances.  Rules  governing  transformations  of  this 
type  were  early  recognized  by  Wislicenus  as  a  result  of  his  researches  on 
ethyl  formylacetate,  and  these  were  confirmed  in  the  main  by  later 
investigations.  They  have  been  formulated  by  H.  Stobbe3  as  follows: 

Solvents  may  be  divided  in  general  into  two  groups  according  to 
their  ability  to  isomerize  tautomeric  substances  under  constant  condi- 
tions of  temperature.  To  the  first  group  belong  water,  alcohol,  and 
other  oxygen  compounds,  or,  in  general,  those  solvents  which  are 
characterized  by  a  tendency  to  dissociate.  Such  solvents  are  effective 
both  in  starting  and  in  accelerating  the  enolization  of  neutral  keto  forms 
and  the  ketonization  of  acid  enol  forms.  In  so  far  as  experimental 
evidence  goes  they  do  not  seem  to  combine  with  the  dissolved  sub- 
stance, and  their  action  must,  therefore,  be  regarded  as  catalytic.  To 
the  second  group  belong  chloroform,  benzene,  and  other  substances 
which  contain  no  oxygen  and  whose  power  of  dissociation  and  isomeriza- 
tion  is  very  weak.  Such  solvents  not  only  do  not  actively  accelerate 
reversible  reactions  of  the  type  under  consideration,  but  in  certain 
instances  they  may  even  serve  as  media  for  conserving  the  transitory 

1  Ber.,  44,  2718  (1911). 

2  For  a   summary  of  these  different  methods  consult  Wislicenus'  monograph 
entitled    "Uber   Tautomerie"  and    also  Annalen  der  Chemie,    291,  176  and   217 
(1896),  Zeitschr.  physikal.  Chemie,  30,  1  (1899);  Knorr,  Annalen  der  Chemie,  306, 
342(1899). 

3  Annalen  der  Chemie,  326,  359  and  following  (1903). 


TAUTOMERISM  AND  DESMOTROPISM  263 

condition  of  a  tautomeric  substance.  At  times  they  may  even  hinder 
isomerization  and  may,  therefore,  perhaps  be  regarded  as  catalysts  in 
a  negative  sense. 

The  fact  that  tautomeric  substances  isomerize  so  readily  on  solution 
and  upon  fusion  is  particularly  remarkable  and  characteristic  and  at 
once  recalls  C.  Laar's  oscillation  hypothesis.  The  results  of  experiment 
show  that  mutual  rearrangements  take  place  more  or  less  readily  and 
finally  result  in  a  condition  of  equilibrium.  It  is  thus  possible  to  con- 
sider the  process  from  the  standpoint  of  the  kinetic  theory  of  gases 
and  to  suppose  the  existence  of  a  condition  of  dynamic  equilibrium 
whereby  in  a  given  interval  of  time  the  same  number  of  molecules  un- 
dergo transformation  in  one  direction  as  in  the  other. 

The  rate  of  this  rearrangement  was  originally  followed  in  a  quali- 
tative sense  only,  but  in  1904  0.  Dimroth  x  made  the  first  quantitative 
measurements.  He  had  discovered  two  triazol  derivatives  as  the  result 
of  condensing  diazobenzene  imides  with  diethyl  malonate  and  had 
interpreted  the  phenomena  by  supposing  that  the  two  substances  were 
desmotropic  modifications  corresponding  to  cyclic  formulas: 


A 

HOC        N 

II          II 
CH3OOC-C  --  N 

Of  these  the  keto  form  was  found  to  be  absolutely  neutral  in  character 
while  the  enol  was  sufficiently  acidic  to  decompose  a  solution  of  potas- 
sium iodide,  freeing  exactly  one  atomic  equivalent  of  iodine  for  every 
equivalent  of  acid.  Since  the  methods  involved  the  use  of  neutral 
reagents,  all  disturbing  influences  were  eliminated  and  Dimroth  was 
able  to  determine  quantitatively  the  amount  of  enol  present  at  any 
moment  during  the  course  of  the  reaction.  He  thus  succeeded  in  apply- 
ing the  laws  governing  the  rates  of  reaction  to  this  particular  transforma- 
tion, and  discovered  that  it  belongs  to  the  general  class  of  mono- 
molecular  reactions. 

Dimroth  then  showed  that  it  was  possible  on  the  basis  of  the  kinetic 
molecular  hypothesis  to  calculate  from  the  constants  for  the  rates  of 
reaction  the  interval  of  time  necessary  for  one  molecule  to  rearraniro 
into  enol  and  then  back  again  into  the  keto  form,  or,  in  other  words 

1  Annalen  der  Chemie,  335,  1  (1904);  338,  143  (1905). 


264  THEORIES  OF  ORGANIC  CHEMISTRY 

the  interval  of  time  necessary  for  one  complete  oscillation  of  a  hydrogen 
atom  in  the  sense  of  Laar's  hypothesis,  viz. : 

R  R 

C=O  C— OH* 

^  II 

H*  C* 


R     R' 

Keto  Enol 

Although  Dimroth  discovered  later  that  his  two  triazol  derivatives 
could  not  be  interpreted  as  desmotropic  modifications,  but  that  they 
must  be  regarded  as  the  structural  isomers, 

C6H5  CONH-CeHfi 

XN\  XN 

HO  •  C        N       ancl         C\  I' 

CHsOOC-C— N  I,OOCH3 

his  deductions  have,  nevertheless,  been  applied  recently  to  a  solution 
of  the  problem  involved  in  the  chemistry  of  ethyl  acetoacetate.  This 
substance  consists  of  an  equilibrium  mixture  of  enol  and  keto  forms : 

OH 
CH3C=€HCOOC2H5     <=±    CH3CO  •  CH2COOC2Hr, 

Each  of  these  substances  has  recently  been  obtained  in  pure  or  almost 
pure  condition,  and  it  has  been  observed  that  if  either  is  allowed  to 
remain  for  some  time  at  room  temperature  it  rearranges — the  enol  into 
the  keto  and  vice  versa.  The  final  product  consists  in  each  case  of  a 
mixture  composed,  according  to  Kurt  H.  Meyer,1  of  7.4  per  cent  enol 
and  92.6  per  cent  kelo,  or,  according  to  L.  Knorr,  of  2  per  cent  enol 
and  98  per  cent  keto.  This  phenomenon  is  explained  on  the  basis 
of  the  kinetic  molecular  hypothesis  by  supposing  that  the  two  forms 
are  constantly  in  a  condition  of  dynamic  equilibrium  with  reference  to 
each  other  The  fact  that  the  amount  of  keto  present  in  the  mixture 
after  equilibrium  has  been  reached  is  proportionally  greater  than 
the  amount  of  enol,  serves  to  show  that  ketonization  proceeds 
much  more  rapidly  than  enolization.  These  rates  of  transformation 
have  now  been  determined  very  exactly  and  under  the  most  varied 
1  Ber.,  44,  2720  (1911);  also  47,  837  (1914). 


TAUTOMERISM  AND  DESMOTROPISM  265 

conditions 1  and  it  has  been  found  that  in  alcohol  solution  and  at  zero 
degrees,  for  example,  the  velocity  constants  are,  respectively: 

Enol        ->        Keto,  KI  =  0.077 
Keto  Enol,  K2  =  0.0105 

These  figures  show  that  under  these  conditions  0.077  or  7.7  per  cent 
of  the  enol  present  is  transformed  into  keto  and  0.0105  or  1.05  per  cent 
keto  is  changed  into  enol.  In  other  words,  in  any  given  interval  of 
time,  if9-,  or,  roughly,  13  molecules  of  /3-hydroxycrotonic  ester  are 
isomerized  to  ethyl  acetoacetate,  while  ^,  or,  roughly,  95  molecules 
of  ethyl  acetoacetate  are  isomerized  to  0-hydroxycrotonic  ester.  Every 
95  minutes  a  single  molecule  of  ketone  has  the  chance  to  become  enol, 
and  every  13  minutes  a  single  molecule  of  enol  has  the  chance  to  become 
keto.  Thus  108  minutes  are  required  for  the  following  series  of 
transformations : 

Keto     ->     Enol     ->     Keto 

and  this  interval  of  time  represents  the  period  of  oscillation  of  the 
hydrogen  atom.  In  aqueous  solution  it  has  been  found  that  the  period 
of  oscillation  requires  100  minutes;  in  liquid  ester,  2400  minutes  or 
17  days.  ' 

These  figures  are  in  contradiction  to  our  conception  of  intramolecular 
atomic  movements  in  the  sense  of  the  kinetic  theory  of  gases.  They 
must,  therefore,  be  regarded  as  representing  the  periods  of  extreme 
deviation  from  the  regular  course  of  the  atom.  In  the  words  of  O.  Dim- 
roth:2  "  Just  as  the  path  or  course  of  individual  molecules  is  supposed, 
according  to  the  kinetic  theory  of  gases,  to  vary  greatly  during  successive 
intervals  of  time  and  to  show  frequently  considerable  divergence  from 
the  average,  so  the  course  of  individual  atoms  only  approximates  a 
mean.  In  the  case  of  tautomeric  substances,  for  example,  it  may  be 
assumed  that  the  range  of  the  hydrogen  atom,  and  therefore  the  extreme 
distance  which  may  separate  it  from  its  point  of  union  with  carbon  and 
oxygen,  respectively,  may  vary  greatly  in  different  molecules.  The 
average  range  is  probably  in  most  cases  less  than  the  distance  between 
a  carbon  and  an  oxygen  atom,  but  may  at  times  be  imagined  as  equal 
to  it."  When  the  latter  condition  exists,  rearrangements  from  enol  to 
keto,  or  vice  versa,  will  take  place.  In  the  instance  just  cited  it  occurs 
in  alcohol  solutions  every  13  and  every  95  minutes,  respectively. 


1  Annalen  der  Chemie,  380,  239  (1911). 

2  Annalen  der  Chemie,  336,  17-18  (1904). 


266  THEORIES  OF  ORGANIC  CHEMISTRY 

Since  this  tendency  for  rearrangement  is  recognized  as  a  characteristic 
property  of  all  tautomeric  and  desmotropic  substances,  these  reactions 
should  be  followed  by  means  of  velocity  determinations  wherever  this  is 
possible.  Such  methods  should  also  be  improved.  To  quote  Dimroth  l 
again:  "  an  exact  description  of  a  desmotropic  substance  requires  a 
knowledge,  not  only  of  the  quantities  in  which  two  isomers  are  present, 
but  of  the  time  which  is  required  in  order  to  establish  a  condition  of 
equilibrium  between  them."  Dimroth  is  himself  convinced  that  the 
innumerable  variations  observed  in  connection  with  phenomena  of  this 
kind  depend  upon  enormous  fluctuations  in  velocity  values.  If,  for 
example,  one  modification  rearranges  much  more  rapidly  than  the 
other  the  reverse  reaction  will  become  negligible,  equilibrium  will  dis- 
appear and  the  phenomenon  will  assume  the  aspect  of  pseudomerism . 
If,  however,  both  modifications  possess  very  great  and  approximately 
equal  isomerization  velocities  a  condition  will  arise  which  corresponds 
to  that  which  Laar's  oscillation  hypothesis  seeks  to  explain.  If, 
finally,  the  isomerization  velocities  are  small  a  condition  of  apparent 
stability  will  result  and  the  phenomenon  will  assume  the  aspect  of 
desmotropism.2 

The  question  as  to  the  cause  and  mechanism  of  tautomeric  rearrange- 
ments has  been  answered  in  a  variety  of  ways.3  The  simplest  explana- 
tion is  one  which  assumes  that  the  whole  molecule  is  changed  because 
of  a  change  in  the  relative  position  of  a  hydrogen  atom ; 


R  R 

C— OH  0=0 

*=*  I 

CH 


•*^s 

?• 


Ri    R2  Ri    R2 

In  this  case  the  migration  of  the  hydrogen  atom  is  supposed  to  have 
nothing  to  do  with  the  phenomenon  of  electrolytic  dissociation  and  the 
reaction  is  assumed  to  be  mono-molecular.  Another  explanation  which 
has  been  quite  widely  accepted  by  workers  in  this  field  supposes  that 

1  Annalen  der  Chemie,  335,  5  (1904). 

2Annalen  der  Chemie,  335,  5-6  (1904). 

3Ber.,  28,  708  (1895);  30,  2388  (1897);  Annalen  der  Chemie,  293,  34,  100 
(1896);  306,  342  (1899);  291,  176  (1896);  Ahrens'  Samml.  Chem.  u.  chem.  techn. 
Vortrage,  2,  230  (1898);  Ber.,  32,  2326  (1899);  Zeitschr.  physikal.  Chemie,  30,  38 
(1899);  Jour.  Chem.  Soc.,  81,  1508  (1902);  85,48  (1904);  Annalen  der  Chemie, 
335,  1  (1904);  338,  143  (1905);  Zeitschr.  Elektrochemie,  11,  137  (1905);  Ber.,  41, 
1080  (1908). 


TAUTOMERISM  AND  DESMOTROPISM  267 

rearrangement  is  due  to  the  alternate  addition  and  splitting  off  of  a 
molecule  of  water  according  to  the  scheme : 


R  R                                    R 

I  I  /OH                            | 

C— OH  C<                                  C— O 

+  H20  <=±         |   X)H  <=>    H20  +     I 

C  CH                                 CH 

Ri    R2  Ri    R2  Ri    R2 

This  should,  of  course,  be  capable  of  experimental  verification. 

A  third  explanation  supposes  that  rearrangements  of  tautomeric  sub- 
stances are  due  to  electrolytic  dissociation.  The  fact  that  tautomeric  sub- 
stances are  frequently  formed  when  solutions  of  sodium  salts  react 
with  alkyl  halides  led  H.  Goldschmidt  in  1890  to  suppose  that  in  such 
cases  rearrangement  is  directly  due  to  the  influence  of  free  ions.1 
Immediately  after  this  P.  Walden  2  and  Mullikan  3  discovered  a  num- 
ber of  tautomeric  substances  which  acted  as  electrolytes  either  in  their 
free  state  or  in  the  form  of  their  salts,  and  in  1895  Knorr  supplied 
additional  evidence  along  the  same  line  as  a  result  of  his  researches 
on  phenylmethylpyrazolones.  Tautomerism  in  the  case  of  these  sub- 
stances was,  in  his  opinion,  due  to  the  electrolytic  dissociation  of 
hydrogen. 

Such  a  conception  has  the  advantage  of  accounting  for  the  remark- 
able ease  with  which  tautomeric  rearrangements  take  place.  The 
mechanism  of  the  change  has  been  explained  in  a  number  of  different 
ways : 

I.  The  hydrogen  ions  may  be  assumed  to  take  no  part  in  the  reaction 
which  involves  a  redistribution  of  affinity  inside  the  anions.  This 
explanation  applies  only  where  the  rate  of  rearrangement  is  proportional 
to  the  concentration  of  the  anions. 


Ri    R2 


II.  The  hydrogen  ions  may  be  regarded  as  taking  part  in  the  change 
in  either  one  of  two  ways : 

iBer.,  23,  257  (1890). 

2Ber.,  24,  2025  (1891). 

s  Am.  Chem.  Jour.,  16,  523  (1893). 


268  THEORIES  OF  ORGANIC  CHEMISTRY 

(a)  By  reaction  with  anions  to  form  undissociated  molecules  of 
ketonic  ester:1 

/R          \/  R 

'^°-lhH.     „        fo 

CH 

/\ 
Ri    R2 


Ri    R 


Such  an  explanation  applies  only  in  cases  where  the  electrolytic  dis- 
sociation of  an  electrically  neutral  body  takes  place  and  does  not  hold 
in  the  case  of  alletropic  mixtures  since  the  rearrangement  of  desmotropes 
is  reciprocal  and  the  keto  modifications  are  neutral  bodies. 

(6)  By  reaction  as  catalysts,2  and  in  this  way  inducing  rearrange- 
ment from  enolions  to  ketonions  and  vice  versa : 


Such  an  interpretation  holds  only  in  cases  where  the  reaction  has  been 
shown  to  be  bimolecular. 

In  conclusion  it  may  be  said  that  any  explanation  as  to  the  cause 
and  mechanism  of  tautomeric  rearrangements  must  in  any  particular 
instance  depend  finally  upon  the  experimental  data.  In  order  to 
understand  better  what  this  implies  it  may  be  well  to  consider  briefly 
the  individual  case  of  ethyl  acetoacetate  since  this  is  one  of  the  most 
interesting  as  well  as  one  of  the  most  widely  investigated  of  all  tauto- 
meric substances. 

This  /3-ketone  ester  was  discovered  by  Geuther,  who  prepared  it  by 
interaction  of  ethyl  acetate  with  sodium,  and  gave  it  the  formula 

OH 
CH3'C=CH.COOC2H5 

in  order  to  account  for  the  fact  that  it  is  capable  of  forming  a  metallic 
derivative  with  sodium.  Later  it  was  prepared  by  Claisen  from  ethyl 

1  Jour.  Chem.  Soc.,  81,  1509  (1902);  Ber.,  32,  2329  (1899). 
2Schaum,  Ber.,  31,  1964  (1898). 


TAUTOMERISM  AND  DESMOTROPISM  269 

acetate  by  the  action  of  sodium  ethylate.     This  reaction  was  interpreted 
in  the  sense  of  Geuther's  formula  by  means  of  the  following  scheme  : 

X)Na 

+  NaOC2H5  =  CH3-C^-OC2H5 
XOC2H5  \OC2H5 

ONa 


/25  \ 

CH3-C<  +       >CH.COOC2H5 

\OC2H5        H/ 

ONa 


C=CH- 


=  2C2H5OH  +  CH3C=CH-COOC2H5 

The  hydroxycrotonic  acid  formula  failed,  however,  to  explain  many  of 
the  reactions  of  this  interesting  substance.  For  example,  methyl  iodide 
reacts  with  the  sodium  salt  of  ethyl  acetoacetate  to  give  a  product  in 
which  the  methyl  group  would  seem  to  be  in  union  with  carbon  and  not 
with  oxygen  as  was  to  be  expected.  All  of  the  reactions  of  this  substance 
point  in  fact  to  the  formula 

CH3CO.CH.COOC2H5 


CHj 


and  it  must,  therefore,  be  regarded  as  a  derivative  of  a  true  ketone, 
CH3COCH2COOC2H5.  Indeed  at  this  time  so  many  of  the  reactions  of 
ethyl  acetoacetate  were  found  to  be  in  harmony  with  the  above  formula 
that  not  only  did  it  displace  the  original  hydroxycrotonic  acid  formula, 
but  even  the  sodium  derivative  of  ethyl  acetoacetate  came  to  be  written 
as  CH3CO  •  CH(Na)  -  COOC2H5. 

In  the  course  of  time,  however,  an  increasing  number  of  substances 
were  discovered  which  corresponded  to  Geuther's  formula.  Thus  the 
main  product  of  the  reaction  between  ethyl  chloroformate  and  the  sodium 
salt  of  ethyl  acetoacetate  is 

CH3— C=CHCOOC2H5 

0-COOC2H5 
although  the  isomeric  acetylmalonic  ester 

CH3CO-CH(COOC2H5)2 

is  also  formed  in  small  quantities.  This  reaction  definitely  proves  that 
ethyl  acetoacetate  is  capable  of  reacting  in  the  sense  of  two  different 


270  THEORIES  OF  ORGANIC  CHEMISTRY 

formulas,  and  that  it  is  therefore  a  tautomeric  substance.  The  question 
as  to  the  mechanism  of  these  reactions  still  remains  to  be  answered. 
Is  the  hydrogen  atom  actually  in  such  a  state  of  continual  vibration  in 
the  molecule  that  the  substance  is  at  any  moment  capable  of  reacting  in 
two  ways  or  has  the  substance  a  fixed  and  definite  structure  which 
suffers  isomerization  prior  to  the  formation  of  derivatives  possessing  a 
different  structure? 

On  the  basis  of  the  latter  assumption  it  was  possible  to  support  anew 
the  hydroxycrotonic  acid  formula  for  ethyl  acetoacetate.  The  forma- 
tion of  alkylated  derivatives,  for  example,  has  been  explained  by  Nef 
on  the  assumption  that  the  alkyl  halide  adds  directly  to  the  ethylene 
carbon  atoms, 

ONa  ONa    I     CH3 

\/       I 
CH3C==CHCOOC2H5  +  CH3I    -+    CH3C CH-COOC2H5 

and  that  the  hypothetical  addition  product  then  decomposes  im- 
mediately to  give  sodium  iodide  and  the  carbon  substituted  ester 
CH3CO-CH(CH3)COOC2H5.  The  objection  to  this  interpretation  as 
to  the  course  of  the  reaction  is  to  be  found  in  the  fact  that  no  addition 
products  of  this  type  have  as  yet  been  isolated.  Although  it  was 
thought  quite  recently  that  substances  of  this  type  had  finally  been 
discovered  it  was  proved  that  colloidal  sodium  halide  combinations 
had  been  mistaken  for  them.1 

In  1900  Claisen  2  succeeded  in  regulating  the  conditions  governing 
the  reaction  between  acid  chlorides  and  ethyl  acetoacetate  so  as 
to  obtain  at  will  either  one  of  two  classes  of  derivatives — viz., 
those  in  which  the  acyl  group  is  in  union  with  oxygen  or  carbon, 
respectively.  Acyl  derivatives  of  the  hydroxycrotonic  acid  type  were 
obtained  almost  exclusively  when  ethyl  acetoacetate  was  treated  with 
acid  chlorides  in  the  presence  of  pyridine,  while  substitutions  of  the 
acyl  group  on  the  methylene  carbon  occurred  when  sodium  alcoholate 
was  used  as  a  condensing  agent.  It  was  discovered  further  that  deriva- 
tives of  the  first  class  could  be  transformed  into  the  second  by  warming 
with  potassium  carbonate  or,  better,  with  the  potassium  salt  of  ethyl 
acetoacetate. 

As  a  result  of  these  investigations  it  seems  probable  that  derivatives 
of  the  hydroxycrotonic  acid  type  form  the  primary  products  in  the 
alkylation  of  ethyl  acetoacetate,  and  that  these  then  suffer  a  secondary 

1  Ber.,  38,  3217  (1905);  39,  1436  (1906). 
2Ber.,  33,  1242,3778  (1900). 


TAUTOMERISM  AND  DESMOTROPISM  271 

rearrangement  under  the  influence  of  the  sodium  salt  of  ethyl  aceto- 
acetate  as  is  expressed  by  the  following  equation : 

ONa  OC2H5 

CH3O=CHCOOC2H5  +  C2H5I    ->     CH3C=CHCOOC2H5  +  Nal 
OC2H5  C2H5 

CH3C=CHCOOC2H5    ->    CH3COCHCOOC2H5  l 
I  II 

This  explanation,  however,  also  lacks  complete  experimental  demon- 
stration, for  while  Claisen  has  succeeded  in  preparing  /3-ethoxycrotonic 
ester,  he  never  was  able  to  transform  either  it  or  the  0-methoxy  derivative 

OCH3 
CH3C=CHCOOC2H5 

into  the  corresponding  a-ethyl  and  a-methyl  acetoacetates. 

The  general  status  of  the  whole  question  may  be  summed  up  by 
saying  that  the  sodium  derivative  of  ethyl  acetoacetate  is  commonly 
supposed  to  have  the  formula 

ONa 
CH3C=CHCOOC2H5 

while  the  /3-ketone  ester  itself  is  regarded  as  an  equilibrium  mixture  of 
both  modifications  (end  and  keto) 

OH 
CH3C=CHCOOC2H5    *±    CH3CO-CH2COOC2H5 

The  latter  fact  has  been  demonstrated  through  the  researches  of  Stobbe  2 
and  Hantzsch3  who  have  recently  succeeded  in  resolving  ethyl  aceto- 
acetate into  its  desmotropic  forms.  Both  keto  and  enol  modifications 
have  also  been  separated  in  pure  condition  from  the  equilibrium  ester  as 
a  result  of  the  efforts  of  Knorr4  in  conjunction  with  his  students, 
O.  Rothe  and  H.  Averbeck.  The  keto  modification  may  be  obtained 
by  extracting  ordinary  ethyl  acetoacetate  with  ether,  alcohol,  hexane, 
and  other  solvents  cooled  at  —78°  when  the  pure  ester  separates  in 

iBer.,  46,  3157  (1912). 

2  Annalen  der  Chemie,  362,  132  (1907). 

3  Ber.,  43,  3049  (1910);  44,  1773  (1911). 
4Ber.,  44,  1147  (1911). 


272 


THEORIES  OF  ORGANIC  CHEMISTRY 


crystalline  form  on  evaporation  of  the  solvent.  The  end  modification, 
on  the  other  hand,  may  be  obtained  by  decomposing  the  sodium  salt 
of  ethyl  acetoacetate  at  —78°  with  dry  hydrochloric  acid  gas.  The 
properties  of  the  pure  substances  and  of  their  equilibrium  mixture  are 
shown  in  the  following  table : 


Ethyl  Acetoacetate 
CH3COCH2COOC2H5 


Ethyl  /3-Hydroxycrotonate 
CH3C(OH)  =  CH-COOC2H3 


melting  at  -39°. 
Crystallizes  from  con- 
centrated solutions  of 
ether  or  ethyl  alcohol 
at  -78°. 

Distills  between  40°-41° 
at  2  mm.  pressure. 


cooled  with  liquid  air  solidi- 
fies to  a  glassy  mass  which 
soon  becomes  crystalline. 


Distills  unchanged  at  33°  in  a 
strong  vacuum  if  small  quan- 
tities are  used. 


Equilibrium  Ester 
(Enol  and  Keto) 


Long,   colorless  needles     Colorless  oil  at    —78°,   when     Liquid. 


Distills  between  39°  and 
40°  at  2  mm.  pressure. 


n}?  =  1.4225 


=  1.4480 
=  1.01 19 


n}? 


1.423-1.4232 


Does  not  react  at  once 
with  FeCl3  at  -40°; 
but  will  react  slowly, 
due  to  the  catalytic 
action  of  FeCl3. 

Does  not  react  instantly 
with  bromine. 

Unchanged  at  —78°  for 
long  periods  of  time  if 
no  catalytic  agent  is 
present. 

Rearranges  to  form  the 
equilibrium  ester  at 
room  temperature  in 
the  course  of  weeks  or 
even  months, 


Reacts    instantly   with    FeCls 
even  at  -78°. 


Reacts  instantly  with  bromine. 


Unchanged  at  —78°  for  long 
periods  of  time  if  no  catalytic 
agent  is  present. 


Rearranges  to  form  the  equi- 
librium ester  at  room  tem- 
perature in  from  10  to  14 
days. 


The  keto  and  end  modifications  of  ethyl  acetoacetate  also  differ 
considerably  in  their  respective  indices  of  refraction  for  sodium  light. 
Knorr,  with  the  assistance  of  his  students,  has  determined  the  indices 
of  refraction  in  the  case  of  mixtures  containing  known  quantities  of  the 


TAUTOMERISM  AND  DESMOTROPISM 


273 


two  forms  and  has  obtained  the  following  values    for  wave    length 
D  at  10°  C. 


Per  Cent 

Per  Cent 

10 

Enol 

ii}f 

Enol 

n™ 

0 

1.4225 

50 

1.4352 

2 

1.4230 

75 

1.4417 

25 

1.4287 

100 

1.4480 

Since  the  index  of  refraction  of  ordinary  ethyl  acetoacetate  has  been 
found  to  be  equal  to  1.423  under  these  conditions,  it  would  seem  to 
follow  that  this  substance  may  definitely  be  regarded  as  an  allelotropic 
mixture  consisting  of  about  2  per  cent  of  the  enol  and  98  per  cent  of  the 
keto  modification. 

Working  along  somewhat  different  lines  Kurt  H.  Meyer1  has  ob- 
tained values  which  seem  to  indicate  the  presence  of  a  rather  higher 
per  cent  of  enol  in  the  equilibrium  mixtures  than  the  2  per  cent  referred 
to  above.  Meyer's  method  was  based  upon  the  fact  that  in  alcohol 
solution  the  enol  modification  reacts  instantly  with  bromine  while  the 
keto  form  does  not.  It  is  thus  possible  to  determine  the  amount  of  enol 
present  in  a  given  equilibrium  mixture  by  direct  titration  with  bromine. 
Certain  difficulties  had  to  be  overcome,  however,  in  the  working  out  of 
this  method  since  it  was  necessary  to  obviate  the  possibility  of  any 
change  in  the  condition  of  equilibrium  during  the  process  of  titration 
if  the  results  were  to  prove  reliable. 

It  was  known,  for  example,  that  the  mere  act  of  dissolving  the  ester 
might  produce  change  in  the  composition  of  the  mixture,  since  solution 
has  frequently  been  shown  to  favor  the  formation  of  one  desmotropic 
modification  at  the  expense  of  the  other.  The  presence  of  bromine  in 
excess  might  also  serve  to  bring  about  such  a  change,  for  this  reagent 
has  been  observed  to  function  as  a  catalyst  in  certain  instances  and  to 
induce  isomerization  in  a  given  direction.  While  these  and  other  obvious 
sources  of  error  undoubtedly  exist,  Meyer  and  Kappelmeier  were, 
nevertheless,  able, — by  working  at  low  temperatures  and  by  avoiding 
the  presence  of  an  excess  of  bromine, — to  reduce  these  errors  to  a 
minimum  and  thus  to  develop  the  method  to  a  very  high  degree  of 
accuracy.  Their  results  show  that  at  ordinary  temperatures  the 
so-called  "  equilibrium  ester  "  contains  7.4  per  cent  of  the  enol  modi- 
fication. The  difference  between  these  figures  and  those  of  Knorr  has 
1  Annalen  der  Chemie,  380,  220  (1911);  Ber.,  44,  2718  (1911). 


274  THEORIES  OF  ORGANIC  CHEMISTRY 

not  as  yet  been  explained.1  Nevertheless  the  two  experiments  taken 
together  firmly  establish  the  fact  that  ordinary  ethyl  acetoacetate  is 
indeed  an  allelotropic  mixture  consisting  of  a  relatively  large  quantity 
of  the  keto  and  a  small  quantity  of  the  enol  modification. 

The  condition  of  equilibrium  in  ordinary  liquid  ethyl  acetoacetate  was 
only  very  slightly  changed  by  heating  at  its  boiling  point.2  It  showed 
in  two  separate  titrations,  7.22  and  6.92  per  cent  of  enol.  Freshly 
distilled  ethyl  acetoacetate,  on  the  other  hand,  contains  20-25  per  cent 
of  enol,  but  reverts  on  long  standing  to  the  ordinary  equilibrium  mixture. 
These  observations  are  in  general  agreement  with  those  of  Schaum 
and  Traube,  who  have  noted  that  freshly  distilled  ethyl  acetoacetate 
possesses  a  different  specific  gravity  and  also  a  different  viscosity  from 
that  of  ordinary  ethyl  acetoacetate. 

Velocity  determinations  show  that  ketonization  takes  place  much 
more  readily  than  enolization.  As  the  result  of  a  series  of  titrations 
Meyer  found  that  the  constants  KI  and  K2  are  equal,  respectively,  to 
0.00055  and  0.000046  and  it  has  since  been  calculated  that  the  period 
required  for  a  complete  oscillation  from  keto  — >  enol  — >  keto  equals 
approximately  17  days.3  In  visualizing  this  process  according  to  the 
terms  of  the  kinetic  theory,  the  hydrogen  atom  must  be  imagined  as  in 
constant  and  rapid  motion.  The  average  distance  traveled  in  any 
single  oscillation  is  not,  however,  sufficient  to  bring  the  hydrogen 
outside  the  sphere  of  influence  of  the  atom  (oxygen  or  carbon)  with 
which  it  is  in  union  and  does  not,  therefore,  result  in  any  change  in  the 
distribution  of  affinity  within  the  molecule.  Since,  however,  individual 
oscillations  must  be  supposed  to  vary  considerably  in  range,  it  may 
happen  that  once  in  every  17  days  the  velocity  of  the  hydrogen  will 
prove  sufficient  to  carry  it  beyond  the  sphere  of  its  own  atom  and 
within  the  radius  of  attraction  of  the  adjacent  (carbon  or  oxygen)  atom. 
Under  these  conditions  an  exchange  of  valencies  would  result. 

The  period  required  for  a  complete  oscillation  from  keto  — > 
enol  — >  keto  is  influenced  in  different  degrees  by  different  solvents. 
Thus  in  aqueous  solution  it  equals  100  minutes  and  in  alcohol  108 
minutes.  Catalytic  agents  have  a  tendency  to  shorten  this  period. 

While  equilibrium  mixtures  of  ethyl  acetoacetate  have  been  found 
to  vary  considerably  under  different  conditions,  equilibrium  may  in 
general  be  said  to  depend  upon  the  nature  of  the  solvent,  the  degree  of 
solubility  in  the  solvent,  the  temperature,  and  the  relative  concentration 
of  both  forms  in  the  solution. 

1  Meyer's  work  has  recently  been  repeated  and  verified.     See  Ber.,  47,  837  (1914). 

2  Compare  Ber.',  44,  2732  (1911). 

3  Annalen  der  Chemie,  380,  235  (1911);   compare  Knorr,  Ber.,  44,  1147  (1911). 


TAUTOMERISM  AND  DESMOTROPISM 


275 


The  per  cent  of  enol  present  in  various  solvents  at  a  temperature 
of  10°  C.  has  been  determined  by  Knorr,  Rothe,  and  Averbeck  1  by 
means  of  optical  methods.  Their  results  are  given  in  the  following 
table: 


Solvent 

Percentage 
concentration 
of  the  solution 

Percentage 
of  enol 

Ethyl  ether  
Carbon  bisulphide  
Chloroform 

30 
30 
50 

11.0 
25.0 
2  0 

Petroleum  ether  

3 

27.5 

Hexane  

3 

31.0 

The  influence  of  concentration  upon  the  equilibrium  of  such  systems 
has  been  investigated  by  Meyer  and  Kappelmeier,  who  have  found  that 
within  the  limits  which  may  be  accurately  studied  by  titration  methods, 
dilution  seems  to  favor  the  formation  of  the  enol  modification.2  The 
results  obtained  with  solutions  of  ethyl  acetoacetate  in  absolute  ethyl 
alcohol  are  recorded  in  the  following  table : 


Percentage 
of  ester  in 
solution 

Percentage 
of  enol  form 

Percentage 
of  ester  in 
solution 

Percentage 
of  enol  form 

65 

7.8 

11.0 

11.6 

57 

8.1 

5.6 

12.5 

34 

8.7 

2.2 

12.7 

25 

10.2 

0.8 

13.2 

19 

10.8 

Changes  in  the  condition  of  equilibrium  due  to  substitutions  in  the 
ethyl  acetoacetate  molecule  have  been  investigated  by  Knorr  and 
co-workers  and  their  results  show  that  while  the  ethyl  ester  contains 
7.4  per  cent  enol  under  ordinary  conditions,  the  corresponding  methyl 
ester  contains  only  4  per  cent.  Ethyl  benzoylacetate  has  also  been 
studied  by  Knorr  3  and  Meyer,4  both  of  whom  have  succeeded  in  separat- 

1  Compare  K.  Meyer,  Annalen  der  Chemie,  380,  226  (1911). 

2  For  solution  in  benzene,  carbon  bisulphide,  and  hexane  see  Ber.,  44,  2723  (1911); 
also  see  Annalen  der  Chemie,  380,  231  (1911). 

3  Ber.,  44,  2767(1911). 
*  Ber.,  44,  2729  (1911). 


276  THEORIES  OF  ORGANIC  CHEMISTRY 

ing  the  end  modification  in  pure  condition.  This  substance  is  described 
as  a  white  solid  which  melts  at  41°  C.  The  ester  in  common  use  consists 
of  an  allelotropic  mixture  which  contains  a  relatively  smaller  percentage 
of  the  enol  than  of  the  keto  modification.  In  the  case  of  the  methyl 
ester  the  enol  present  equals  16.7,  and  in  the  case  of  the  ethyl  ester, 
29.2  per  cent.  Heat  tends  to  increase  the  relative  amount  of  enol 
present  in  this  mixture.  The  rate  of  isomerization  from  enol  — > 
keto  — »  enol  is  greater  than  in  the  case  of  ethyl  acetoacetate  but 
seems  to  vary  in  much  the  same  way  under  the  influence  of  different 
solvents,  concentration,  etc.  The  following  table  gives  the  relative 
amounts  of  enol  present  in  methyl  acetoacetate  and  related  compounds 
and  is  of  interest  for  purposes  of  comparison : 


Per  Cent 
of  enol 

Methyl  ester  of  acetoacetic  acid  .  .      .    . 

4  4 

Methyl  ester  of  methyl  acetoacetic  acid  

3.16 

Ethyl  ester  of  brom  acetoacetic  acid  
Diethyl  ester  of  acetondicarboxylic  acid    .        ... 

4.0 
16  8 

Acetylacetone  1 

80  4 

Benzoylacetone  

98-99 

It  may  be  added  that  molecular  weight  determinations  of  ethyl 
acetoacetate  in  chloroform  by  the  freezing-point  method  show  that  it  is 
still  mono-molecular  even  at  —  62 °.2 

In  summary  it  may  be  said  that  an  exact  knowledge  of  the  character 
of  enol-keto  mixtures  can  be  obtained  only  by  the  application  of  all 
available  physical  and  chemical  methods.3  Among  the  chemical 
methods  the  so-called  "  brom-t  it  ration  method  "  of  K.  H.  Meyers  has 
proved  to  be  of  great  value.  The  method  of  ozonide  decomposition, 
which  has  been  developed  by  J.  Scheiber  and  P.  Herold4  is  also  of 
importance  since  in  cases  where  the  structure  of  the  compound  under 
investigation  is  very  complex  it  affords  a  point  of  attack  in  determining 
the  constitution  of  the  enolization  products.  Among  the  physico- 
chemical  methods,  refractometric  methods, — thanks  to  the  exact  work 
of  K.  von  Auwers  and  his  students, — have  been  found  to  give  excellent 
results  and  at  the  same  time  to  be  of  general  application. 

^er.,  44,  2771  (1911). 

2  Annalen  der  Chemie,  362,  147(1907). 

3  Annalen  der  Chemie,  416,  169  (1917). 

4  Annalen  der  Chemie,  405,  295  (1914). 


CHAPTER  XII 
IONIZATION   ISOMERISM 

THE  subject  matter  presented  under  this  title  represents  a  special 
phase  of  tautomerism  and  desmotropism  and  forms  part  of  the  more 
general  topic  of  molecular  rearrangements  to  be  considered  later.  It 
owes  its  development  historically  to  the  study  of  the  constitution  of 
aliphatic  nitro-compounds : 

According  to  Victor  Meyer  the  structure  of  aliphatic  nitro-com- 
pounds may  be  correctly  represented  by  the  formula  R-CH^NC^ 
since  all  of  the  chemical  relationships  of  these  substances  are  adequately 
represented  in  this  way.  Such  a  formula  assumes,  of  course,  that 
metallic  derivatives  have  the  constitution 

R-CHNO2 

(M- metal) 

M 

but  this  was  in  full  accord  with  the  chemical  theory  of  the  time,  which 
supposed  that  hydrogen  atoms  in  union  with  carbon  could  be  directly 
replaced  by  alkali  metals. 

In  1888  A.  Michael l  came  to  the  conclusion  that  if  there  were  a 
choice  between  carbon  and  oxygen  in  the  position  to  be  occupied  by  a 
metallic  atom  in  the  process  of  salt  formation,  the  choice  would  always 
fall  to  the  oxygen.  This  conclusion  was  reached  as  the  result  of  a  study 
of  the  derivatives  of  ethyl  acetoacetate,  diethyl  malonate,  and  ether 
similar  substances  containing  acidic  'methylene  groups.  Reasoning 
from  analogy,  Michael  assigned  the  following  formula  to  the  sodium 
derivative  of  nitromethane : 

0 

H2C=NONa 
This  conception  soon  found  a  zealous  advocate  in  J.  U.  Nef,2  who 

1  Jour,  prakt.  Chemie,  37,  507  (1888). 

2  Annalen  der  Chemie,  270,  330  (1892);  280,  263  and  290  (1894);  Ber.,  29,  1222 

(1896). 

277 


278  THEORIES  OF  ORGANIC  CHEMISTRY 

believed  that  such  substances  were  to  be  regarded  as  the  salts  of  a 
hypothetical  acid: 

O 

H2C=NOH 

Experimental  confirmation  of  this  assumption  was  not  obtained 
until  1895  when  Holleman,1  Hantzsch,  and  Schultze,2  and  also  Konowa- 
low3  succeeded  in  isolating  isomeric  derivatives  of  certain  aromatic 
nitromethanes,  as,  for  example,  CeHsCH^NC^.  Isomerism  of  this 
kind  was  explained  by  supposing  that  desmotropic  modifications  such  as 

C6H5CH2N02     and     C6H5CH=NO.OH 

were  formed.  One  of  these  substances  was  found  to  be  a  solid  and  was 
regarded  as  the  labile  form  since  it  gradually  rearranged  to  give  a  stable 
liquid  modification.  The  solid  reacted  with  phenylisocyanate,  gave  a 
color  reaction  with  ferric  chloride,  and  behaved  in  many  ways  as  if  an 
hydroxyl  group  were  present,  while  the  liquid  form,  on  the  other  hand, 
showed  none  of  these  reactions.  Both  modifications  behaved  in  a  very 
remarkable  way  towards  soda  solutions.  Solid,  labile  phenylnitro- 
methane  dissolved  readily  in  cold  aqueous  sodium  carbonate.  The 
stable  liquid  variety,  however,  went  into  solution  slowly  and  reluctantly, 
and  its  solution  when  acidified  gave  a  precipitate  of  the  isomeric  labile 
form.  This  could  be  interpreted  by  assuming  that  rearrangement  of  the 
stable  into  the  labile  modification  preceded  the  act  of  solution.  At 
all  events  solution  was  evidently  accompanied  by  a  radical  change  in  the 
constitution  of  the  substance. 

According  to  Hantzsch  the  labile  modification  of  phenylnitromethane 
may  be  explained  on  the  basis  of  either  of  the  following  two  formulas : 

O  O 


C6H5CH=NOH  ,    or       C6H5-CH—  NOH 

since  each  represents  an  hydroxyl  group  as  present  in  the  molecule. 

Of  these  the  first  is  the  one  most  generally  accepted,  but  there  is 
actually  no  way  at  present  of  distinguishing  definitely  between  them. 
The  stable  modification,  which  is  characterized  by  neutral  reactions,  is 
commonly  assumed  to  possess  the  formula 


*Rec.  trav.  chim.  des  Pays-Bas,  14,  129  (1895);  15,  356  (1896);   16,  162  (1897); 
Ber.,  33,  2913  (1900). 

2Ber.,  29,  699,  2251  (1896);  also  compare  Hantzsch,  Ber.,  33,  2542  (1900). 
3  Ber.,  29,  2193  (1896). 


IONIZATION  ISOMERISM  279 

These  conclusions  were  naturally  carried  over  into  the  field  of  the 
purely  aliphatic  nitro-compounds  and  the  sodium  derivative  of  nitro- 
methane,  for  example,  was  represented  by  the  formula : 

H2O=NOONa 

This  immediately  raised  the  question  as  to  the  constitution  of  free 
nitromethane  itself.  If  it  is  a  true  nitro-compound,  i.e.,  CHsNC^,  it 
must  then  be  regarded  as  reacting  tautomerically  in  the  process  of 
salt  formation. 

In  an  effort  to  solve  this  problem  in  the  case  of  w-nitrophenyl- 
nitromethane  A.  F.  Holleman  l  conceived  the  idea  of  applying  con- 
ductivity measurements.  He  had  observed  that  when  an  aqueous  solu- 
tion of  m-nitrophenyl-nitromethane  is  treated  with  an  equivalent 
quantity  of  hydrochloric  acid,  the  solution  is  at  first  colored  and  then 
becomes  colorless.  This  color  change  occupies  only  a  few  minutes  and 
during  this  time  the  conductivity  of  the  solution  which  is  originally 
relatively  high,  decreases  gradually  but  very  noticeably.  Even  as 
early  as  1895  Holleman  suspected  that  m-nitrophenyl-nitromethane 
existed  in  two  modifications  and  that  of  these  the  labile  variety  formed 
salts.2 

Following  this  Hantzsch  and  his  students  determined  the  conductiv- 
ity of  a  freshly  prepared  solution  of  phenylisonitromethane  and  found 
that  while  such  a  solution  conducts  electricity,  its  power  to  do  so  gradu- 
ally decreases  and  finally  completely  disappears.  This  was  interpreted 
as  marking  the  gradual  change  of  the  isonitro  into  the  true  nitro-com- 
pound. Thus  the  sodium  derivatives  of  nitromethane  and  nitroethane 
may  be  assumed  to  correspond  respectively  to  the  formulas 

H2C=NO-ONa     and     CH3CH=NO-ONa 

and  their  aqueous  solutions  when  acidified  should  give  primarily  the 
free  isonitro-compounds.  In  aqueous  solution  both 

H2C=NO-OH    and     CH3CH=NO-OH 

1  Ber.,  33,  2913  (1900);  also  Ostwald,  Jour,  prakt.  Chemie,  31,  433  (1885);  Zeitschr. 
physikal.  Chemie,  3,  170  and  418(1889)  and  "Handbuch  der  physikal.  Chemie"; 
Walker,  Zeitschr.  physikal.  Chemie,  4,  319  (1889);     Bader,    ibid.,    6,    289    (1890); 
Walden,  ibid.,  8,  433  (1891);  Bredig,   ibid.,  13,  289   (1894);   Holleman,   Rec.  trav. 
chim.  des  Pays-Bas,  14,  129  (1895);   16,  162  (1897);  Henrich,  Ber.,  37,  1406  (1904). 

2  Rec.  trav.  chim.  des  Pays-Bas,  14,  129  (1895). 


280  THEORIES  OF  ORGANIC  CHEMISTRY 

should  conduct  the  electric  current,  but  in  the  event  of  their  transforma- 
tion into  the  true  nitro-compounds 

CH3N02    and     CH3CH2NO2 

this  property  should  vanish. 

To  test  these  conclusions  Hantzsch  devised  the  following  quantitative 
experiment:  starting  with  a  dilute  aqueous  solution  of  sodium  nitro- 
ethane,  he  added  the  exact  equivalent  of  hydrochloric  acid,  whereupon  a 
reaction  took  place  according  to  the  following  equation : 

CH3CH=NO  -  ONa+HCl  ->  NaCl+CH3CH=NO  •  OH 

Since  the  process  is  instantaneous  the  result  of  mixing  the  two 
solutions  is  to  give  immediately  a  mixture  of  Na+,  Cl~,  H+  and 
(CH3CH=NO  •  O)  ~~  ions  and  such  a  solution  ought  to  be  a  better  con- 
ductor than  a  solution  of  pure  sodium  chloride  of  the  same  concentra- 
tion. It  was  found  that,  as  a  matter  of  fact,  the  conductivity  of  the 
reaction  mixture  was  at  first  considerably  greater  than  that  of  a  solution 
of  pure  sodium  chloride  of  the  same  concentration  under  similar  condi- 
tions, but  that  this  higher  conductivity  was  not  maintained  and  gradu- 
ally decreased  until  finally  it  was  exactly  equal  to  that  of  a  pure  salt 
solution.  This  could  happen  only  as  a  result  of  the  combination  of  the 
positive  and  negative  ions  of  the  isonitro-compound  to  form  undissociated 
nitroethane,  viz., 

(CH3CH=NO-Or+H+  -»  CH3CH2N02 

The  reverse  process  may  also  be  followed  quantitatively  by  means 
of  conductivity  measurements.  While  strong  acids  react  instantly  with 
bases  to  form  salts,  true  nitro-compounds  usually  react  slowly  since 
they  must  first  be  isomerized  into  the  corresponding  isonitro-derivatives. 
In  order  to  fo?low  the  progress  of  this  reaction  Hantzsch  mixed  together 
equimolecular  solutions  of  nitroethane  and  sodium  hydroxide,  measuring 
the  conductivity  of  the  reaction  mixture  from  time  to  time.  His 
results  showed  that  in  the  beginning  the  conductivity  of  the  solution 
was  approximately  that  of  pure  sodium  hydroxide,  but  that  the  hydroxyl 
ions  decreased  in  quantity  as  more  and  more  salt  was  formed. 

CH3CH2NO2  +  Na+  +OH-    ->     (CH3CHNO  •  0)~  +  Na+  +  H20 

The  reaction  was  accompanied  by  a  gradual  decrease  in  the  conductivity 
of  the  mixture,  which  continued  until  the  conductivity  equaled  that  of  a 
solution  of  the  sodium  derivative  of  nitroethane  of  the  same  concen- 
tration. Hantzsch  was  thus  able  to  demonstrate  that  free  nitroethane 
is  a  true  nitro-compound  and  that  while  it  seems  to  react  with  bases  to 


IONIZATION  ISOMERISM  281 

form  salts  it  does  not  itself  actually  function  in  this  reaction,  but  is 
first  isomerized  into  the  corresponding  enol  form.  Substances  of  this 
kind,  which  are  not  actually  acids,  but  which  seem  to  react  like  acids, 
are  called  pseudoacids. 

In  order  to  account  for  the  relatively  great  reactivity  of  the  aci- 
nitro  compounds,  O.  Baudisch  1  formulates  the  reaction  by  which  such 
substances  are  formed  from  the  corresponding  nitro-compounds  in  the 
following  way: 

/  OKv  /  0_Kv 

+  KOH  /H\      |      \  /Hx  \ 

CH3-N02  >      H-^C-N       OH    or      H-^C— N          fc)H 

\H/     ||     /  IH/I    ||       f 

\  O    /  \  0        / 

I  II 

The  first  formula  corresponds  to  that  which  Werner  has  used  to  express 
the  constitution  of  ammonium  salts.  The  second  indicates  by  means  of 
a  waving  line  that  one  of  the  valencies  of  nitrogen  is  different  from  the 
others,  and  by  the  arrow  that  the  carbon  atom  approaches  a  trivalent 
condition.  Even  these  additions  fail  to  express  fully  the  actual  rela- 
tionships between  the  atoms  in  the  molecule,  and  at  best  it  must  be 
recognized  that  the  constitutional  formulas  in  current  use  give  an 
inadequate  representation  of  the  substances  for  which  they  stand. 

Certain  organic  compounds  containing  pentavalent  nitrogen  behave 
towards  acids  in  a  manner  analogous  to  that  which  has  just  been 
described  and  are,  therefore,  called  pseudobases.  An  example  of  such 
a  substance  is  to  be  found  in  the  cyclic  quaternary  base, 


/\ 
CH3  OH 

which  is  derived  from  methyl  phenylacridinium  chloride 


<     | 


CH3  Cl 
.,  49,  1162  (1916). 


282  THEORIES  OF  ORGANIC  CHEMISTRY 

by  replacing  the  chlorine  atom  by  an  hydroxyl  group.  Organo-ammo- 
nium  bases  are  usually  readily  soluble  in  water  and  their  solutions  con- 
duct the  electric  current.  The  above  substance,  however,  exhibits 
none  of  these  properties,  being  insoluble  in  water  and  completely  neutral 
in  all  of  its  chemical  reactions.  In  fact  it  behaves  more  like  phenyl 
methyl  acridol 

C6H5  OH 

X 


Nc6H, 

"N" 

CH3 


than  phenyl  methyl  acridinium  hydroxide.1 

In  order  to  understand  these  discrepancies  Hantzsch  studied  the 
reaction  by  means  of  conductivity  measurements.  His  results  show  that 
the  conductivity  of  the  solution  which  is  formed  by  mixing  together 
equimolecular  quantities  of  the  chloride  and  potassium  hydroxide  is  at 
first  approximately  equal  to  that  of  a  solution  of  pure  potassium  hydrox- 
ide of  the  same  concentration,  but  that  this  gradually  decreases  in  value 
until  it  is  finally  equal  to  zero.  Hantzsch  explains  this  by  supposing 
that  the  decomposition  of  methyl  phenylacridinium  chloride  is  accom- 
panied by  the  formation  of  a  true  ammonium  base  and  that  this  rear- 
ranges immediately  to  give  an  isomeric  substance  in  which  the  hydroxyl 
group  is  in  union  with  carbon. 


OH 

i 

c 

C6H4      |       ,6H4     ->      C6H4 

N  N 


I 

c 


UH3  ( 


CH3  Cl  CH3  OH  CH3 

Such  a  substance  shows  no  tendency  to  dissociate  and  is,  in  fact,  a  true 
pseudobase.2 

Other  illustrations  of  this  type  of  substance  are  to  be  found  among 
the  derivatives  of  quinoline  and  isoquinoline.  Thus  when  caustic 
alkali  or  moist  silver  hydroxide  reacts  with  any  of  the  salts  of  these 
substances  the  primary  products  of  the  reaction  are  true  ammonium 

1  Compare  rearrangement  of  NH2  instead  of  OH  in  derivatives  of  ammonium 
amide,  H4N-NH2,  Decker,  Ber,  39,  749  (1906);  46,  969  (1913). 
2Ber.,  32,  3109  (1899). 


IONIZATION  ISOMERISM 


283 


bases  (II),  but  these  rearrange  immediately  in  aqueous  solution  to  give 
pseudobases  (III)  : 


Quinoline  salt 
I 


Pseudobase 
III 


Rearrangement  products  in  which  the  hydroxyl  groups  are  represented 
as  in  union  with  carbon  are  referred  to  by  Decker  as  oxydihydro  or 
carbinol  bases.2 

A.  Kaufmann3  has  since  shown  that  these  carbinol  bases  behave 
like  acyclic  combinations  and  ascribes  to  them  the  tautomeric  structure 
of  amino  aldehydes.  According  to  his  interpretation  the  pseudoam- 
monium  base  III  should  be  regarded  as  an  ortho  alkyl  amino  derivative 
of  cinnamic  aldehyde  IV: 


CH 


CHO 


IV 


After  Willstatter  had  recognized  that  the  anthocyanines  represent  a 
large  class  of  naturally  occurring  derivatives  of  the  oxonium  bases  he 
discovered  that  the  colored  cyanidine  chloride  isomerizes  into  a  colorless 
compound  under  the  action  of  alkali.  In  other  words,  the  colored  base 
changes  into  a  colorless  pseudobase  ;4 

iRantzsch,  Ber.,  32,  575  (1899). 

2Ber.,  26,  3327  (1892);  see  also  Jour,  prakt.  Chemie,  47,  28  (1893);  Ber.,  33, 
1715  (1900);  Ber.,  36,  2588,  2589,  3068  (1902);  Roser,  Annalen  der  Chemie,  264, 
362  (1889);  Freund,  Ber.,  22,  2337  (1889). 

3  Kaufmann  and  Strubin,  Ber.,  44,  680;   Kaufmann  and   Play-Janini,  Ber.,  44, 
2670  (1911).     See  also  Koenig,  Jour,  prakt.  Chemie,  83,  409  (1911). 

4  Compare  Annalen  der  Chemie,  408,  21  (1915). 


284 


THEORIES  OF  ORGANIC  CHEMISTRY 


HO 


H  ->  H 


Cyanidine  chloride 

o 


OH 


OH 

Cyanidine,  violet  color  base 

OOH 


OH 


Hor  H 


H 


OH/\ 
H     OH 


Colorless  pseudobases 


In  such  cases  the  molecule,  which  undergoes  dissociation  in  solution, 
must  be  supposed  to  possess  an  entirely  different  constitution  from  the 
undissociated  molecule.  Pseudoacids  and  pseudobases  represent  organic 
combinations  which  are  characterized  by  this  type  of  isomerism,  and 
Hantzsch  uses  the  term  "  ionization-isomerism  "  1  in  order  to  differen- 
tiate this  from  other  forms  of  tautomeric  change.  He  has  classified  a 
great  many  substances  among  the  pseudoacids  which  had  previously 
been  regarded  as  reacting  directly  v/ith  bases  to  form  salts,  and  includes 
in  this  category  all  nitro-compounds  in  which  at  least  one  hydrogen  atom 
is  in  union  with  the  carbon  atom  to  which  the  nitro  group  is  attached, 
as,  for  example,  CH3CH2NO2,  (CH3)2CH-NO2,  CH(N02)3,  etc.; 
nitramines,  RNH-NO2;  nitrosamines,  R-NH-NO;  oximido-ketones, 
>CO — C=NOH;  quinone  oximes;  hydroxyazo  compounds;  nitro- 

I 

phenols;  etc.  In  all  these  and  many  other  cases  metallic  derivatives 
are  referred  not  to  the  usual  formula  of  the  substance  but  to  an  isomeric 
formula  (enol  modification),  such  as,  for  example: 


CH3-C 


NOOH 


NO2 


NOOH 


STH 


etc. 


Compare  Hantzsch  and  coworkers,  Ber.,  32,  575,  3066  (1899);  also  Ber.,  32, 
600,  607,  628,  641,  3137,  3148  (1899);  33,  278,  752,  2542  (1900);  34,  2506,  3142 
(1901);  36,  226,  249,  877,  883,  1001  (1902);  37,  1076,  2705,  3434  (1904);  38,  998, 
1004,  2143,  2161  (1905);  39,  139,  153,  1084,  1365,  2472,  2478,  3072,  3080,  4153;  also 
3149  (1906);  also  Zeitschr.  physikal.  Chemie,  48,  289  (1904). 


IONIZATION  ISOMERISM 


285 


The  color  bases  or  carbinols  derived  from  the  rosaniline  dyes  may  be 
regarded  according  to  Hantzsch  as  pseudobases.  Thus  in  the  case  of 
crystal  violet,  conductivity  determinations  on  the  system  R-NCl-f 
NaOH  at  zero  degrees  have  demonstrated  that  at  the  beginning  of  the 
reaction  the  conductivity  of  the  mixture  was  greater  than  that  of 
sodium  chloride  of  the  same  concentration.  The  observed  conductivity 
at  this  time  corresponded  almost  exactly  to  that  of  the  system 
RN++Cr+Na++OH~.  This  solution  was  colored  and  reacted 
strongly  alkaline.  It  was  observed  that  as  the  reaction  proceeded 
these  properties  slowly  disappeared  and  that  the  solution  became 
colorless,  the  alkalinity  became  less,  and  the  conductivity  decreased 
until  it  finally  became  equal  to  that  of  sodium  chloride.  Hantzsch 
interpreted  this  by  supposing  that  the  primary  product  of  the  reaction 
is  a  true  ammonium  salt  which  gradually  isomerizes  to  give  a  colorless, 
insoluble,  undissociated  carbinol  combination  or  pseudobase.1 


N 
(R)2OH 

True  ammonium  base 


The  maximum  value  for  the  conductivity,  as  compounded  additively 
from  the  sum  of  the  conductivities  of  the  ions  Na++Cl~+XN++OH~, 
was  realized  only  in  the  case  of  one  single  substance — namely,  crystal 
violet.  Under  similar  conditions  and  even  at  zero  degrees  all  other  dyes 
examined  gave  low  conductivity  values  and  some  approached  or  equaled 
that  of  pure  sodium  chloride.  From  these  facts  Hantzsch  draws  the  con- 
clusion that  the  isomerization  of  the  true  color  base  (of  the  ammonium 
type)  into  the  pseudobase  (of  the  carbinol  type)  takes  place  so  rapidly 
that,  even  in  the  time  required  for  the  first  conductivity  measurement, 
the  change  is  either  almost  or  wholly  complete. 

A  base  which  is  characterized  by  solubility,  strong  basicity,  etc., 
has  recently  been  discovered  by  Homolka.  This  substance  is  formed 
as  an  intermediate  when  pararosaniline  (new  fuchsine)  is  decomposed 
by  potassium  hydroxide  and,  according  to  Hantzsch,  may  be  regarded  as 


Ber.,  33,  282  (1900). 


286 


THEORIES  OF  ORGANIC  CHEMISTRY 


an  imidobase  which  is  derived  from  the  true  ammonium  base  by  loss  of 
water : 


(C6H4NH2)2 

II 
C 


NaOH 


(C6H4NH2)2        (C6H4NH2)2 

II  II 

C  C 


(CGH4NH2)2 

II 
C—  OH 


-  H2O 


+  H20 


N— OH 
H2 

True  base   


Imido  base 


II 


(Homolka's  base) 
III 


NH2 


Carbinol 
(Pseudo  base) 

IV 


Thus  "  when  a  dye  (I)  is  treated  with  alkali  the  primary  product  is 
always  a  true  ammonium  base  (II).  This  may  either  isomerize  slowly 
to  give  the  pseudobase  (IV);  or,  due  to  the  presence  of  an  excess  of 
alkali  it  may  quickly  lose  water  and  form  an  imido  base  (III).  The 
latter  may  then  slowly  add  water  to  give  the  pseudobase."1 

Hantzsch  and  Osswald  discovered  that  other  groups  rearrange  in  the 
same  way  as  hydroxyl  and  that  in  the  case  of  cyclic  quaternary  cyanides 
the  reaction  may  be  expressed  as  follows : 


— CN 


R2N— CN 

According  to  Hantzsch  intramolecular  rearrangements  of  this  type  may 
be  assumed  in  all  cases  where  dyes  of  this  class  show  conductivities  which 
are  lower  than  the  ideal  maximum  values.  He  supposes  that  such 
reactions  are  instantaneous  in  cases  where  the  initial  conductivity  of  the 
reaction  mixture  equals  that  of  sodium  chloride.2  Criteria  other  than 
those  which  have  been  mentioned  must,  of  course,  be  considered,  but 
none  of  those  which  have  been  suggested  by  Hantzsch  is  entirely 
satisfactory  and  some,  at  least,  are  generally  discredited  at  the  present 
time. 

iBer.,  33,  760  (1900);  37,  3434  (1904). 

2Ber.,  32,  578-579(1899). 


IONIZATION  ISOMERISM  287 

To  recapitulate  briefly:  "when  neutralization  phenomena  do  not 
take  place  instantaneously  but  represent  slow  processes  extending  over  a 
considerable  period  of  time  the  presence  of  pseudoacids  or  pseudobases 
may  be  assumed."  It  does  not  follow,  however,  that  the  reverse  process, 
whereby  pseudoacids  or  bases  are  themselves  formed  from  the  salts  of 
the  true  acid  or  base,  is  also  a  slow  process.  Indeed  this  change,  as  for 
example, 

CH3CH=NO-ONa+HCl  ->  CH3CH2NO2+NaCl 

may  take  place  with  such  rapidity  that  it  cannot  be  followed 
quantitatively. 

It  has  been  observed  in  the  case  of  violuric  acid  and  other  oximido- 
ketones  that  these  combinations  when  heated  exhibit  abnormally  large 
coefficients  of  conductivity  and  also  abnormally  large  dissociation  con- 
stants. It  therefore  seems  probable  that  such  substances  possess  at 
high  temperatures  different  constitutions  from  those  which  are  regularly 
assigned  to  their  colored  ions  and  salts.  According  to  Hantzsch l 
"  The  presence  of  ionization-isomerism  may  be  assumed  in  the  case  of 
all  tautomenc  substances  which  upon  heating  show  increases  in  the 
value  of  their  coefficients  of  conductivity  and  of  their  dissociation 
constants." 

Hantzsch's  theory  in  regard  to  pseudoacids  and  pseudobases  has 
been  outlined  without  reference  to  the  objections  which  have  been 
offered  and  continue  to  be  offered  to  it  by  different  investigators.  It 
may  be  said  in  conclusion  that  such  objections  have  not,  for  the  most 
part,  been  directed  against  the  main  conceptions  embodied  in  the  theory 
which  appear  to  be  fundamentally  sound  and  represent  a  valuable  exten- 
sion of  the  theory  of  tautomerism  and  molecular  rearrangements.  On 
the  other  hand,  many  of  the  methods  which  have  been  suggested  as  a 
means  for  recognizing  phenomena  of  this  sort  have  been  rejected.  The 
universality  with  which  the  theory  has  been  developed  by  Hantzsch 
has  also  been  subject  to  attack.  References  to  this  controversy  may  be 
found  in  a  footnote  2  since  it  is  impossible  to  consider  the  matter  more 
fully  at  this  time. 

1  LOG.  cit. 

2Zawidzki,  Ber.,  36,  3334  (1903);  37,  154;  Hantzsch,  Ber.,  37,  1076;  Zawidzki, 
Ber.,  37,  2298  (1904);  Bamberger,  2468;  Hantzsch,  1084  and  2705  (1905);  Kauf- 
mann,  Zeitschr.  physikal.  Chemie,  47,  618  (1904);  Ber.,  37,  2468  (1904);  Ley  and 
Hantzsch,  Ber.,  39,  3149  (1906);  Euler,  Ber.,  39,  1607  (1906);  Hantzsch,  2093; 
Euler,  2265;  Hantzsch,  2703;  H.  Lunden,  Zeitschr.  physikal.  Chemie,  64,  532  (1906); 
Hantzsch,  ibid...  56,  57  (1906). 


CHAPTER  XIII 

THE     APPLICATION     OF    PHYSICO-CHEMICAL     PRINCIPLES 
TO   ORGANIC   CHEMISTRY 

DURING  the  first  half  of  the  last  century  the  epoch-making  dis- 
coveries of  men  like  Gay-Lussac,  Avogadro,  Ampere,  Faraday,  Mitscher- 
lich,  Bunsen,  Dulong,  and  Petit  served  to  enrich  both  chemistry  and 
physics.  At  that  time  most  chemists  were  familiar  with  physical 
methods,  but,  as  organic  chemistry  gradually  differentiated  itself  from 
other  branches  of  chemistry  and  began  to  develop  into  a  separate  science, 
it  became  more  and  more  apparent  that  a  knowledge  of  physical  methods 
need  not  be  the  necessary  equipment  of  a  productive  investigator  in 
this  field.  In  one  of  his  lectures  1  W.  Nernst  describes  how  the  physical 
apparatus  which  had  originally  been  present  in  all  chemical  laboratories 
gradually  became  more  and  more  superfluous  until  finally  it  vanished 
completely.  For  a  long  period  of  time  organic  chemists  were  absorbed 
with  problems  of  structural  organic  chemistry.  They  learned  that 
carbon  is  capable  of  forming  a  very  great  number  of  compounds  with  a 
relatively  small  number  of  elements  and  they  gradually  acquired  such 
an  exact  insight  into  the  constitution  of  organic  molecules  that  they 
were  able  to  describe  the  position  and  function  of  every  atom.  In 
this  way  structural  and  constitutional  formulas  were  developed,  and 
only  then  did  it  become  obvious  that  a  definite  relation  exists  between 
certain  physical  properties  of  organic  compounds  and  their  structure  or 
constitution. 

In  any  study  of  the  relationship  which  exists  between  the  physical 
properties  of  a  substance  and  its  chemical  constitution  it  is  necessary  in 
the  first  place  to  understand  what  is  meant  by  the  conception  of  additive 
properties.  If  two  atoms  of  carbon  combine  with  six  atoms  of  hydrogen 
and  one  atom  of  oxygen  to  form  C2HeO,  for  example,  it  is  obvious  that 
the  molecular  weight  of  the  compound  will  equal  the  sum  of  the  atomic 
weights  and  may  be  determined  additively,  viz., 

(2X12)   +   (6X1)   +   (1X16)    =  46 

1  Address  at  the  dedication  of  the  Institute  for  Physical-  and  Electro-Chemistry, 
Gottingen,  1896. 

288 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  289 

It  does  not  in  the  least  matter  how  these  atoms  are  bound  together  in 
the  molecule,  the  sum  of  their  weights  will  always  be  the  same.  Thus 
weight,  or  mass,  is  a  perfect  additive  property  of  the  atoms  present 
in  any  compound. 

Other  properties  of  the  atoms  have  long  been  regarded  as  additive 
in  character  as,  for  example,  molecular  volume.  This  becomes  clear 
by  reference  to  the  following  table,  where  V  represents  the  volume  in 
cubic  centimeters  which  1  gram  molecule  of  the  substance  occupies 
when  measured  at  its  boiling  point: 


Substance 

V 

Constant 
Difference 

Formic  acid 

42 

Acetic  acid  

64 

22 

Propionic  acid 

86 

22 

Butyric  acid  

108 

22 

It  is  obvious  that  in  this  series  of  fatty  acids  the  constant  molecular 
difference  of  one  carbon  and  two  hydrogen  atoms  (CH2  =  14)  corre- 
sponds to  a  constant  numerical  increase  in  the  value  of  V,  the  molecular 
volume.  H.  Kopp  further  made  the  discovery  that  in  other  series,  — 
viz.,  hydrocarbons,  alcohols,  esters,  aldehydes,  and  ketones,  —  the  addi- 
tion of  CH2  also  increases  the  molecular  volume  of  a  substance  by  22, 
so  that  in  general  it  may  be  said  that  equal  differences  in  composition 
(in  this  case  CH2)  correspond  to  equal  differences  in  the  numerical 
value  of  the  molecular  volume. 

Kopp  also  discovered  that  certain  pairs  of  compounds,  as,  for 
example,  C4HioO  and  CeHeO,  CsH^O,  and  C?H80,  etc.,  have  the  same 
molecular  volume.  In  terms  of  the  conceptions  then  held  it  was  pos- 
sible to  imagine  that  in  such  compounds  four  atoms  of  hydrogen  were 
exactly  replaced  by  two  atoms  of  carbon.  Reasoning  in  this  way  Kopp 
concluded  that,  in  general,  one  atom  of  carbon  can  replace  two  atoms 
of  hydrogen  without  changing  the  molecular  volume  of  the  compound. 
Since,  moreover,  the  molecular  volume  of  CH2  may  be  represented 
numerically  by  22,  and  since  two  atoms  of  hydrogen  equal  one  atom  of 
carbon,  it  follows  that  the  atomic  volume  of  carbon  will  equal 


while  the  atomic  volume  of  hydrogen  will  equal 


CHAPTER  XIII 

THE    APPLICATION     OF    PHYSICO-CHEMICAL     PRINCIPLES 
TO   ORGANIC   CHEMISTRY 

DURING  the  first  half  of  the  last  century  the  epoch-making  dis- 
coveries of  men  like  Gay-Lussac,  Avogadro,  Ampere,  Faraday,  Mitscher- 
lich,  Bunsen,  Dulong,  and  Petit  served  to  enrich  both  chemistry  and 
physics.  At  that  time  most  chemists  were  familiar  with  physical 
methods,  but,  as  organic  chemistry  gradually  differentiated  itself  from 
other  branches  of  chemistry  and  began  to  develop  into  a  separate  science, 
it  became  more  and  more  apparent  that  a  knowledge  of  physical  methods 
need  not  be  the  necessary  equipment  of  a  productive  investigator  in 
this  field.  In  one  of  his  lectures  1  W.  Nernst  describes  how  the  physical 
apparatus  which  had  originally  been  present  in  all  chemical  laboratories 
gradually  became  more  and  more  superfluous  until  finally  it  vanished 
completely.  For  a  long  period  of  time  organic  chemists  were  absorbed 
with  problems  of  structural  organic  chemistry.  They  learned  that 
carbon  is  capable  of  forming  a  very  great  number  of  compounds  with  a 
relatively  small  number  of  elements  and  they  gradually  acquired  such 
an  exact  insight  into  the  constitution  of  organic  molecules  that  they 
were  able  to  describe  the  position  and  function  of  every  atom.  In 
this  way  structural  and  constitutional  formulas  were  developed,  and 
only  then  did  it  become  obvious  that  a  definite  relation  exists  between 
certain  physical  properties  of  organic  compounds  and  their  structure  or 
constitution. 

In  any  study  of  the  relationship  which  exists  between  the  physical 
properties  of  a  substance  and  its  chemical  constitution  it  is  necessary  in 
the  first  place  to  understand  what  is  meant  by  the  conception  of  additive 
properties.  If  two  atoms  of  carbon  combine  with  six  atoms  of  hydrogen 
and  one  atom  of  oxygen  to  form  C2HeO,  for  example,  it  is  obvious  that 
the  molecular  weight  of  the  compound  will  equal  the  sum  of  the  atomic 
weights  and  may  be  determined  additively,  viz., 

(2X12)   +   (6X1)   +   (1X16)    =  46 

1  Address  at  the  dedication  of  the  Institute  for  Physical-  and  Electro-Chemistry, 
Gottingen,  1896. 

288 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES 


289 


It  does  not  in  the  least  matter  how  these  atoms  are  bound  together  in 
the  molecule,  the  sum  of  their  weights  will  always  be  the  same.  Thus 
weight,  or  mass,  is  a  perfect  additive  property  of  the  atoms  present 
in  any  compound. 

Other  properties  of  the  atoms  have  long  been  regarded  as  additive 
in  character  as,  for  example,  molecular  volume.  This  becomes  clear 
by  reference  to  the  following  table,  where  V  represents  the  volume  in 
cubic  centimeters  which  1  gram  molecule  of  the  substance  occupies 
when  measured  at  its  boiling  point: 


Substance 

V 

Constant 
Difference 

Formic  acid 

42 

Acetic  acid  

64 

22 

Propionic  acid 

86 

22 

Butyric  acid  

108 

22 

It  is  obvious  that  in  this  series  of  fatty  acids  the  constant  molecular 
difference  of  one  carbon  and  two  hydrogen  atoms  (CH2  =  14)  corre- 
sponds to  a  constant  numerical  increase  in  the  value  of  V,  the  molecular 
volume.  H.  Kopp  further  made  the  discovery  that  in  other  series,  — 
viz.,  hydrocarbons,  alcohols,  esters,  aldehydes,  and  ketones,  —  the  addi- 
tion of  CH2  also  increases  the  molecular  volume  of  a  substance  by  22, 
so  that  in  general  it  may  be  said  that  equal  differences  in  composition 
(in  this  case  CH2)  correspond  to  equal  differences  in  the  numerical 
value  of  the  molecular  volume. 

Kopp  also  discovered  that  certain  pairs  of  compounds,  as,  for 
example,  C4HioO  and  CeHeO,  CsH^O,  and  CrHgO,  etc.,  have  the  same 
molecular  volume.  In  terms  of  the  conceptions  then  held  it  was  pos- 
sible to  imagine  that  in  such  compounds  four  atoms  of  hydrogen  were 
exactly  replaced  by  two  atoms  of  carbon.  Reasoning  in  this  way  Kopp 
concluded  that,  in  general,  one  atom  of  carbon  can  replace  two  atoms 
of  hydrogen  without  changing  the  molecular  volume  of  the  compound. 
Since,  moreover,  the  molecular  volume  of  CH2  may  be  represented 
numerically  by  22,  and  since  two  atoms  of  hydrogen  equal  one  atom  of 
carbon,  it  follows  that  the  atomic  volume  of  carbon  will  equal 


while  the  atomic  vokime  of  hydrogen  will  equal 


290  THEORIES  OF  ORGANIC  CHEMISTRY 

The  atomic  volumes  of  other  elements  have  been  obtained  in  a  similar 
way  and  it  has  been  found  that  O  =  7.8,  Cl  =  22.8,  Br  =  29.1, 1  =  39.6,  etc. 
It  is  thus  possible  to  calculate  the  molecular  volume  of  a  given  compound 
by  finding  the  sum  of  the  atomic  volumes  of  the  different  atoms  com- 
posing it.  For  example,  the  molecular  volume  of  C2H60  should  equal 

(2X11)  +  (6X5.5)  +  (1X7.8)=62.8 

and  it  has  been  found  experimentally  to  equal  62.3. 

In  a  great  number  of  instances  the  calculated  values  agree  very 
closely  with  the  values  which  have  been  determined  by  experiment, 
but  frequently  they  do  not.  The  most  conspicuous  differences  of  this 
sort  were  first  noted  in  connection  with  substances  which  contained 
oxygen.  In  the  case  of  valeric  acid,  for  example,  the  molecular  volume 
as  calculated  equals  126.5  and  as  found,  130.5.  Kopp  explained  this 
discrepancy  as  due  to  a  difference  in  the  nature  of  the  union  of  the 
oxygen  atoms  and  pointed  out  that  oxygen  has  a  different  atomic  volume 
when  linked  to  carbon  as  carbonyl  than  when  linked  to  hydrogen  as 
hydroxyl.  In  the  first  case  the  atomic  volume  of  oxygen  equals  12.2, 
and  in  the  latter,  7.8.  By  bearing  this  fact  in  mind  and  making  the 
necessary  correction  for  valeric  acid,  C^gCOOH,  the  value  calculated 
for  the  molecular  volume  becomes 

(5X11)  +  (10X5.5)  +  (12.2)  +  (7.8)  -130.0 

and  corresponds  closely  with  the  experimental  value,  130.5. 

It  was  also  found  in  the  case  of  other  elements  that  the  values 
representing  atomic  volumes  vary  according  to  the  form  of  combination 
in  which  the  atom  is  found.  Properties  controlled  in  this  way  by 
constitution  came  to  be  known  as  "  constitutive  properties  "  in  order 
to  distinguish  them  from  those  properties  which  are  purely  additive  in 
character.1 

These  and  other  investigations  have  made  it  possible  to  calculate 
with  considerable  accuracy  the  numerical  values  of  many  of  the  proper- 
ties of  a  given  organic  substance,  supposing  that  both  the  structure 
of  the  molecule  and  the  additive  and  constitutive  properties  of  the 
individual  atoms  are  known.  Indeed,  in  cases  where  a  substance  may 
be  represented  by  two  or  more  structural  formulas,  preference  is  usually 
given  to  that  formula  which  shows  the  closest  agreement  between  the 

1  Traube's  work  in  this  field  is  summed  up  in  Ahrens'  "Samml.  Chem.  u.  Chem.- 
techn.  Vortrage."  Vol.  IV  (1899).  See  also  Ber.,  25,  2524  (1892);  27,  3173,  3179 
(1894);  28,  410,  2722,  2728,  2924,  3292  (1895);  29,  2732  (1896);  30,  265  (1897); 
31,  157  (1898);  40,  130,  723,  734  (1907). 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  291 

calculated  and  the  experimental  values  of  the  physical  constants  of  the 
substance.  The  molecular  refraction  and  absorption  of  light  have  been 
of  special  service  in  solving  many  of  the  difficult  problems  of  constitu- 
tion. Indeed,  so  important  has  been  the  part  played  by  spectro- 
chemistry in  the  theory  of  organic  chemistry  that  it  is  very  desirable 
to  consider  this  subject  in  some  detail  at  this  point. 

The  fact  that  a  ray  of  light  is  always  bent  from  a  direct  course  when 
it  passes  from  a  less  dense  (air)  into  a  more  dense  medium  (glass,  water, 
etc.)  forms  the  basis  of  the  science  of  spectroscopy.  Different  sub- 
stances differ  as  to  their  power  of  refraction.  Snell  discovered  in  1861 
that  a  constant  ratio  exists  between  the  sine  of  the  angle  of  incidence 
(i)  and  the  sine  of  the  angle  of  refraction  (r).  This  ratio  is  called  the 
index  of  refraction  (n)  and  is  constant  for  the  same  pair  of  media, 
whatever  the  value  of  i  happens  to  be.  It  is,  moreover,  equal  to  the 
ratio  of  the  velocities  of  light  (v  and  vi)  in  the  two  media.  Thus: 

sin  i     v 

n  =  — —  =  — - 

sin  r    vi 

The  refractive  index  of  a  substance  varies  considerably  as  a  result 
of  changes  in  physical  state.  In  such  changes  density  is  by  far  the  most 
important  factor  and  several  equations  have  been  proposed  in  the 
effort  to  establish  a  definite  relation  between  density  and  refraction. 
The  first  of  these  was  based  upon  the  emission  theory  of  light,  where 

n2-! 


d 


=  r  =  a  constant 


since  according  to  Newton  the  expression  n2  —  1  represents  a  measure  of 
the  refractive  power  of  a  substance.  With  the  development  of  the 
undulatory  theory  of  light  this  first  formula  fell  into  disuse.  It  was 
attacked  by  Laplace  who  held  that  the  ratio  here  represented  depended 
upon  conditions  of  temperature  and  was  not,  therefore,  a  constant. 
Arago  and  Biot  believed,  on  the  other  hand,  that  it  gave  constant 
values  for  gases.  Gladstone  and  Dale  as  well  as  Landolt l  and  Wullner 
demonstrated  that  in  the  case  of  both  liquids  and  solids  the  expression 

-2—. —  gave  different  values  at  different  temperatures  and  was  not 
a 

therefore  a  constant.  They  maintained,  however,  that  it  could  be  used 
in  a  somewhat  modified  form,2  namely: 

n-l 

— - —  =  7-  =  a  constant 
a 

1  Poggeridorf  s  Annalen  der  Chemie,  123,  595  (1864). 

2  Poggendorf's  Annalen  der  Chemie,    133,  1  (1868). 


292  THEORIES  OF  ORGANIC  CHEMISTRY 

and  that  it  then  gave  constant  values  for  liquids  and  solids  at  all  tem- 
peratures. This  was  referred  to  as  the  Gladstone-Dale  formula  since 
it  was  empirically  established  by  these  investigators.  It  came  to  be 
quite  universally  accepted  and  is  still  in  more  or  less  general  use  among 
English  chemists. 

Since  it  is  customary  in  chemistry  to  express  constants  in  terms  of 
gram  molecules  (m)  instead  of  units  of  mass,  specific  refraction  gave 
place  to  molecular  refraction,  viz., 

=  a  constant 

This  expression  has,  however,  been  superseded  by  a  third  which  was 
advanced  by  H.  A.  Lorentz  l  and  L.  Lorenz  2  and  which  is  in  quite  general 
use  by  European  chemists  at  the  present  time 

n2-! 

=  a  constant 


(n2+2)d 

H.  A.  Lorentz  deduced  this  formula  from  Maxwell's  electromagnetic 
theory  of  light.  L.  Lorenz,  on  the  other  hand,  demonstrated  that  it 
could  be  derived  from  the  undulatory  theory  of  light  on  the  assumption 
that  the  volume  occupied  by  any  given  substance  is  not  completely 
filled  with  matter,  but  that  between  the  spherical  molecules  there  are 
interstices  through  which  light  travels  with  the  same  velocity  as  through 
a  vacuum.  The  molecular  refraction  of  a  substance  equals  in  terms 
of  the  Lorentz-Lorenz  formula 

n2-! 


where  X  signifies  the  particular  wave  length  at  which  the  value  for  n 
was  determined.  This  is  important  since  the  index  of  refraction  is  dif- 
ferent for  different  parts  of  the  spectrum.  The  yellow  sodium  line  is 
frequently  used  in  such  determinations  or  else  the  lines  a,  /8,  7  of  the 
hydrogen  spectrum.  Under  these  circumstances  M  becomes  MD) 
Ma,  Mp,  My,  respectively. 

If  a  ray  of  ordinary  light  undergoes  refraction  in  a  given  medium 
it  is  broken  up  into  homogeneous  rays  of  different  wave  lengths.  This 
phenomenon  is  known  as  dispersion.  Thus  when  white  light  passes, 
for  example,  through  a  hollow  prism  which  is  filled  with  water  it  is 
broken  up  into  the  various  colors  of  the  spectrum.  The  distance 

1  Wiedem,  Ann.,  9,  641  (1880). 

2  Ibid.,  11,  70  (1880). 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  293 

from  red  to  violet  has  been  found  to  be  always  the  same  under  like 
conditions  for  any  given  medium  and  in  the  case  of  water  is  equal  to 
one  centimeter.  It  is,  however,  different  for  different  substances, 
and  in  the  case  of  carbon  bisulphide,  for  example,  equals  6.5  cm.  when 
measured  under  exactly  the  same  conditions  as  give  a  value  of  1  cm. 
for  water.  The  former  substance  is  thus  said  to  possess  a  greater  power 
of  dispersion  than  water.  This  property  is  dependent  upon  the  chemical 
nature  of  substances  and  is,  in  fact,  more  readily  influenced  by  con- 
stitution than  is  the  case  with  refractivity. 

Dispersion  is  usually  measured  in  terms  of  the  difference  between 
the  indices  of  refraction  of  red  and  violet  light  of  known  wave  lengths, 
as,  for  example,  the  hydrogen  lines  y(B)  and  a(H),  that  is  by  ny  —  na. 
The  dispersion  of  a  substance  depends  upon  its  density,  but  Bruhl 1 
found  that  the  formulas, 


dfic  dispersion 

and 

'nJ2  —  1     na2  —  1  \  m 


/ny2—l     na2—l\,,v  ,. 

(2X0 2li^);7~  =  molecular  & 


give  values  which  are  independent  of  temperature,  density,  and  state 
of  aggregation. 

According  to  Bruhl  changes  in  structure  or,  in  other  words,  changes 
in  the  combinations  of  the  atoms,  are  usually  much  more  strikingly 
apparent  from  values  representing  the  dispersions  than  from  corre- 
sponding values  representing  the  refractions  of  organic  compounds. 
For  this  reason  in  modern  chemical  literature  the  dispersion  of  a  sub- 
stance is  usually  given  along  with  its  specific  and  molecular  refractions. 
It  may  be  added  that  Auwers  and  Eisenlohr  2  have  recently  recom- 
mended that  multiples  of  100  times  the  specific  refraction  be  used 
instead  of  molecular  refractions.  These  values  are  represented  by  the 
symbols  Sa,  Sz>,  etc.,  and  are  supposed  to  be  a  better  guide  in  interpret- 
ing the  phenomena  of  optical  exaltations  than  are  the  corresponding 
refractions. 

In  determining  the  index  nx,  from  which  both  specific  and  molecular 
refractions  and  dispersions  are  calculated,  it  is  very  important  that  the 
temperature  should  be  kept  constant,  and  usually  at  20°.  The  Pulfrich 

1  Zeitschr.  physikal.  Chemie,  7,  140  (1891). 

!  Ber.,  43,  809  (1910);  also  see  W.  A.  Roth  and  F.  Eisenlohr,  "  Refraktometrisches 
Hilfsbuch,"  Leipzig,  1911,  Veit  &  Company. 


294  THEORIES  OF  ORGANIC  CHEMISTRY 

ref ractometer  is  especially  convenient  for  colorless  or  only  slightly  colored 
substances,  since  it  affords  an  easy  means  of  regulating  temperatures, 
while  sodium  and  hydrogen  (red  and  violet)  may  be  used  as  the  sources 
of  light.  At  a  given  temperature  the  observed  angle  should  remain 
constant.  Should  it  change  and,  for  example,  become  increasingly 
greater,  this  would  indicate  that  polymerization  was  taking  place  and 
the  experiment  would  cease  to  have  any  value. 

The  specific  gravity — or  what  is  the  same  in  this  case,  the  density- 
must  also  be  determined  very  accurately,  and  for  this  purpose  Ostwald's 
pyknometer,  which  holds  about  3  cc.,  is  found  most  serviceable.  The 
temperature  at  which  density  is  determined  must  obviously  be  the  same 
as  that  for  the  index  of  refraction.  The  density  compared  with  water  at 
4°  C.  must  also  be  given  and  atomic  refractions  must  be  calculated  on 
this  basis.  In  calculating  densities  it  is  unnecessary  to  correct  for  a 
vacuum,  although  this  is,  of  course,  the  more  accurate  procedure.  Also 
Auwers  and  Eisenlohr  recommended  molecular  weights  calculated  to 
four  figures,  rather  than  those  represented  by  round  numbers.  Because 
of  the  possibility  of  polymerization  it  should  always  be  stated  whether 
the  operation  immediately  followed  the  purification  of  the  substance, 
and  if  not  how  much  later. 

The  refraction  of  organic  compounds  has  been  the  subject  of  very 
careful  study  for  the  past  fifty  years,  and  as  a  result  the  relation  which 
exists  between  this  physical  property  of  a  substance  and  its  chemical 
structure  is  better  known  than  in  the  case  of  any  other  physical  proper- 
ties. The  first  efforts  of  investigators  in  this  field  were  focused  upon 
the  additive  aspects  of  the  problem.  The  question  as  to  whether  the 
refraction  of  light  is  a  property  of  the  mass  of  the  atoms  in  the  molecule 
or  whether  it  depends  upon  the  form  of  combination  as  well,  was  investi- 
gated as  early  as  1864  by  Landolt  who  found  that  isomeric  eompounds 
possess  almost  identical  refractions.  In  comparing  the  molecular 
refraction  of  glycerine,  CsHgOs  (34-32),  with  that  of  mixtures  corre- 
sponding to  the  same  empirical  formula — as,  for  example,  CH3COOH+ 
CH3OH  and  HCOOH+C2H5OH— he  discovered  almost  identical  values 
in  all  cases  and,  therefore,  concluded  that  refraction  was  a  purely 
additive  function  of  the  atoms.  Reasoning  on  this  basis  it  was  possible 
to  calculate  the  values  represented  by  the  atomic  refractions  of  the 
different  elements.  Thus  since  the  molecular  refraction  of  methyl 
alcohol,  CH4O,  is  13.17  and  that  of  acetaldehyde,  C2H40,  is  18.58,  and 
since  these  two  substances  differ  by  but  a  single  carbon  atom,  it  follows 
that  the  difference  in  their  molecular  refractions,  or  5.41,  must  represent 
the  atomic  refraction  of  an  atom  of  carbon.  In  the  same  way  the 
difference  in  the  molecular  refractions  of  acetaldehyde,  C2H40,  and 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  295 

ethyl  alcohol,  C2H6O,  which  differ  by  two  hydrogen  atoms,  gives  the 
atomic  refraction  for  hydrogen,  viz., 

20.7 -18.58  ='2.12-7-2  =  1.06 

The  mean  of  a  great  many  experiments  has  furnished  the  following  values 
for  atomic  refractions :  C  =  5,  H  =  1.3,  0  =  3. 

The  molecular  refractions  of  such  substances  as  alcohols,  ethers, 
and  acids,  as  calculated  from  the  sum  of  their  atomic  refractions,  agree 
very  closely  with  the  values  which  have  been  obtained  as  the  result  of 
experiment.  In  the  case  of  other  organic  compounds,  however,  the  dif- 
ferences between  the  calculated  and  the  experimental  values  for  molecu- 
lar refractions  are  so  great  that  Landolt  was  forced  to  the  conclusion 
that  the  property  of  refraction  is  not,  as  a  matter  of  fact,  purely  additive 
in  character.  He  found,  however,  that  in  the  case  of  any  given  homolo- 
gous series  the  molecular  refractions,  like  the  molecular  volumes  of  the 
substances,  increase  regularly  with  the  increase  in  th«?  number  of  carbon 
atoms.  Thus  in  the  case  of  alcohols,  ethers,  and  acids  the  increase  in 
molecular  refraction  in  any  series  is  almost  a  constant.  It  equals  7.2 
and  corresponds  to  an  increase  of  CEb. 

Landolt,  and  quite  independently  Gladstone  and  Dale,1  were  then 
able  to  demonstrate  that  changes  in  the  atomic  grouping  in  the  case  of 
isomeric  substances  exercise  a  small  but  perfectly  definite  and  measur- 
able effect  upon  molecular  refraction,  an  effect  which  cannot  possibly 
be  accounted  for  on  the  basis  of  experimental  error.  "  Every  liquid 
has  a  specific  refraction  composed  of  the  specific  refractions  of  its  com- 
ponent elements,  modified  by  the  manner  of  combination  but  unaf- 
fected by  changes  of  temperature."  This  marks  the  definite  discovery 
of  constitutive  differences  in  refraction. 

Since  1880  knowledge  along  these  lines  has  been  actively  developed 
and  constitutive  influences  have  come  to  be  better  understood.  This  is 
in  large  measure  due  to  the  investigations  of  Briihl,2  who  has  added 
greatly  to  the  material  collected  by  earlier  investigators,  and  has  applied 
this  material  in  the  elucidation  of  many  problems  of  structural  chemistry. 
Briihl  has  been  able  in  the  first  place  to  demonstrate  that  fluctuations  in 
molecular  refraction  do  not  depend  upon  the  neutral,  acid,  or  basic 
properties  of  the  substances  in  question,  and  in  the  second  place  to  assign 
a  definite  value  to  the  constitutive  influence  of  double  bonds  between 
carbon  atoms.  The  latter  was  found  to  be  fairly  constant,  although 
it  varied  in  different  compounds  between  1.63  and  2.17.  Thus,  for 

»Phil.  Trans.,  148,  8  (1858);  163,  323  (1863). 

2  Zeitschr.  physikal.  Chemie,  7,  140  (1891);  Jour,  prakt.  Chemie,  60,  152  (1894). 


298  THEORIES  OF  ORGANIC  CHEMISTRY 

The  difference  between  these  two  formulas  may  be  expressed  in  terms  of 
atomic  refractions  and  increments  as  follows: 


C6Hi0O20<     and    C6HioO"OO<  [f 

If  the  molecular  refraction  of  each  of  these  is  calculated  for  light  of  wave 
length  D  the  following  results  are  obtained : 


Keto  Form  Enol  Form 

C6    =  6X2.418  =  14.508  C6    =   6X2.418  =  14.508 

H10  =10X1.100  =  11.000  H10  =10X1.100  =  11.000 

O2    =  2X2.211=  4.422  O"   =  1X2.211=  2.211 

O<=   1X1.643=   1.643  O'    =  1X1.525=   1.525 

O<=  1X1.643=  1.643 
IT    =  1X1.733=   1.733 


Mol .  refraction    =  3 1 . 573  Mol .  refraction    =  32 . 620 

The  molecular  refraction  for  ordinary  ethyl  acetoacetate  as  deter- 
mined at  room  temperature  has  been  found  to  be  Jlf  =  32.00.1  This 
value  is  obviously  intermediate  between  the  calculated  values  for  the 
keto  and  end,  respectively,  although  it  approximates  the  former 
more  closely  than  the  latter.  These  results  thus  tend  to  confirm  chem- 
ical investigation  in  the  matter  since  they  indicate  that  liquid  ethyl 
acetoacetate  consists  of  a  mixture  which  contains  a  large  amount 
of  the  keto  modification  together  with  small  quantities  of  ethyl 
/3-hydroxycrotonate. 

In  the  case  of  certain  groups  of  unsaturated  compounds  abnormal 
molecular  refractions  and  dispersions  have  been  observed,  which  greatly 
exceed  the  calculated  values  even  after  all  possible  increments  have  been 
added.  Such  unusual  deviations  are  referred  to  as  optical  anomalies 
or  exaltations,  and  are  frequently  observed  in  the  case  of  substances 
with  one  or  more  ethylene  linkages  in  immediate  proximity  to  the 
benzene  ring.  Thus,  for  example,  CeH5CH2CH=CH2  shows  normal, 
while  C6HoCH=CHCH3  shows  abnormal  molecular  refraction.  Even 
more  striking  illustrations  of  phenomena  of  this  kind  have  been 
discovered  by  Brtihl 2  in  the  case  of  cinnamyl  derivatives  as,  for 
example : 

ifier.,  44,  3530  (1911). 
2Ber.,  40,  883  (1907). 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  299 


Calculated 

Found 

Dispersion 

My—  Ma 

C«H6CH=CH.CHO 

CsHsO'F 

Found  =  4.  17 

Cinnamic  aldehyde 

Ma  =  39.78 

Ma-43.51 

Calc.    =1.65 

My—  Ma 

x,O 

CnHi2O"O<  F 

Found  =  3.  82 

CeHs  •  OH==OH  •  Cv 
^OCzR* 

Ma  =  50.58 

Ma  =  53.62 

Calc.    =1.88 

Cinnamic  ester 

C6H5-CH=CHCH 

My—  Ma 

II      ^o 

CH-C^ 

NOH 

CnH100-0'  f 
Ma  =  50.06 

Ma  =  60.42 

Found  =  9  .  70 
Calc.    =2.04 

Cinnamylidene  acetic  acid  dis- 

solved in  acetone 

Other  instances  of  this  sort  were  discovered  by  Eijkmann  among  the 
derivatives  of  propenyl,  by  Tschugaeff  among  trimethylene  derivatives, 
and  by  Wallach  among  substances  possessing  so-called  semi-cyclic  double 
bonds  as  illustrated,  for  example,  in  combinations  containing  the  atomic 
grouping : 


These  last  two  discoveries  have  both  been  confirmed  by  the  work  of 
Auwers  and  Eisenlohr. 

In  reviewing  the  situation  Briihl  pointed  out  that  optical  anomalies 
are  most  frequently  met  with  in  the  case  of  substances  containing  two 
unsaturated  ethylene  linkages  (or  one  ethylene  and  one  carbonyl)  in 
adjacent  positions.  In  attempting  to  explain  the  phenomena  Briihl 
applied  the  conceptions  of  Thiele's  theory  of  partial  valency,  which 
was  attracting  a  great  deal  of  attention  at  the  time,  and  pointed  out 
various  relationships  which  tended  in  his  opinion  to  establish  connection 
between  optical  anomaly  and  the  presence  of  conjugated  systems  of 
double  bonds  in  the  molecule.1  Auwers  and  Eisenlohr  have  since 
shown,  however,  that  at  least  some  of  Bruhl's  conclusions  must  be  dis- 
counted, because  many  of  the  relationships  to  which  he  refers  have 
proved  far  more  complicated  in  character  than  he  could  possibly  have 
sensed  from  the  data  known  at  that  time.  The  general  status  of  the 


Ber.,  40,  900  and  1160  (1907). 


300 


THEORIES  OF  ORGANIC  CHEMISTRY 


problem  may  be  summed  up  by  saying  that  B^uhl  was  able  to  show 
conclusively  that  the  molecular  refraction  of  a  substance  depends  upon 
the  relative  position  of  the  double  bonds  functioning  in  the  molecule;  and 
in  those  cases  where  more  than  one  unsaturated  linkage  is  present  he 
demonstrated  that  an  arrangement  represented  by  systems  of  so-called 
conjugate  double  bonds  corresponds  to  abnormally  high  molecular 
refractions  and  dispersions.  Illustrations  of  this,  which  might  be 
greatly  augmented  in  number,  are  to  be  found  in  the  following  table.  It 
should  be  noted  that  figures  expressing  anomalies  are  printed  in  heavy 
type: 


Mf 


CHr=CH  •  CH2  •  CH2  •  CH=CH2 
Diallyl  C6H10f 


CH3.CH=CH  •  CH=CH  •  CH3 
Isodiallyl  C6H10Ir 


CH30— S  \—  CH=CH— CH3 

AnetholeCiCHi2O<|T 

CH3O— (~        \—  CH2— CH=CH2 
Methylchavicole  Ci0Hi2O  <  p 


Found  =28. 77 
Calc.    =28.89 


-0  12 

Found  =  30. 38 
Calc.    =28.89 


+1.49 

Found  =47. 70 
Calc.    =45.89 


+1.81 

Found  =  45. 95 
Calc.    =45.89 


+0  06 


Found  =  1.00 
Calc.    =1.05 


-0  05 

Found  =  1 . 57 
Calc.    =1.05 


+0  52 

Found  =2. 95 
Calc.    =1.75 


+1.20 

Found  =2. 04 
Calc.    =1.75 


+0  29 


Perfectly  definite,  albeit  relatively  small,  optical  anomalies  have  also 
been  observed  in  the  case  of  substances  which  contain  an  unsaturated 
atom  in  direct  union  with  a  carbonyl  or  ethylene  group,  as  in  the  radical 


O1 
CL 


Ber.,  44,  3514,  3525  (1911);  also  3188  and  3679. 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES 


301 


While  the  refractions  of  substances  containing  unsaturated  atoms 
in  conjugate  positions  usually  exhibit  exaltations,  there  are,  neverthe- 
less, important  exceptions  to  this  general  rule.  It  has  been  observed, 
for  example,  that  substitution  in  certain  positions  seems  to  diminish 
the  optical  exaltation  of  a  substance;  and  this  fact  becomes  even  more 
strikingly  apparent  if  figures  are  compared  which  express  100  times  the 
specific  refractions  and  dispersions,  viz.,  2D  and  SY— Sa.  An  illustra- 
tion of  this  is  to  be  found  in  the  case  of  the  two  isomeric  diisopropenyls 
which  differ  only  as  regards  the  relative  positions  occupied  by  the 
methyl  groups  and  whose  optical  exaltations  are  given  in  the  following 
table : 


Exaltation 

Exaltation 

MD 

Zn 

My~  Ma 

ZT—  S« 

CH3  •  CH=CH—  CH=CH—  CH3 

+  1.71 

+2.08 

+0.52 

+0.63=50% 

H2C=C—  C=CH2 

1       1 

+0.82 

+1.00 

+0.37 

+0.45  =  35% 

CH3CH3 

Diisopropenyl  C6Hi0]~ 

A  comparison  of  the  figures  shows  an  appreciable  decrease  in  exaltation 
in  the  case  of  the  second  isomer. 

Indeed  Auwers  and  Eisenlohr  l  are  of  the  opinion  that,  in  general, 
disturbances  in  conjugate  systems  due  to  substitutions  tend  to  diminish 
the  optical  exaltation  of  the  substance.  This  effect  upon  a  conjugate 
system  is  not  restricted  to  alkyl  groups,  but  extends  to  hydroxy,  methoxy, 
and  ethoxy  groups  as  well.  These  authors  conclude  that  a  sharp  line 
cannot  be  drawn  between  normal  and  abnormal  compounds,  but  that 
there  is  a  transition  from  one  to  the  other.  Thus  the  disturbances  pro- 
duced by  substitutions  may  be  such  as  to  completely  neutralize  the 
optical  exaltation,  and  under  such  circumstances  a  substance  which 
possesses  a  conjugate  system  of  double  bonds  might  show  perfectly 
normal  optical  properties.  These  points  must  all  be  very  carefully 
borne  in  mind  in  the  application  of  spectroscopy  to  the  determination 
of  structure  if  serious  errors  are  to  be  avoided.  The  following  table 
shows  the  influence  of  substitution  upon  several  types  of  conjugate 


Ber.,  43,  806  (1910);  Jour,  prakt.  Chemie,  82,  65  (1910);  Jahrb.  der  Radioak- 
tivitat,  9,  333  (1912). 


302 


THEORIES  OF  ORGANIC  CHEMISTRY 


systems.     The  symbols  ^S-Refr.  and  E2-Disp.  represent  values  100 
times  as  great  as  the  specific  refractions  and  dispersions  respectively: 


Class  of 

Compound 

Conjugate  System 

E-2-Refr. 

#Z-Disp. 

Aliphatic  hydrocarbons 

—  CH=CH—  CH=CH— 
—  CH=CH—  C=CH 

1 

1.90 
1.10 

50% 

R 

Styroles 

-CH. 
^)C—  CH=CH— 
—  CH/ 
-CH. 

yc—  C=CH— 

—  CH/ 
R 
-CH. 

>c—  c=c— 

-CH/         |       | 
R    R' 

1.10 
0.70 

0.45 

45% 
30% 

20% 

Hydroaromatic 
hydrocarbons 

CH—  CH=C—  R' 

II         I 

R  —  C  OH2  —  OH2 

CH2—  CH=C—  CH=CH— 

1                     1 

1                     1 
C  H2  —  C  H2  —  C  H2 
R 

1 
r1  —  r<TT           p      f  TT 

,  0.8-1.2 
0.25 

40% 
20% 

V^  Vxll                   ^  V^Xl  

1                        1 
CH2—  CH2—  CH2 

CH=CH  C—  R 

II 

11 
CH2  —  OH2  —  OH 

—  CH=CH—  CH=O 

_C=CH—  CH=O 

1 

1.80 

50% 

Aldehydes  < 

Acyclic    \ 

1 
R 
—  CH=C—  CH=O 

1 

•  1.25 

45% 

Cyclic 

R 
-CH. 
JC—  CH=0 
—  CH/ 

1.00 

45% 

APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  303 


Class  of  Compound 

Conjugate  System 

#S-Refr. 

EZ-Disp. 

Acylic 

—  CH=CH—  C=O 

0.90 

1 
R 

30-^0% 

_c=ct 

[_C=O 

0.85 

Ketones 

Acyclic 
and 

1 
R' 

R 

Cyclic 

_CH=( 

:—  c=o 

0.50 



R'   R 

Acids 

—  CH=CH—  C=O 

| 

1.10 

40% 

OH 

—  CH=C—  C==O 

1 

0.80 

.... 

1 
R    OH 

Esters 

CH=CH—  C=O 

0.80 

30% 

OR 

:_c=o 

0.50 

20% 

R'    OR 

In  cases  where  ethylene,  or  other  unsaturated  linkages  possess  an 
atom  in  common,  as,  for  example,  in  the  system  C=C=C,  the  sub- 
stance shows  normal  or  nearly  normal  refraction.  The  same  is  true 
when  the  double  bonds  are  widely  separated  in  the  molecule. 

Where  two  or  more  conjugate  systems  of  double  bonds  are  present 
in  the  same  molecule  two  distinct  arrangements  are  possible,  viz., 


and 


-,     0=C— C=C— C=O  etc., 


i=, 


etc. 


=O 


These  and  analogous  systems  are  called,  respectively,  "  cumulative 
conjugate  systems  "  and  "  crossed  conjugate  systems."  The  effect 
of  such  systems  on  refraction  is  radically  different,  the  former  pro- 
ducing an  extraordinary  increase  in  optical  exaltation,  while  the  latter 
causes  optical  depression.  These  relationships  are  much  more  compli- 
cated than  in  the  case  of  simple  conjugate  systems  and  no  exact  rules 


304 


THEORIES  OF  ORGANIC  CHEMISTRY 


have  as  yet  been  formulated  in  regard  to  them.  At  best  exaltations 
can  only  be  appraised  roughly  and  cannot  as  yet  be  represented  by  exact 
increments.  The  following  table  gives  the  mean  values  for  certain 
types  of  atomic  groupings  as  determined  by  Auwers  and  Eisenlohr,1 
and  such  values  may  serve  in  certain  instances  as  a  basis  for  calculations : 


Class  of  Compound 

Conjugate  System 

JMW, 

^-Disp. 

—  CH=C—  CH=CH—  CH=CH— 

1 

3.4 

130% 

Hydrocarbons 

1 
R 

—  CH/       ||      NDH— 
c 

1.0 
3.3 

40% 

Aldehydes 

—  CH=C—  CH=CH—  CH=O 

R 

150% 

—  CH=CH—  CH=CH—  C=O 

1 

3.3 

145% 

1 
R 
—  CH=C—  CH=CH—  C=O 

1                          1 

2.7 

110% 

Ketones 

1                          1 
R                         R 
—  CH=C—  CH=C—  C=O 

1                 1       1 

2.1 

95% 

1                 1       1 
R              R    R 

—  CH/         |j        \2H— 
O 

1.0 

45% 

—  CH=CH—  CH=CH—  C=O 

1 

2.4 

120% 

1 
OR 
—  CH=C—  CH=CH—  C=O 

2.0 

100% 

Esters 

R                        OR 
_CH=C—  CH=C—  C=O 

1                 1      1 

1.5 

75% 

1                 1      1 
R               R     OR 

—  CH=CH—  C—  C=O 

II      1 

0.5 

25% 

II      1 
—  CHOR 

Jour,  prakt.  Chemie,  84,  37  (1911). 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  305 

The  statement  that  the  presence  of  cumulative  conjugate  systems 
leads  to  an  increase  in  the  optical  exaltation  of  a  substance  is  not  without 
important  exceptions.  Benzene,  which  in  a  sense  may  be  said  to  have 
such  an  arrangement  of  atoms  in  the  molecule,  shows  no  exaltation  and 
may  therefore  be  said  to  possess  a  neutral  conjugate  system.  Deriva- 
tives of  benzene  vary  greatly  as  to  their  refraction,  showing  in  some 
cases  a  very  slight  and  in  other  cases  a  very  great  exaltation.  For 
example,  certain  phenols  and  their  ethers  exhibit  such  slight  divergence 
in  optical  properties  that  they  cannot  be  said  to  possess  constitutive 
differences.  This  is  also  true  in  the  case  of  certain  isomeric  alkyl 
derivatives  of  benzene.  If,  on  the  other  hand,  the  substituent  in  the 
benzene  ring  consists  of  an  unsaturated  group,  active  conjugate  systems 
may  arise. 

K.  Auwers  has  recently  investigated  the  subject  of  position  isomerism 
in  its  relation  to  spectro-chemistry  and  has  obtained  results  which  are 
very  valuable  in  their  bearing  upon  problems  of  constitution.  Aromatic 
aldehydes,  ketones,  and  esters, 


\ 


— CH=O 


were  selected  for  observation  and  the  effect  of  substitution  in  the  ortho, 
meta,  and  para  positions  was  studied  in  the  case  of  a  large  number 
of  different  substituents.  As  the  above  formulas  indicate  these  sub- 
stances all  show  active  conjugate  systems.  It  was  found  that  the 
introduction  of  methyl  in  the  ortho  and  meta  positions  produced  only 
insignificant  changes  in  the  optical  constants  of  the  substance,  but  that 
substitutions  in  the  para  position  were  accompanied  by  marked 
exaltations.  This  is  a  very  general  rule  and  holds  in  the  case  of 
all  substitutions  of  methyl  which  have  been  observed  up  to  the 
present  time. 

Substitutions  of  the  methoxy  group  also  produce  the  greatest  effect 
when  they  take  place  in  the  para  position ;  but  in  this  case  the  influence 
of  the  substituent  in  the  ortho  and  meta  positions  is  more  marked  than 
in  the  case  of  methyl.  The  values  given  in  the  following  table  which 
show  the  dispersions  of  the  various  substances  are  especially  interesting 
in  this  connection.  It  will  be  noted  that  the  highest  values  are  always 
those  of  para  substitution  products  and  that  they  therefore  afford  a 
definite  method  for  identifying  these  substances.  The  ortho  and  meta 
substitution  products  cannot  be  distinguished  in  this  way. 


306 


THEORIES  OF  ORGANIC  CHEMISTRY 
TABLE  I — METHYL  DERIVATIVES 


No. 

Formula 

«. 

n,- 

•*, 

^ 

£V" 

EZy-Za 

1 

/               \    C*C\    TT 

S  /-CO*H 

1.046 

1.5452 

+0.99 

+  1.01 

+45 

+49 

CH3 

2 

</        ^-CO-H 

1.038 

1.5483 

+0.93 

+  1.01 

+48 

+53 

CH3 

3 

<(        )>-CO-H 

1.022 

1.5416 

+  1.10 

+  1.18 

+49 

+54 

4 

CH3./~     XCO-H 

1.018 

1.5460 

+  1.37 

+  1.47 

+58 

+64 

5 

/  VCQ.CH 

X       / 

1.027 

1.5338 

+0.60 

+0.65 

+32 

+35 

CH3 

6 

~S-CO-CH3 

1.014 

1.5320 

+0.51 

+0.57 

+33 

+36 

CH3 

7 

\  /-CO-CH-, 

1.007 

1  .  5306 

+0.66 

+0.73 

+37 

+41 

8 

1.004 

1.5342 

+0.95 

+  1.01 

+42 

+47 

CH3 

9 

/               N   C*C\   fW 

N  /-COCH3 

0.995 

1.5294 

-1-0.72 

+0.77 

+36 

+39 

CH3 

10 

/  \.CO-CH 

\  X 

1.010 

1.5272 

+0.43 

+0.48 

+29 

+31 

11 

CHS.<^>CO.CJH6 

0.991 

1.5275 

+0.77 

+0.82 

+38 

+42 

12 

<^        ^).CO-CH(CH3)2 

0.984 

1.5177 

+0.51 

+0.53 

+28 

+31 

13 

CH,<^>.CO.CH(CH3)2 

0.969 

1  .  5192 

+0.80 

+0.86 

+40 

+44 

14 

<^>.CO.OC2HS 

1  .  047 

1  .  5056 

+0.43 

+0.49 

+26 

+26 

Ctt, 

15 

<^>.CO.OC2H5 

1.033 

1.5077 

+0.49 

+0.53 

+27 

+30 

CH3 

16 

\-CO-OC,H, 

1.028 

1.5057 

+0.58 

+0.62 

+28 

+31 

17 

CH3.<("  ^.CO-OC.H, 

1.026 

1.5081 

+0.73 

+0.77 

+33 

+35 

APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  307 

TABLE  II — METHOXY  DERIVATIVES 


Formula 

d*° 

D 

£2 

E*D 

**r-=. 

*v-* 

No. 

% 

% 

1 

<^>CO.H 

1.046 

1.5452 

+0.99 

+1.01 

+45 

+49 

OCHj 

2 

<f~^>-CQ.H 

1.133 

1.5598 

+  1.10 

+1.21 

+79 

+91 

OCH, 

3 

/        >.CQ.H 

1.118 

1.5538 

+  1.17 

+  1.27 

+67 

4 

CH30.<^>.CO.H 

1.123 

1.5731 

+  1.81 

+1.97 

+95 

5 

<^        )>-CO-CH, 

1.027 

1.5338 

+0.60 

+0.65 

+32 

+35 

OCH3 

6 

<^        ^>-CQ.CH3 

1.088 

1  .  5395 

+0.89 

+0.97 

+53 

+59 

OCH3 

7 

/  ^ 

1.095 

1.5410 

+0.73 

+0.82 

+51 

+55 

8 

CH3O  •  <^        y  •  CO  •  CH3 

1.099 

1  .  5564 

+1.33 

+1.45 

+75 

+81 

9 
9a 

/  VcO-OCH 
\  / 

1.087 
1.047 

1.5163 
1.5056 

+0.43 
+0.43 

+0.45 
+0.49 

+24 
+26 

+24 
+26 

OCH3 

10 

~S-CO-OCH3 

1.156 

1.5340 

+0.67 

+0.72 

+39 

+43 

OCH3 

lOa 

<^        )>.CQ.CC2H5 

1.111 

1.5213 

+0.69 

+0.73 

+37 

+42 

OCH3 

11 

/  \  m  or  H 
\  /-CO-°C^ 

1.100 

1.5152 

+0.68 

+0.73 

+38 

+43 

12 

CHS0.<^>.CO.OC2HS 

1.103 

1.5544 

+0.97 

+1.04 

+52 

+57 

308 


THEORIES  OF  ORGANIC  CHEMISTRY 


If  the  two  preceding  tables  are  compared  it  appears  that  the  dif- 
ferences between  the  ortho  and  the  meta  substitution  products,  on  the 
one  hand,  and  the  para  substitution  products,  on  the  other  hand,  are 
best  shown  by  the  specific  refractions  in  the  case  of  methyl  and  by  the 
respective  dispersions  in  the  case  of  the  methoxy  derivatives.  This 
is  due  to  the  fact  that  the  relation  between  the  densities  and  the  specific 
refractions  is  different  in  the  case  of  these  two  groups  of  substances. 
If,  for  example,  the  density  decreases  while  the  index  of  refraction 
remains  approximately  the  same,  the  change  will  be  accompanied  by  an 
increase  in  the  molecular  refraction,  but  by  no  appreciable  change  in  the 
value  representing  the  dispersion.  If,  on  the  other  hand,  as  happens 
in  the  case  of  the  methoxy  substitution  products,  the  density  decreases 
very  slightly  while  the  index  of  refraction  increases  rapidly,  the  change 
will  be  accompanied  by  an  increase,  both  in  the  molecular  refraction 
and  dispersion. 

Brlihl  had  observed  that  the  substitution  of  hydroxyl  in  the  benzene 
ring  is  often  accompanied  by  optical  exaltation  and,  on  the  assumption 
that  the  hydroxyl  group  is  unsaturated,  had  explained  the  phenomenon 
as  due  to  the  formation  of  an  active  conjugate  system.  Later  the 
question  was  reopened  by  Auwers  who  discovered  that  phenol  and  its 
homologues  differ  only  very  slightly  in  optical  properties  from  the 
corresponding  hydrocarbons  from  which  they  are  derived.  It  is  true 
that  polyhydric  phenols  show  exaltations,  but  from  the  point  of  view  of 
constitution  the  most  important  exaltations  are  those  which  result 
from  the  introduction  of  hydroxyl  into  derivatives  of  benzene  which 
already  possess  unsaturated  groups  in  the  side  chain.  Under  these 
conditions  the  "  unsaturated  "  hydroxyl  group  is  optically  more  effective 
than  the  other  unsaturated  substituents  as  a  comparison  of  the  follow- 
ing two  compounds  shows : 


Formula 


OC2H5 


C2H5O— C=O 


+0.8-1 


+0.56 


+0.90 


+0.58 


+56% 


+23% 


+64% 


+25% 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  309 

In  the  case  of  the  salicylic  ester  the  marked  exaltation  may  be 
explained  on  the  assumption  that  the  following  system  is  present  in  the 
molecule : 

H  OC2H5 

This  supposes  that  salicylic  ester  has  the  first  of  the  two  possible  formulas, 
OH  OH 


and 


or  else  that  more  of  the  first  than  of  the  second  kind  of  molecules  are 
present  in  an  equilibrium  mixture  of  the  two. 

It  is  obvious  from  what  has  been  said  that  spectro-chemistry  affords 
a  more  delicate  and  exact  method  for  determining  important  questions 
of  constitution  than  any  other  single  method  which  is  known  at  the 
present  time. 

In  continuing  the  study  of  benzene  derivatives  which  possess  active 
conjugate  systems  as,  for  example, 


Auwers  came  to  the  conclusion  that  in  spite  of  the  fact  that  one  of  the 
three  ethylene  linkages  of  benzene  is  active  in  such  compounds,  the 
other  two  continue  to  be  optically  neutral  in  character.1  While  such  a 
condition  seems  at  first  sight  improbable  and  is  difficult  to  account  for 
in  terms  of  theory,  it  is,  nevertheless,  in  general  agreement  with  the 
observed  relationships.  The  following  considerations  serve  to  illustrate 
this  point  : 

The  substitution  of  a  saturated  radical  (alkyl)  in  the  ortho  position 
to  the  unsaturated  substituent  should  under  such  conditions  cause  no 
optical  disturbances  since  the  constitution  of  the  derivative  in  question 
must  correspond  to  either  one  or  the  other  of  the  following  formulas 

CH3  CH3 

/      V 


I.  II. 

1  Annalen  der  Chemie,  408,  230  and  following  (1915). 


310  THEORIES  OF  ORGANIC  CHEMISTRY 

and  the  methyl  is  not,  therefore,  in  a  position  to  exert  any  appreciable 
influence  upon  either  the  active  conjugate  system  C=C — C=0,  or 
upon  the  neutral  system  C=C— C=C.  If  substitution  takes  place  in 
the  meta  position,  the  resulting  product  may  be  represented  by  either 

CH3 


or 


III.  IV. 

In  this  case  the  optical  effect  of  substitution  cannot  be  predicted  since  a 
substance  constituted  like  III.  would  not  differ  optically  from  the  sub- 
stance from  which  it  was  derived,  while  one  corresponding  to  IV.  would 
be  expected  to  show  increased  optical  exaltation.  This  would  follow 
from  the  fact  that  the  substitution  of  methyl  in  the  following  position 

C=C— C=C 


would  tend  to  disturb  the  equilibrium  of  the  neutral  system.     Substitu- 
tion in  the  para  position 


=O 


might  be  expected  from  every  point  of  view  to  produce  disturbances 
which  would  be  accompanied  by  increased  optical  exaltation.  This 
deduction  is  in  complete  harmony  with  the  facts  of  observation  since 
para  alkyl  derivatives  always  possess  higher  exaltations  than  the 
substances  from  which  they  are  derived.1 

These  relationships  become  much  more  complicated  if  two  substitu- 
ents  in  the  benzene  ring  are  unsaturated  as  is  the  case,  for  example,  if 
the  substituting  radical  is  hydroxyl  or  alkoxyl  instead  of  methyl.  Even 
under  these  circumstances,  however,  para  substitution  products  may  be 
distinguished  by  the  fact  that  they  possess  much  greater  powers  of 
refraction  and  dispersion  than  the  corresponding  ortho  and  meta  deriva- 
tives. Such  an  increase  in  optical  exaltation  may  be  readily  understood 
by  reference  to  the  following  formula: 


R  X 

in  which  a  continuous  system  of  four  conjugate  linkages  is  represented. 

aBer.,  46,  2768  (1912). 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  311 

In  the  case  of  ortho  and  meta  derivatives 


(/-  "S-c-o 

X 


I. 

it  is  difficult  to  predict  which  will  show  the  greater  optical  exaltation 
since  the  former  possesses  a  continuous  system  of  four  pairs  of  conjugate 
double  bonds  while  the  latter  contains  two  separate  conjugate  systems. 
It  may  be  reasoned  that  ortho  derivatives  (I.)  should  show  the  stronger 
optical  exaltations  since  a  single  conjugate  of  the  type 

=:::,,.,,()— C=C 

i 

is,  so  far  as  present  observation  shows,  optically  weak.  This  conclusion 
is  in  harmony  with  the  fact  that  o-methoxy-benzophenone  and  o-meth- 
oxy-benzaldehyde  are  both  characterized  by  strong  optical  exaltations. 
The  experimental  evidence  is,  however,  too  slight  at  the  present  time  to 
allow  of  safe  generalizations  in  regard  to  this  or  other  similar  phenomena.1 
While  it  is  remarkable  that  benzene  shows  no  optical  exaltation  in 
spite  of  the  presence  of  three  pairs  of  conjugate  double  bonds,  it  is  even 
more  amazing  that  furane,  thiophene,  pyrrol,  and  cyclopentadiene,  i.e., 


0  S  NH  CH2 

all  show  optical  depressions.  The  fact  that  this  statement  includes 
cyclopentadiene  eliminates  the  possibility  of  any  explanation  based 
upon  the  particular  influence  of  the  oxygen,  sulphur,  and  nitrogen  atoms. 
The  cause  for  this  anomalous  condition  must,  therefore,  be  sought  in 
the  nature  of  the  five-membered  ring.  Following  the  argument  used 
in  the  case  of  benzene  it  may  be  assumed  that  the  free  affinity  on  the 
unsaturated  atoms  which  normally  would  produce  optical  exaltation  is 
in  some  way  neutralized  in  maintaining  the  equilibrium  of  the  ring. 
In  the  formation  of  these  particular  rings  neutralization  must  be  con- 
ceived to  be  so  complete  that  in  individual  instances  it  actually  pro- 
duces the  effect  of  optical  depression.2  The  phenomenon  is  analogous 

iAuwers,  Annalen  des  Chemie,  408,  212  (1915). 
2Ber.,  40,  1157  (1907);  46,  3077  (1912). 


312 


THEORIES  OF  ORGANIC  CHEMISTRY 


No. 

Formula 

**„ 

**„ 

E\*° 

E\  "° 

Literature 

1 

.  K 

-0.45 

-0.47 

+  2 

+     4 

Auwers,    Ber., 
45,3078(1912). 

^>CH2 

1  

2 

i=  x° 

-0.97 

-1.10 

-  8 

+     8 

Nasini  and  Car- 

!=/ 

rara,         Gazz. 

CH2-OH 

chim.,24,1,278 

I 

(1894). 

3 

/ 

-0.39 

-0.42 

[+29]1 

+     2 

Gennari,  Gazz. 

CH3 

chim.,    24,     I, 

253  (1894). 

4 

!  y° 

-0.19 

-0.19 

+  5 

+     5 

Nasini  and  Car- 

' / 

rara. 

CH3 

CH30-C=0 

5 

So 

+0.56 

+0.60 

+43 

+  48 

Gennari,      and 

!=/ 

also      Auwers, 

C2H5O.C=0 

Ber.,  44,  3690 

(1911). 

6 

0 

+0.60 

+0.72 

[+56P 

+  46 

Gennari. 

C3H,0-C=0 

7 

.  t> 

+0.59 

+0.63 

+38 

+  42 

Gennari. 

8 

2E 

+0.55 

+0.59 

+38 

— 

Gennari. 

9 

'   [~> 

+0.59 

+0.62 

+34 

_ 

Gennari. 

C2H6O-OC      CH3 

10 

H^0 

+0.42 

+0.43 



+  20 

Briihl,     Jour. 

. 

prakt.  Chemie. 

CH3 

[2],      60,     143 

CH=0 

(1894). 

11 

l^>° 

+1.54 

+1.69 

+95 

+110 

Briihl,  Annalen 
der      Chemie, 

236,  7  (1886). 

APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES 


313 


No. 

Formula 

EZ 

EZD 

ESr-Za 

Literature 

% 

» 

12 

.  v 

-1.30 

-1.31 

-  3 

A 

N  a  s  i  n  i      and 

/S 

\==/ 

Scala,      Rend. 

• 

Ace.  dei  Line., 

1886,      621; 

Knops,  Annal- 

en  der  Chem- 

ie,     248,     204 

(1888);  Briihl, 

Zeitschr.  phys- 

CH3 

ikal.     Chemie, 

22,  392  (1897). 

13 

1          /^ 

-0.44 

-0.41 

+  8 

+     8 

Nasini  and  Car- 

1         ; 

rara. 

CH3 

CH3 

14 

=\s 

— 

-0.39 

— 

— 

Grischkewitsch- 

. 

Trochimowski, 

CH(OC2H6)2 

Chem.    Cen- 

tralbl.,      1911, 

II,  1239. 

CH3.OC       CH3 

15 

r  ^ 



+0.25 

— 

— 

Silberfarb, 

*  / 

Chem.     C  e  D  - 

CH3 

tralbl.,  1914,  I, 

1663. 

CH3 

16 

1          ^S 

— 

+0.99 

— 

— 

Grischke  wi  tsch- 

. 

Trochimowski. 

CH—  O 

to  that  which  is  observed  in  the  case  of  benzene  since  substitution  of  the 
hydrogen  of  the  ring  produces  an  analogous  effect.  Thus,  for  example, 
the  introduction  of  an  unsaturated  group  into  substances  containing  a 
five-membered  ring  always  lessens  the  optical  depression  of  the  sub- 
stance and  may  even  be  so  strong  as  to  produce  slight  exaltation.  This 
is  demonstrated  by  a  comparison  of  the  results  set  forth  in  the  preceding 
table.  It  is  explicable  on  the  assumption  that,  just  as  in  the  case  of 
benzene,  substitutions  tend  to  disturb  the  equilibrium  represented  in 


314 


THEORIES  OF  ORGANIC  CHEMISTRY 


the  neutralization  of  free  affinity  due  to  ring  structure,  and  that  this 
change  to  a  condition  where  free  affinity  is  actually  operative  in  the 
molecule,  corresponds  to  an  increase  in  optical  exaltation  or,  what  is  the 
same,  a  decrease  in  optical  depression.  The  experimental  material  is  at 
present  so  slight  that  it  is  difficult  to  make  any  generalizations  as  to  the 
relationships  which  exist  in  five-membered  rings.  The  results  shown 
in  the  table  on  p.  312  would,  however,  seem  to  indicate  that  the  appear- 
ance of  a  so-called  active  conjugation  in  such  a  compound,  is  attended 
by  marked  spectrochemical  changes. 

The  data  in  the  preceding  table  show  that  optical  depression  varies 
considerably  in  the  case  of  different  types  of  ring  compounds.  This 
would  seem  to  indicate  that  while  these  substances  are  similar  in  con- 
stitution, their  structure  is  by  no  means  identical.  The  benzene 
derivatives  of  furane  and  thiophene 


H 


o 


Cumarone 


Thionaphthene 


together  with  their  various  substitution  products,  show  practically  the 
same  specific  exaltation,  and  even  the  introduction  of  methyl  or  methoxy 
in  the  /3-position  produces  no  appreciable  effect  upon  the  spectrochemical 
relationships  of  these  substances.  This  is  true,  for  example,  in  the  case 
of  the  esters  of  cumarilic  acid : 


In  the  case  of  the  chromenes,  on  the  other  hand, 


the  substitution  of  methyl  in  the  corresponding  position  (*)  produces  a 
very  marked  effect  and  is  accompanied  by  a  definite  decrease  in  the 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  315 


exaltation  of  the  substance.  This  phenomenon  has  been  observed, 
not  only  in  connection  with  derivatives  of  chromene,  but  also  in  the 
of  the  styroles 

CH 
W 


CH3 


/ 


which  closely  resemble  the  chromenes  in  constitution  in  spite  of  the  fact 
that  they  are  open  chain  compounds. 


No. 


Formula 


CH 


(CH3)2 


+  1.15 


+1.24 


O 
C-CH3 


+0. 


+0.93 


O 


+54 


+47 


+59 


+53 


CH 


+1.12 


+  1.21 


O 
C-CH3 


CH, 


+0.78 


+0.82 


O 


+57 


+32 


+64 


+36 


As  a  result  of  a  general  physical  examination  of  the  cyclo-hexanes, 
-hexenes,  -hexanols,  and  -hexanons,  Auwers  1  has  discovered  that  the 

i  Annalen  der  Chemie,  410,  287  (1915). 


316  THEORIES  OF  ORGANIC  CHEMISTRY 

index  of  refraction  increases  in  passing  from  a  saturated  to  an  unsatur- 
ated  substance  and  decreases  in  passing  from  an  alcohol  to  the  corre- 
sponding ketone.  A  comparison  of  isomeric  substances  belonging  to 
this  series  shows,  moreover,  that  the  molecular  refraction  is  greater 
the  further  the  side  chains  are  from  each  other  or  from  the  double 
bond  or  from  the  substituent  containing  oxygen.  The  specific  refrac- 
tions of  symmetrical  compounds  therefore  usually  show  exaltations 
while  in  the  case  of  unsymmetrical  compounds  they  are  either  normal 
or  depressed.  The  molecular  dispersions  of  all  saturated  and  unsatur- 
ated  endocyclic  compounds  in  this  series  are  normal,  and  isomers  show 
no  optical  differences  within  the  limits  of  experimental  error.1 

Substances  which  contain  systems  of  adjacent  double  bonds,  as,  for 
example,  —  C=C=C  — ,  were  said  by  Bruhl  to  show  no  optical  exalta- 
tion, but  this  statement  has  recently  been  questioned  by  K.  von  Auwers. 
Since  the  compounds  which  were  selected  for  investigation  by  Bruhl 
were  open  to  certain  objections,  Auwers  undertook  a  systematic  study 
of  certain  of  the  allenes  and  ketenes  with  a  view  to  determining  the 
optical  effect  of  so-called  cumulative  systems  of  the  types  — C=C=C— 
and — C=C=0.  As  a  result  of  this  work2  he  was  able  to  demon- 
strate that  the  refractions  and  dispersions  of  such  compounds  are  not 
normal,  but  that  they  always  show  slight  exaltations.  The  variation 
from  normal  in  such  cases  is  very  small  as  compared  with  the  exaltations 
which  have  been  observed  in  the  case  of  substances  which  contain 
conjugated  systems.  Indeed,  in  certain  instances,  as,  for  example,  in 
the  case  of  diphenyl  and  diethyl-ketene,  they  may  even  be  so  slight  as 
to  be  negligible.  The  rule  that  a  decrease  in  the  degree  of  saturation  of 
the  atoms  present  in  chemical  compounds  corresponds  to  an  increase  in 
their  respective  refractions  and  dispersions  has  been  found  to  be  of  very 
general  application.  The  effect  of  ring  formation  is  of  particular  interest 
in  this  connection.  Bruhl  originally  believed  that  this  effect  was  negli- 
gible, but  a  more  exact  investigation  of  the  phenomenon  has  led  to  the 
discovery  of  an  ever-increasing  number  of  instances  where  ring  formation 
is  attended  by  marked  changes  in  optical  properties.  This  is  especially 
true  in  cases  where  a  condition  of  tension  is  assumed  to  result  from  the 
closing  of  the  ring,  as,  for  example,  in  trimethylene  and  tetramethylene 
derivatives  where  instances  of  optical  anomalies  have  been  observed 
frequently.  It  should  be  added,  however,  that  ethylene  oxide  and  its 
derivatives  represent  notable  exceptions  to  this  general  rule. 

K.  von  Auwers  has  recently  made  the  spectrochemical  effects  of  ring 

1  Annalen  der  Chemie,  410,  330  (1915). 
2Ber.,  61,  1124  (1918). 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES          317 

closure  the  subject  of  an  extended  investigation  1  and  as  a  result  of  this 
work  he  has  been  able  to  formulate  the  following  three  rules.  These 
rules  apply  particularly  to  the  formation  of  benzene  and  its  derivatives: 

1.  In  all  cases  where  a  saturated  chain  closes  to  form  a  ring,  the 
spectrochemical  character  of  the  original  substance  remains  unchanged. 

2.  In   cases  where   an  unsaturated  acyclic  chain  closes  to  form  a 
ring,  the  optical  effect  of  the  unsaturated  groups  is  weakened.     This 
change  is  greater,  the  greater  the  degree  of  unsaturation  of  the  original 
chain.      In  other  words  the  optical  effect  of  unsaturated  linkages  is  less 
in  closed  than  in  open  chains. 

3.  The  presence  of  alkyl  and  other  substituents  in  unsaturated  chain 
compounds  acts  to  retard  ring  formation. 

It  may  be  noted  in  passing  that  the  spectrochemical  behavior  of 
saturated  iso-  and  heterocyclic  compounds,  as  well  as  of  unsaturated 
iso-cyclic  compounds,  may  be  easily  understood  and  may  even  be  pre- 
dicted on  the  basis  of  the  above  rules. 

It  is  evident  from  even  this  brief  survey  of  the  subject  that  the  experi- 
mental material  available  in  this  field  is  often  very  complex  and  very 
difficult  of  interpretation,  and  that  the  very  slightest  changes  in  the 
arrangements  of  the  atoms  within  the  molecule  are  reflected  in  the 
optical  properties  of  the  substance.  Optical  methods  should,  therefore, 
always  be  used  in  conjunction  with  chemical  methods  and  never  by  them- 
selves, and  it  should  never  be  forgotten  that  all  such  data  are  based  upon 
the  laws  of  structural  chemistry  and  that  it,  therefore,  stands  or  falls 
with  them.  If  these  considerations  are  borne  in  mind,  if  sufficient  care 
is  taken  in  determining  refractions  and  dispersions,  and  if  comparative 
experiments  are  never  neglected,  spectrochemical  methods  will  not  only 
be  found  invaluable  in  elucidating  fine  points  in  regard  to  questions  of 
structure,  but  will  prove  essentially  helpful  in  developing  the  theory  of 
structural  organic  chemistry.2 

The  relation  of  optical  rotation  to  constitution  may  now  be  con- 
sidered briefly.  As  is  well  known,  substances  which  possess  an  asym- 
metric structure  have  the  power  to  rotate  the  plane  of  polarized  light. 
This  property  has  been  particularly  associated  with  compounds  which 
contain  asymmetric  carbon  atoms,  supposing,  of  course,  that  such  atoms 
are  not  rendered  inactive  either  by  racemization  or  by  intramolecular 
compensation.  That  a  definite  relation  exists  between  the  constitution 
of  a  given  substance  and  its  specific  and  molecular  rotatory  power  has 

1  Annalen  der  Chemie,  416,  98  (1917). 

2Ber.,  50,  329  (1917);  61,  1087,  1106  (1918);  62,  584  (1919);  Annalen  der 
Chemie,  420,  84  (1920);  421,  1  (1920). 


318  THEORIES  OF  ORGANIC  CHEMISTRY 

been  fully  demonstrated.1  In  some  of  the  very  early  investigations 
undertaken  in  this  field  Guye  was  led  to  conclude  that  the  rotatory 
power  of  a  substance  was  materially  influenced  by  the  mass  (or  weight) 
of  the  substituents  in  union  with  an  asymmetric  carbon  atom,  but  this 
was  not  conclusively  proved,  and  further  study  tended  to  show  that 
various  other  factors,  such  as  homology,  position  isomerism,  etc.,  af- 
fected rotation.2  Thus,  for  example,  in  the  case  of  isomeric  substances 
possessing  different  rotations  the  variation  was  supposed  to  be  due  to 
the  relative  positions  of  the  substituents  in  the  molecule. 

Soon  after  this  Freundler  discovered  that  the  optical  rotation  of  a 
substance  is  very  much  increased  by  the  substitution  of  a  phenyl  group. 
This  observation  was  confirmed  by  Tschugaeff,3  who  showed  further 
that  this  influence  is  most  felt  in  cases  where  the  phenyl  group  occupies 
a  position  adjacent  to  the  asymmetric  carbon  atom,  and  that  the  strength 
of  this  influence  decreases  in  proportion  to  the  distance  separating  the 
phenyl  group  from  the  asymmetric  carbon  atom. 

The  influence  of  unsaturated  double  bonds  has  since  been  made  the 
subject  of  extended  study  by  P.  Walden,4  A.  Haller,5  Hilditch,6  H.  Rupe  7 
and  others,  and  as  the  result  of  this  study  a  number  of  rules  which  may 
be  assumed  to  govern  optical  rotation  have  been  formulated.8  The  list 
of  substances  which  formed  the  basis  of  this  investigation  includes  such 
compounds  as  the  menthyl  esters  of  a  number  of  different  acids,  and 
derivatives  of  citronellal,  carvoxime,  myrtenol,  and  methylene-camphor. 
Before  proceeding  to  a  detailed  consideration  of  this  subject  it  will  be  of 
advantage  to  pause  and  review  a  few  of  the  fundamental  physical 
conceptions  upon  which  the  determination  of  optical  activity  is  based. 

The  rotatory  power  of  a  substance  is  measured  by  means  of  an 
instrument  called  a  polariscope.  The  angle  of  rotation  for  polarized 

1  Landolt  "  Das  optische  Drehungsvermogen  organischer  Substanzen,"  2d 
Edition,  1896. 

2Zeitschr.  physikal.  Chemie,  12,  723  (1893);  Bull.  soc.  chimie,  16,  177  (1896); 
Annales  Chimie  et  phys.  6,  142  (1895);  Compt.  rend.,  128,  1370  (1895);  Bull.  soc. 
chimie,  11,  316  (1894);  Zeitschr.  physikal.  Chemie,  17,  245  (1895). 

3Ber.,  31,  1777  (1898). 

4  Zeitschr.  physikal.  Chemie,  20,  569  (1896). 

5  Compt.  rend.,  128,  1370;  129,  1005  (1899). 

6  Jour.  Chem.  Soc.,  93,  1,  700,  1388,  1618  (1908);   95,  331,    1570,  1579  (1909); 
97,  223,  1091,  2110  (1910);  99,  218,  224  (1911). 

7  Annalen  der  Chemie,  327,  157  (1903);   369,  311  (1909);    373,  121  (1910);  396, 
87,  136  (1913);   398,  372  (1913);   402,  149  (1914);  409,  327  (1915);  414,  99  (1917); 
420,  1  (1920). 

8  Trans.  Faraday  Soc.,  10,  1  (July,  1914). 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  319 

light  of  definite  wave  lengths,  represented  by  aD  for  sodium  light, 
depends  upon  the  length  of  the  rotatory  column  and  also  upon  the 
temperature.  In  order  to  compare  substances  in  cases  where  very 
unequal  masses  are  contained  in  this  column  the  observed  rotation 
must  in  every  case  be  divided  by  the  specific  gravity  of  the  substance 
at  the  temperature  at  which  the  observation  is  taken.  The  specific 
rotation  of  an  optically  active  substance  may  then  be  calculated  by 
means  of  the  expression 

r  i  t    a*>' 
la]D=Ld 

where  aDl  represents  the  observed  angle  of  rotation  of  sodium  light 
at  the  given  temperature;  /,  the  length  of  the  column  through  which  the 
light  passes,  and  d,  the  density  of  the  substance  at  the  temperature 
at  which  the  observation  was  made.  The  product  of  the  specific  rota- 
tion of  a  substance  by  its  molecular  weight  represents  its  molecular 
rotatory  power  [M],  but  since  the  values  which  are  obtained  in  this  way 
are  apt  to  be  large  it  is  convenient  to  divide  the  molecular  weight  by 
100,  when  the  expression  becomes 


The  values  which  represent  the  molecular  rotations  of  different 
substances  are  frequently  misleading  for  purposes  of  comparison  because 
the  numbers  which  represent  the  molecular  weights  are  relatively  so 
large  that  they  serve  to  veil,  rather  than  to  accentuate,  the  observed 
small  differences  in  rotation.  Indeed  H.  Rupe  1  is  of  the  opinion  that  a 
formula  for  determining  optical  rotations  has  yet  to  be  devised  which 
shall  take  into  consideration  the  relative  values  which  are  expressed 
by  means  of  molecular  weights  and  not  at  the  same  time  allow  these  to 
overshadow  other  factors  of  equal  or  greater  importance  in  the 
equation. 

In  order  to  acquire  the  most  complete  knowledge  which  it  is  possible 
to  obtain  in  regard  to  the  relation  between  chemical  constitution  and 
optical  activity  it  is  necessary  to  determine  the  specific  rotations  of 
substances  under  the  influence  of  polarized  light  of  four  different  wave 
lengths,  viz.,  the  red  C  line,  the  yellow  D  line,  the  green  E  line,  and  the 
blue  F  line.  From  the  data  which  are  obtained  in  this  way  it  is  possible 

1  Annalen  der  Chemie,  420,  21  (1920). 


320  THEORIES  OF  ORGANIC  CHEMISTRY 

to  calculate  the  specific  rotatory  dispersions  of  different  compounds 
and  by  a  comparison  of  these  results  to  arrive  at  more  or  less  definite 
conclusions  as  to  molecular  structure.  Numerous  instances  of  optical 
anomaly  are  met  with  in  the  field  of  rotation  dispersions,  but  this 
phase  of  the  subject  must  wait  to  be  considered  a  little  later  in  the 
discussion. 

H.  Rupe  determined  the  optical  rotations  of  a  large  number  of 
menthol  esters  derived  from  both  saturated  and  unsaturated  acids  and 
also  of  derivatives  of  the  citronellals,  carvoximes,  and  methylene- 
camphors.  The  immediate  object  of  this  investigation  was  to  compare 
the  rotatory  powers  of  saturated  groups  with  those  of  similar  groups 
possessing  different  degrees  of  unsaturation.  Another  object  was  to 
determine  the  effect  of  substitution.  In  the  latter  case  it  was  soon 
apparent  that  the  introduction  of  methyl,  or,  in  general,  of  alkyl  groups 
in  place  of  hydrogen  was  attended  by  insignificant  changes  in  optical 
rotation  and  that  even  the  presence  of  a  number  of  such  groups  in  the 
same  part  of  the  molecule  failed  to  produce  any  appreciable  effect. 
The  introduction  of  unsaturated  groups  such  as  phenyl  was,  on  the 
other  hand,  accompanied  by  marked  changes  in  rotation.  The  most 
valuable  results  were  obtained  by  comparing  compounds  which  contain 
long  aliphatic  chains  since  in  such  cases  the  molecular  weight  of  the 
substance  was  found  to  act  as  a  kind  of  ballast.  In  general,  the  results 
showed  that  a  condition  of  unsaturation  generally  corresponds  to  an 
increase  in  rotation  and  that  this  effect  is  relatively  great  when  the 
unsaturated  group  is  in  direct  union  with  the  asymmetric  carbon  atom 
and  may  become  negligible  in  cases  where  several  other  groups  intervene. 
A  conjugate  system  of  double  bonds  has  a  more  powerful  influence  upon 
rotation  than  two  separate  pairs  of  ethylene  linkages. 

The  introduction  of  a  phenyl  group  into  a  molecule  containing  an 
asymmetric  carbon  atom  has  a  very  pronounced  effect  upon  rotation. 
This  effect  is  greatest  when  the  phenyl  group  occupies  a  position  adjacent 
to  the  asymmetric  atom,  decreases  gradually  as  the  distance  between  the 
two  becomes  greater  and  finally,  in  certain  instances,  even  swings  over 
to  the  opposite  sign.  Rupe  attempts  to  explain  this  phenomenon  which 
has  been  observed  in  a  great  many  cases  by  suggesting  that  the  heavy 
phenyl  group  exercises  greater  leverage  as  the  distance  between  it  and 
the  asymmetric  carbon  atom  is  increased. 

The  menthyl  esters  of  sorbinic,  dimethylsorbinic,  and  cinnamylacry- 
lic  acids  were  compared  with  the  corresponding  saturated  compounds  in 
an  effort  to  determine  the  influence  of  conjugate  systems  upon  rotation. 
The  results  of  this  investigation  are  given  in  the  following  table: 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  321 


Menthyl  ester  of  sorbinic  acid  and  its  reduction  products : 


CH3-CH  :  CH-CH  :  CH.CO-O-Ci0H19. 
CH3-CH2-CH  :  CH.CH2.CO-O-Ci0H19. 
CH3  •  CH2  -  CH2  •  CH2  •  CH2  •  CO  •  O  •  Ci0H19 . 


Menthyl  ester  of  cinnamyl-acrylic  acid  and  its  reduction  products: 


C6H5-CH  :  CH-CH  :  CH •  CO •  O •  C10H19 . 
C6H5.CH2-CH  :  CH  •  CH2  •  CO  •  O  •  C10H19 
C6H5  •  CH2CH2  •  CH2  •  CH2  •  CO  •  O  •  C10Hi9 . 


Menthyl  ester  of  dimethyl  sorbinic  acid  and  its  reduction  products: 


CH3-C  :  CH-C  :  CH •  CO •  O •  d0H19 .  . 

CH3        CH3 
CH3-CH-CH  :  C-CH2-CO-O-d0H19. 


CH3  CH 


CH3  •  CH  •  CH2  •  CH  •  CH2  •  CO  •  O  •  Ci0H19 
CH3          CH3 


88.53 
65.11 
64.86 


75.14 
47.54 
33.86 


59.80 


68.51 


57.38 


It  is  apparent  that  a  decrease  in  optical  rotation  accompanies  the 
change  from  an  unsaturated  to  a  saturated  compound  in  the  case  of  the 
menthyl  esters  of  sorbinic  and  of  cinnamylacrylic  acid.  A  comparison 
of  the  rotations  of  the  esters  of  sorbinic  and  dimethylsorbinic  acids 
shows  that  the  substitution  of  methyl  for  ethylene  hydrogen  has  a  dis- 
turbing influence  which  is  manifested  by  a  pronounced  decrease  in  rota- 
tion. This  is  analogous  to  the  effect  of  methyl  in  the  combination 

c=c— c=c 


CH; 


which  has  been  referred  to  in  connection  with  the  discussion  of  molecular 
refractions.  In  both  cases  the  change  in  the  optical  properties  which 
is  produced  by  the  substitution  of  methyl  is  marked. 

In  cases  where  a  phenyl  group  forms  part  of  a  conjugate  system  which 
is  in  union  with  an  asymmetric  carbon  atom  the  substance  frequently 
shows  an  enormous  increase  in  rotation  as  compared  with  a  correspond- 
ing saturated  compound.  This  is  strikingly  illustrated  in  the  case  of  a 
number  of  substances,  the  rotations  of  which  are  given  in  the  accompany- 
ing table: 


322 


THEORIES  OF  ORGANIC  CHEMISTRY 


MD 


I.  Benzylidene  camphor  and  its  reduction  products: 

yC  :  CH-CeHs  .............................    429.25      1020 

O.HI/  | 

XC  :O 

/C  :  CH-CeHn  .............................  323 

C8H14<  | 

XC  :0 

/CH-CH2-C6H6  ............................        —  248 

C8H14<    | 
XCO 

II.  Condensation  production  from  citronellal  and  C6H5MgBr 
and  its  reduction  products: 

C6Hu-CH(CH3)-CH  :  CH.C6H5  ....................      63.24       135.4 

C6Hn-CH(CH3)-CH2-CH2-C6H5  ....................        7.62 

III.  Cinnamylidene  camphor  and  phenylpropyl  camphor: 

,C  :  CH-CH  :  CH.C6H6  ....................    296.  11 

CaHux        I 

XC  :O 

/CH-CH2-CH2-CH2-C6H6   ..................      66.35 

C8H14<   | 

XC  :0 

IV.  Optically  active  ester  of  cinnamic  and  hydrocinnamic  acids: 

C6H5-CH  :  CH-CO2H  .............................      76.95       220.8 

C6H5-CH2.CH2-CO2H  .............................      58.48       168.4 


Here  again  the  proximity  of  the  unsaturated  group  to  the  asymmetric 
carbon  atom  represents  an  important  factor  in  determining  the  optical 
rotation  of  a  substance,  as  is  apparent  from  a  comparison  of  the  following 
derivatives  of  citronellal: 


[a\D 


C6Hn-CH(CH3)-CH=CH-C6H6 
C6H1i-CH(CH3)-CH2-CH=CH-C6H5 


63.24 
3.33 


135.4 
7.6 


In  general  it  may  be  said  that  the  action  of  any  group  which  influences 
the  optical  rotatory  power  of  a  substance  is  a  function  of  its  distance 
from  the  asymmetric  carbon  atom. 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  323 

A  carbonyl  group  in  conjunction  with  an  ethylene  group  also  tends 
to  increase  the  optical  rotation.  Thus,  according  to  Hilditch,1  the 
influence  of  a  carbonyl  group  is  approximately  as  great  as  that  of  an 
ethylene  group  and  the  combination  CO-CO-O  is  just  as  effective  as 
C6H5-CO-0. 

The  unusually  high  values  which  are  obtained  in  the  case  of  p-phenyl- 
ene-bisimino-camphor, 


[<*]D 

MD 

yC  :N-C6H4-N  :Cx 

1528 

6100 

^C  :  O            O  :  Cr 

are  very  significant  when  noted  in  connection  with  the  fact  that  this 
substance  contains  an  unbroken  chain  of  conjugations  between  carbon, 
nitrogen,  and  oxygen  atoms. 

The  application  of  data,  which  have  been  obtained  as  the  result  of 
the  measurement  of  rotations,  to  the  determination  of  constitution  is 
subject  to  the  same  limitations  as  have  been  described  in  connection 
with  a  discussion  of  molecular  refractions  and  dispersions.  The  informa- 
tion which  is  gained  in  this  way  will  always  serve  to  support  and  amplify 
the  knowledge  of  atomic  relationships  which  have  been  acquired  as  the 
result  of  purely  chemical  investigations.  The  brief  survey  of  Rupe's 
work  in  this  field  which  has  just  been  made  demonstrates  further  that 
in  highly  competent  hands  the  conclusions  drawn  from  a  comparative 
study  of  the  rotatory  power  of  different  substances  may  also  play  an 
important  role  in  the  actual  determination  of  constitution. 

To  add  one  more  illustration  to  those  which  have  been  given,  Rupe, 
Jager,  and  others2  have  obtained  two  alcohols  which  correspond  to 
the  formulas; 

(CH3)2C=CH  -  CH2  •  CH2  -  CH  -  CH2  •  CH  -  CH3 
and  CH3  OH 

(CH3)2C=CH  -  CH2  -  CH2  •  CH  •  CH2  •  CH(OH)  •  CH2  •  CH3 

CH3 

1  Jour.  Chem.  Soc.,  99,  224  (1911). 
2  Tran.  Faraday  Soc.,  10,  1  (1914). 


324  THEORIES  OF  ORGANIC  CHEMISTRY 

by  the  action  of  methyl-  and  ethyl-magnesium  bromide,  respectively, 
upon  citronellal.  These  alcohols  both  lose  water,  and  in  so  doing  pass 
over  into  two  hydrocarbons  which  correspond  to  either  the  formulas 
la  or  Ib  and  Ila  or  116,  respectively : 

la.     (CH3)2  :  C=CH  >  CH2  •  CH2  •  CH  •  CH=CH  •  CH3 

CH3 

16.     (CH3)2  :  C=CH  •  CH2  •  CH2  •  CH  •  CH2  •  CH=CH2 

CH3 

Ila.     (CH3)2  :  C=CH  •  CH2  •  CH2  •  CH  •  CH=CH  •  CH2  •  CH3 

CH3 

116.     (CH3)2C=CH  •  CH2  •  CH2  •  CH  •  CH2  -  CH=CH  •  CH3 

CH3 


It  was  observed  that  the  hydrocarbon  which  was  obtained  by  the  use 
of  Mg(CH3)Br  showed  a  higher  value  for  rotation,  [a]D=  —10.30, 
than  the  hydrocarbon  which  was  obtained  by  the  use  of  Mg(C2H5)Br 
where  [a]D=  —6.64.  On  the  basis  of  these  measurements  Rupe  and 
Jager  concluded  that  the  former  compound  possessed  the  constitution 
la  and  the  latter,  the  constitution  116,  and  the  correctness  of  this 
deduction  was  confirmed  by  the  decomposition  of  the  substance  under 
the  action  of  ozone.  In  this  way  the  definite  relation  which  exists 
between  the  rotatory  power  and  the  constitution  of  a  substance  has 
again  been  established. 

H.  Rupe,1  A.  Akermann,  and  H.  Kagi  have  recently  obtained  some 
very  important  results  which  are  based  upon  the  clever  mathematico- 
physical  analysis  of  A.  Akermann  2  and  which  inaugurate  a  definite 
advance  in  a  knowledge  of  the  relationship  which  exists  between  the 
molecular  structure  and  the  optical  rotation  of  organic  compounds. 
Rupe3  had  discovered  that  the  difference  in  dispersion  represented  by 
[a]p  —  [oi\c  is  constant  for  substances  which  belong  to  the  same  general 
class  of  chemical  compounds.  If  this  constant  specific  rotation-dis- 

1  Annalen  der  Chemie,  420,  1  and  33  (1920). 

2  Dissertation,  Basel,  1920. 

3  Annalen  der  Chemie,  409,  336  (1915). 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  325 

persion  is  regarded  as  a  function  of  the  wave  length,  it  becomes  possible 
to  calculate  the  value  for  Xa  by  means  of  the  following  equation  l 

2       (      2  2\Hf Me [«]*,         22         1  1 

Va=(vcT — VDZ) F-T—  —. h  Vif  •  Xa—-o,  X«=- 

[a\c — [a]D  v2'  v 

where  v  represents  the  reciprocal  wave  length  and  Xa,  their  so-called 
characteristic. 

Since  the  values  calculated  for  Xa  have  been  found  to  be  fairly 
constant  in  the  case  of  a  great  number  of  substances,  it  has  become 
customary  to  compare  the  rotation-dispersions  of  different  substances 
in  terms  of  this  constant.  For  example,  if  a  substituting  group  pro- 
duces no  change  in  the  value  of  Xa,  the  behavior  of  the  resulting  com- 
pound is  said  to  be  normal  with  respect  to  dispersion,  while  if,  on  the 
other  hand,  the  value  of  Xa  is  altered  by  the  introduction  of  a  new 
group  or  by  a  difference  in  the  configuration  of  the  asymmetric  complex, 
the  dispersion  of  the  resulting  compound  is  said  to  be  anomalous.  In 
the  latter  case  it  is  always  assumed  that  constitutive  changes  have  taken 
place  within  the  molecule. 

In  order  to  determine  the  exact  influence  of  different  kinds  of  sub- 
stituting groups  upon  the  power  of  rotation  of  a  given  asymmetric  com- 
plex, Akermann  has  deduced  a  law  governing  dispersions  which  is  based 
upon  a  study  of  the  behavior  of  substances  possessing  normal  dispersions. 
The  mathematical  expression  of  this  law  not  only  includes  variable 
wave  lengths  and  deviations,  but  also  contains  one  or  more  constants 
which  are  typical  of  the  particular  compound  under  investigation. 
Among  the  constants  selected  by  Akermann  are  k0,  derived  from  the 
specific  rotation;  km,  the  so-called  coefficient  of  optical  activity;  and 
X02,  derived  from  the  molecular  rotation.  The  influence  of  the  solvent 
upon  rotation  cannot,  of  course,  be  disregarded,  but  in  the  case  of 
benzene,  which  was  used  exclusively  by  Rupe  and  his  students,  it  was 
observed  that  the  solvent  had  no  effect  upon  the  values  Xa  and  >02 
within  the  limits  of  experimental  error.  The  values  representing  the 
specific  rotations  were,  however,  affected  by  this  solvent. 

The  relationships  developed  by  Akermann  may  be  expressed  graphic- 
ally in  the  form  of  a  curve  by  plotting  squares  of  the  wave  lengths  as 
abscissa  against  the  specific  rotations  for  the  different  wave  lengths  as 
ordinates.2  Differences  in  the  form  and  position  of  the  so-called 
rotation-dispersion  curves  which  are  obtained  in  this  way  serve  to 
make  changes  in  constitution  instantly  apparent  and  a  comparison  of  a 

1  A.  Hagenbach,  Zeitschr.  Physikal.  Chemie,  89,  581  (1915). 

2  Compare  A.  Hagenbach,   Zeitschr.  physikal.  Chemie,  89,  581    (1915);   Rupe- 
Akermann,  Annalen  der  Chemie,  420,  5  (1920). 


326  THEORIES  OF  ORGANIC  CHEMISTRY 

large  number  of  such  curves  has  demonstrated  that  the  value  for  km 
is  approximately  constant  in  a  large  number  of  cases.  For  example, 
the  introduction  of  saturated  aliphatic  groups  or  even  of  aromatic 
groups  where  the  aromatic  residue  is  separated  by  means  of  a  methylene 
group  from  the  asymmetric  carbon  atom  produces  no  change  in  the  value 
km.  If,  however,  the  aromatic  residue  is  in  direct  union  with  the 
asymmetric  carbon  atom,  a  change  involving  an  increase  in  the  value 
of  km  occurs.  A  similar  effect  is  brought  about  by  the  introduction  of 
one  or  more  ethylene  or  acetylene  residues,  although  the  difference  is 
less  marked  in  the  latter  than  in  the  former  case.  The  value  of  km 
is  also  materially  changed  if  one  or  more  groups  which  contain  oxygen 
are  introduced  within  the  direct  sphere  of  influence  of  the  asymmetric 
carbon  atom. 

Rupe  and  Akermann  assume  that  the  coefficient  of  optical  activity 
(km)  is  normal  in  all  cases  where  the  asymmetric  complex  contains  only 
saturated  hydrocarbon  residues,  none  of  which  possesses  additional 
asymmetric  atoms.  Normal  and  variable  coefficients  of  optical  activity 
must  not,  however,  be  confused  with  normal  and  anomalous  rotation- 
dispersions,  since  substances  possessing  normal  rotation-dispersions  may 
show  either  normal  or  variable  values  for  km. 

The  coefficient  of  optical  activity  is  not  influenced  ordinarily  by  the 
number  of  atoms  or  groups  which  constitute  the  molecule,  but  is,  on 
the  other  hand,  dependent  upon  the  space  relationships  of  these  atoms. 
If  a  substance  possesses  both  a  normal  coefficient  of  optical  activity 
and  a  normal  rotation-dispersion,  it  may  be  assumed  that  this  is  con- 
ditioned by  a  certain  definite  arrangement  of  the  individual  atoms  or 
groups  of  atoms  with  relation  to  each  other  and  also  with  relation  to  the 
diameter  of  the  molecule  as  a  whole.  Since  changes  in  the  relative  space 
relationships  of  the  atoms  within  the  molecule  correspond  to  dif- 
ferences in  the  molecular  volumes  of  the  substances,  such  changes  can 
be  readily  followed  by  means  of  physical  measurements.  As  is  well 
known  changes  in  molecular  volume  have  been  observed  to  accompany 
the  substitution  of  ethylene  and  acetylene  residues,  of  ketone  and 
aldehyde  groups,  and  the  process  of  ring  closure.  Since,  moreover, 
the  coefficient  of  optical  activity  also  depends  upon  changes  in  the 
space  relationships  of  the  atoms  within  the  molecule,  it  is  possible  at 
the  present  time  to  predict  with  relatively  great  accuracy  the  effect 
of  such  changes  upon  the  rotation-dispersion  of  asymmetric  complexes. 
The  various  degrees  of  anomalous  rotation-dispersion  cannot  be  con- 
sidered further  at  this  time  and  the  reader  is,  therefore,  referred  to 
Rupe's  original  paper  for  additional  information. 
1  Annalen  der  Chemie,  420,  57  (1920). 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  327 

The  relation  between  thermochemistry  and  constitution  must  now  be 
considered,  but  here  again  the  way  must  first  be  cleared  by  a  few  pre- 
liminary remarks.  The  internal  energy  of  a  body  is  that  which  is 
stored  up  within  the  individual  molecules  and  is  quite  distinct  from 
external  energy  whether  this  is  kinetic  or  potential  in  character.  In 
the  case  of  an  organic  compound  the  internal  energy  bears  a  definite 
relation  to  the  constitution  of  the  molecule  and  is  therefore  different 
for  different  substances.  It  may  be  readily  measured  by  the  heat  of 
combustion  of  any  given  compound  since  equal  weights  of  the  same 
substance  always  give  the  same  quantity  of  heat  when  burned  under 
the  same  conditions,  while  equal  quantities  of  different  substances 
give  different  heats.  Even  isomeric  or  polymeric  bodies  have  different 
heats  of  combustion.  This  is  true  also  of  different  allotropic  forms  of  the 
same  element,  as,  for  example,  graphite,  carbon,  and  diamond.1  These 
relationships  were  observed  at  a  very  early  date  and  suitable  calori- 
metric  methods  have  since  been  developed  and  brought  to  a  high  degree 
of  accuracy.2  The  heat  of  combustion  is  determined  experimentally 
at  constant  pressure,  but  the  figures  obtained  in  this  way  must  be 
corrected  to  constant  volume,  since  in  the  process  of  burning  changes 
in  volume  and  in  state  of  aggregation  take  place  which  involve  change  in 
energy.  The  heats  of  combustion  of  gases  obtained  by  experiment  may 
be  used  for  comparative  purposes.  This  is  also  true  of  the  heats  of 
substances  in  the  liquid  state,  provided  that  the  respective  boiling  points 
are  approximately  the  same,  since,  under  these  conditions,  differences 
in  the  molecular  heats  of  vaporization  will  be  negligible  (Trouton's 
rule).  From  these  values  the  respective  heats  of  combustion  in  the 
gaseous  state  may  be  calculated  by  adding  the  molecular  heats  of 
evaporation  of  the  respective  substances.  It  is,  however,  difficult  to 
draw  accurate  conclusions  from  a  comparison  of  the  heats  of  combustion 
of  substances  in  the  solid  state.  Great  caution  is  needed  in  such  cases 
since  there  are  no  general  rules  governing  heats  of  fusion  and  these 
values  vary  considerably  even  for  different  modifications  of  the  same 
element  or  compound. 

The  application  of  thermochemical  data  in  the  determination  of 

1Roth,  Ber.,  46,  896  (1913). 

2Berthelot  "Thermochimie,"  Vol.  IV;  Stohmann,  Jour,  prakt.  Chemie,  39,  503 
(1889);  Lonquinine,  "  Hauptmethoden  der  Bestimmung  der  Verbrennungswarme," 
Berlin,  1897;  E.  Fischer,  Sitzungsberichte  der  Berlin  Akad.  der  Wissensch.,  20,  687 
(1904);  24,  129  (1908);  Zeitschr.  physikal.  Chemie,  69,  218  (1909);  Auwers  and 
Rothe,  Annalen  der  Chemie,  373,  239  (1910);  Rothe,  Annalen  der  Chemie,  373,  249 
(1910);  also  Annalen  der  Chemie,  386,  102(1911);  407,  109,112,  134,  145  (1915); 
Ber.,  46,  260  (1913);  Jour,  prakt.  Chemie,  70,  521  (1904);  96,  123  (1917);  97, 
137  (1918). 


328  THEORIES  OF  ORGANIC  CHEMISTRY 

constitution  may  be  seen  from  the  following  example:  the  heat  of 
combustion  of  1  gram  molecule  of  methane  (16)  has  been  found  to  be 
211.9  Cal.  The  heats  for  carbon  (as  diamond)  and  for  hydrogen  have 
also  been  determined,  and  correspond,  respectively,  to  the  following 
values. 

C  +02  =  CO2+94.3Cal. 


H2+O  =  H2O  +67.  5  Cal. 

The  products  of  combustion  in  the  case  of  methane  are  one  molecule 
of  CO2  and  two  molecules  of  H2O.  From  these  facts  it  is  now  possible 
to  calculate  the  heat  of  combustion  of  methane,  viz.  : 

94.3+2X67.5  =  229.8  Cal. 

This  value  exceeds  that  determined  experimentally  by  17.9  Cal.  Since 
the  first  law  of  thermodynamics  states  that  energy  is  never  lost  as  heat, 
but  merely  transformed  into  some  other  equivalent,  it  is  necessary  to 
account  in  some  way  for  the  apparent  disappearance  of  17.9  Cal. 
This  may  be  done  by  taking  into  account  the  following  facts:  the 
process  by  which  CH4  is  transformed  into  C02+H2O  does  not 
consist  merely  in  the  combination  of  C  and  0  to  form  CO2,  and  of 
H  and  0  to  form  H20,  but  requires  as  a  necessary  preliminary  to  this 
action  the  decomposition  of  the  methane  molecule.  The  amount  of 
work  which  must  be  done  in  order  to  sever  the  union  between  C  and  H 
to  free  these  atoms  is  obviously  equal  to  17.9  Cal.  and  corresponds  to  the 
energy  which  is  given  off  when  C  and  H  unite  to  form  CH4.  In  general 
it  may  be  said  that  the  difference  between  the  heat  of  combustion  of  a 
given  substance  and  the  sum  of  the  heats  of  combustion  of  the  elements 
composing  it  represents  the  heat  of  formation  of  the  compound.  This 
latter  value  corrected  to  constant  volume,  and  equal  in  the  case  of 
methane  to  17.9  Cal.,  is  frequently  used  in  organic  chemistry  instead  of 
the  heat  of  combustion. 

While  the  heat  of  formation  of  CH4  is  positive  it  often  happens, 
especially  in  the  case  of  unsaturated  compounds,  that  this  value  is 
negative  in  character.  For  example,  acetylene  shows  a  difference 
between  the  heats  of  combustion  as  found  (310.0  Cal.)  and  calculated 
(256.1  Cal.)  of  —53.9  Cal.  In  interpreting  these  facts  it  is  necessary 
to  assume  not  that  energy  is  required  to  break  the  double  bond  between 
the  carbon  atoms,  but  that  energy  is  actually  set  free  during  this  process. 
Experience  shows,  in  fact,  that  substances  possessing  double  and  triple 
bonds  are  more  unstable  and,  therefore,  more  readily  broken  down  than 
is  the  case  with  saturated  compounds.  This  difference  has  been  ex- 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  329 

plained  by  supposing  that  a  certain  degree  of  tension  is  developed  within 
the  molecule  by  the  union  of  the  second  pair  of  valencies  of  carbon 
to  form  the  double  bond,  and  that  such  an  atomic  arrangement  must, 
therefore,  be  regarded  as  representing  a  definite  amount  of  stored-up 
(i.e.,  potential)  chemical  energy.1  If  the  equilibrium  in  a  system  of  this 
kind  is  disturbed  in  any  way  this  energy  is  set  free.  Thus  in  the  process 
of  burning  it  appears  as  heat  and  serves  to  increase  the  total  heat  of 
combustion  of  the  substance. 

It  follows  as  a  necessary  corollary  that  in  the  formation  of  acetylene 
energy  must  be  supplied  from  the  outside.  This  is  true,  in  general,  of 
all  so-called  endothermic  compounds  as  distinguished  from  exothermic 
compounds  whose  heat  of  formation  is  positive. 

The  illustrations  serve  to  show  that  the  heat  of  combustion  of  organic 
compounds  is  not  merely  additive,  but  is  also  constitutive  in  character. 
The  numerical  value  of  the  additive  and  constitutive  heats  of  various 
groups  of  atoms  has  been  investigated  by  J.  Thomsen,  Welter,  Horst- 
rnann,  and  others.  Thomsen  assumes  that  the  four  valencies  of  carbon 
are  equivalent  and  that,  at  least  in  the  case  of  hydrogen  all  four  atoms 
are  bound  with  equal  strength  to  carbon.  On  this  basis  he  argues  that 
the  amount  of  heat  developed  by  the  formation  of  one  double  bond  is 
15,465  Cal.  less  than  in  the  formation  of  two  single  bonds  between 
carbon  atoms.  In  the  same  way  Thomsen  has  calculated  that  in  the 
union  of  two  carbon  atoms  by  means  of  a  triple  bond  43,922  Cal.  less 
heat  are  developed  than  in  the  union  of  four  carbon  atoms  by  means 
of  three  single  bonds.  These  figures  are  not,  however,  of  wide  applica- 
tion and  even  in  their  own  limited  field  they  have  no  general  significance. 

It  might  be  supposed  that  the  atomic  heats  of  the  elements  could 
be  calculated  in  the  same  way  as  in  the  case  of  refractions, — the  values 
depending,  of  course,  upon  the  form  of  combination  of  the  atoms, — and 
that  theoretical  values  could  then  be  deduced  for  the  heat  of  combustion 
of  a  substance,  calculated  upon  the  basis  of  its  particular  structure 
While  the  mass  of  experimental  data  which  has  been  steadily  accumulat- 
ing for  years  undoubtedly  tends  definitely  in  this  direction,  yet,  as 
Wrede 2  has  recently  pointed  out,  "  any  attempt  which  is  made  to 
calculate  the  heat  of  combustion  of  any  substance  from  its  constituents 
by  means  of  a  formula  and  by  the  use  of  empirical  constants  must  be 
regarded  as  premature." 

1  Compare  Nernst,  Theoret.  Chemie,  6th  Edition,  p.  328;    also  Stohmann  and 
Schmidt,  Zeitschr.  physikal.  Chemie,  21,  314  (1896);  Jour,  prakt.  Chemie,  53,   345 
(1896). 

2  See  Thomsen,  Zeitschr.  anorg.  Chemie,  40,  185  (1904)  and  Auwers,  Annalen  der 
Chemie,  385,  103  and  following  (1911). 


330  THEORIES  OF  ORGANIC  CHEMISTRY 

But  while  it  is  as  yet  impossible  to  calculate  the  heat  of  combustion 
or  the  heat  of  formation  of  a  substance  from  the  sum  of  the  additive  and 
constitutive  heats  of  its  atoms  in  the  same  way  as  in  the  case  of  refrac- 
tion and  dispersion,  it  is,  nevertheless,  possible  to  apply  thermochemical 
data  to  the  determination  of  constitution.  These  relationships  have 
recently  been  the  subject  of  systematic  investigation  by  E.  Fischer  and 
Wrede,  as  well  as  by  Auwers  and  Roth,  and  a  number  of  important  con- 
clusions have  been  reached. 

It  was  at  first  supposed  that  in  the  case  of  an  homologous  series  the 
respective  heats  of  combustion  differed  always  by  a  constant  which 
corresponded  to  CH2,  but  it  has  since  been  found  that  these  values  are 
only  approximately  constant  and  that  they  frequently  vary  considerably 
from  the  mean.  Isomeric  compounds  belonging  to  the  same  general 
class  have  almost  exactly  the  same  heats  of  combustion,  but  here  again 
small  variations  have  been  observed.  For  example,  primary  alcohols 
have  greater  heats  of  combustion  than  secondary  and  tertiary  alcohols, 
and  substituted  malonic  acids  have  higher  heats  than  the  corresponding 
succinic  acids.  In  instances  where  one  isomer  represents  a  stable,  the 
other  an  unstable  modification  as  in  the  case  of  fumaric  and  maleic 
acids  it  has  been  found  that  the  labile  form  possesses  the  greater  heat 
of  combustion.1  Straight  chain  unsaturated  compounds  show  greater 
heats  than  isomeric  ring  compounds.  This  is  true,  for  example,  in  the 
case  of  the  following  substances: 


CH3(CH2)3—  CH=CH2 

CH2—  CH2—  CH2 

Hexene  Cyclohexane 

Important  results  have  been  obtained  by  the  application  of  thermo- 
chemical data  to  the  study  of  unsaturated  compounds.  The  calori- 
metric  relationships  between  benzene  and  hydrobenzene  have  already 
been  referred  to  and  there  are  many  other  instances  of  theoretical  con- 
ceptions which  have  been  confirmed  in  this  way.  Thus,  for  example, 
in  formulating  his  theory  of  partial  valencies,  Thiele  assumed  that  of 
two  unsaturated  systems 

I        >-^       I  [J  II 

CH=C—  C=CH—  CH2    and    CH=CH—  CH2—  CH=CH 
H  H 

the  latter  would  have  the  greater  heat  of  combustion  since  it  was  to  be 
regarded  as  the  more  unsaturated.     It  is  interesting  to  note  that  this 
assumption  has  been  confirmed  as  the  result  of  recent  investigations  by 
1  Compare  Roth,  Ber.,  46,  260,  326  (1913). 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES 


331 


Auwers  and  Roth.  These  authorities  in  co-operation  with  Eisenlohr 
have  also  examined  these  substances  spectrographically  and  have  found 
that  a  remarkable  analogy  exists  between  optical  and  thermal  properties. 
That  conjugate  systems  in  general  represent  less  chemical  energy 
and,  therefore,  possess  greater  stability  and  lower  heats  of  combustion 
than  other  systems  of  double  bonds  has  been  demonstrated  very  clearly 
by  an  examination  of  the  terpenes.  For  example,  such  substances  as 


CH3— 


Limonene 


Dipentene 


CH3 


Sylvestrene 


have  greater  heats  of  combustion  than  such  substances  as 

CH3 

yCH; 

— ^      / 


CH3— 


CH3 


a-Phellandrene 


CH3 


Carvestrene 


CH3 


a-Terpinene 


CH3 


and  this  is  ascribed  to  the  fact  that  members  of  the  second  series  possess 
conjugate  systems  of  double  bonds.  A  similar  relation  holds  in  the 
sty  role  series,1  although  the  differences  are  not  so  striking  here  as  in  the 
case  of  the  terpenes. 

Since  substances  containing  conjugate  systems  have  been  found  to 
have  low  heats  of  combustion  and  at  the  same  time  to  show  optical 
exaltation  it  has  been  possible  by  taking  the  two  sets  of  facts  in  con- 
junction to  settle  certain  doubtful  questions  in  regard  to  constitution. 
For  example,  a  substance  is  usually  assumed  to  possess  a  conjugate 
system  of  double  bonds  if  it  exhibits  both  optical  exaltation  and  a  low 
heat  of  combustion.  It  is,  therefore,  advisable  to  determine  both 
sets  of  physical  constants  in  all  cases  where  this  is  at  all  practicable. 

In  conclusion  it  may  be  added  that  another  analogy  between  thermal 
and  optical  properties  is  to  be  observed  in  the  effect  of  the  substitution  of 
i  Annalen  der  Chemie,  373,  287  (1910). 


332  THEORIES  OF  ORGANIC  CHEMISTRY 

hydrogen  which  is  attached  to  the  central  carbon  atoms  of  a  conjugate 
system.  It  has  already  been  pointed  out  that  substitution  in  this  posi- 
tion, as  represented  in  the  combination 


P  _  p  _  p  _  p 

—  v>  —  ^j  \^>  —  v^ 

II         H    H 
CH3 

often  completely  neutralizes  the  optical  exaltation  of  the  system.     For 
example,  a-phellandrene 


CH 


exhibits  optical  disturbances  of  this  character  and  also  shows  a  corre- 
sponding decrease  in  the  anomaly  of  its  heat  of  combustion. 

In  studying  the  heats  of  combustion  of  ring  compounds  it  has  been 
found  that  three-  and  four-membered  rings  give  relatively  high  values 
when  compared  with  five-  and  six-membered  rings.  The  facts  of 
observation  are,  therefore,  in  general  agreement  with  the  assumption 
of  Baeyer's  tension  theory.  That  the  energy  relations  between  ring 
compounds  are  not  all  so  simple  as  Baeyer's  theory  would  suppose  is 
obvious  from  a  consideration  of  the  following  facts.  In  investigating 
the  heats  of  combustion  of  tetramethylene  derivatives  Stohmann  found 
that  the  respective  heats  of  these  substances  are  greater  than  those  of 
the  corresponding  trimethylene  derivatives.  These  observations  have 
recently  been  confirmed  by  Roth  and  Ostling  who,  as  a  result  of  very 
careful  investigation,  have  discovered  that  ring  systems,  exclusive  of 
the  presence  of  double  bonds,  may  be  arranged  in  the  order  of  their 
energy  content  as  follows: 

4>3>7>6>5 

Marked  asymmetry  in  the  arrangement  of  the  substituents  in  the  mole- 
cule seems  to  increase  the  energy  of  the  compound,  and  the  presence  of  a 
double  bond  between  the  atoms  constituting  the  ring  has  in  general  the 
same  effect. 

It  may  be  said  in  conclusion  that  since  the  heats  of  combustion  of 
relatively  few  substances  have  as  yet  been  determined,  it  may  be 
anticipated  that  the  further  comparison  of  thermal  and  optical  properties 
will  lead  to  results  of  the  utmost  importance  in  developing  the  structural 
theory  of  organic  chemistry. 


APPLICATION  OF  PHYSICO-CHEMICAL  PRINCIPLES  333 

The  relation  of  other  physical  constants,  such  as  magnetic  rotation  l 
and  conductivity,2  to  constitution  cannot  be  discussed  at  this  point 
since  their  detailed  consideration  would  require  more  space  than  could 
with  propriety  be  allowed  within  the  limits  of  the  present  volume. 

!See  Perkin,  Jour.  Chem.  Soc.,  55,  680  (1889);  Zeitschr.  physikal.  Chemie,  .21, 
451,  561  (1896);  27,  447  (1898). 

2Kohlrausch  and  Holborn,  "  Leitvermogen  der  Elektrolyte,"  Leipzig. 


CHAPTER  XIV 
THE  THEORETICAL  SPECULATIONS   OF   JOHN  ULRIC  NEF 

IN  1890  the  American  chemist  J.  U.  Nef  l  brought  forward  original 
views  which,  supported  as  they  were  by  experimental  data,  attacked 
the  very  root  and  fiber  of  the  theories  in  regard  to  organic  reactions 
which  were  current  at  that  time.  Nef  not  only  objected  to  the  prevail- 
ing views  in  regard  to  tautomerism,  but  even  refused  to  recognize  the 
new  progressive  achievements  in  that  field  which  dealt  with  the  stereo- 
chemistry of  nitrogen.  Above  all,  he  denied  two  fundamental  assump- 
tions which  had  been  and  still  were  invaluable  to  organic  chemists, 
namely,  the  assumption  of  the  constant  quadrivalence  of  carbon  in 
organic  combinations  and  the  assumption  of  substitution  or  metalepsis 
as  representing  the  mechanism  of  a  great  number  of  chemical  reactions. 
Nef  attacked  the  prevailing  conception  in  regard  to  metalepsis  most 
energetically  and  maintained  that  it  and  the  other  hypotheses  brought 
forward  in  connection  with  it  during  a  period  of  thirty  years  were  not 
only  illogical,  but  were  responsible  for  the  confusion  of  thought  which 
then  reigned  in  organic  chemistry.  Nef  represented  in  speech  the 
purest  type  of  a  revolutionist  and  the  question  naturally  arises  as  to 
whether  his  conclusions  were  sound  and  in  the  end  better  than  the 
conceptions  advanced  by  earlier  investigators.  In  answer  to  this 
question  it  may  be  said  that  whether  his  views  are  accepted  or  not, 
it  cannot  be  denied  that  he  was  a  conscientious  and  close  student  of  his 
science,  that  he  honestly  endeavored  to  present  sound  theory,  and 
that,  being  an  excellent  experimentalist,  he  was  able  to  support  his 
speculations  with  original  and  reliable  data.  With  these  introductory 
remarks  Nef's  speculations  will  now  be  presented  to  the  reader  in  the 
form  in  which  he  himself  has  presented  them  in  an  excellent  paper 
which  was  published  by  the  Journal  of  the  American  Chemical  Society 
in  1904,2  and  which,  through  the  courtesy  of  the  publishers,  has  been 
incorporated  in  the  present  volume.  A  critical  discussion  of  the  funda- 

1Annalen  der  Chemie,,  258,  261  (1890);  266,  52  (1891);  270,  267  (1892);  280, 
291  (1894);  287,  265  (1895);  298,  202  (1897);  309,  126  (1899);  318,  1,  137  (1901); 
335,  191,  247  (1904);  357,  214  (1907). 

2  Jour.  Am.  Chem.  Soc.,  26,  1549  (1904);  compare  also  30,  645  (1908). 

334 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF         335 

mental  conceptions  embodied  in  this  paper  as  well  as  a  consideration  of 
later  developments  from  it,  will  be  reserved  for  consideration  in  the 
next  chapter. 

The  progress  of  organic  chemistry  since  1858  is  due  chiefly  to  the 
development  of  a  few  very  simple  ideas  concerning  the  valence  of  the 
elements,  ideas  which  were  first  clearly  and  fully  presented  at  the  time 
of  Kekule. 

Hydrogen,  oxygen,  and  nitrogen  are  the  elements  which  most  fre- 
quently combine  with  carbon  to  form  the  so-called  organic  compounds. 
Since  the  compounds  of  one  atom  of  oxygen,  nitrogen,  or  carbon  with 
hydrogen  possess  the  empirical  formulas,  O=H2,  N=H3,  C=H4,  the 
conception  naturally  presents  itself  that  the  capacity  of  the  various 
elements  for  holding  hydrogen  atoms  varies.  Oxygen  is  capable  of 
holding  two  such  atoms,  nitrogen  holds  three,  and  carbon  four  atoms  of 
hydrogen.  Therefore,  we  assume,  taking  hydrogen  as  our  unit,  that  the 

i 

valence  of  the  element  oxygen  is  two,  — O — ;  of  nitrogen,  three,  — N — ; 

i  ' 

and  of  carbon,  four,  — C — .     Without  going  into  much  detail  concerning 

i 

the  nature  of  the  valence,  or  what  is  the  same  thing  concerning  the 
nature  of  the  forces  inherent  in  our  atoms,  we  assume  briefly  that 
every  atom  of  an  element  possesses  one,  two,  three,  four,  or  more  such 
units  of  force  and  we  call  the  element  univalent,  bivalent,  trivalent, 
quadrivalent,  etc.,  according  to  the  number  of  such  units  it  possesses. 
It  is  by  virtue  of  the  existence  of  these  units  of  force  that  the  compounds 
made  up  of  the  same  or  of  various  elementary  atoms  exist.  We  assume 
that  in  such  a  molecular  compound  the  atoms  are  bound  one  to  another 
in  a  definite  way  by  means  of  their  affinity  units. 

Since  the  development  of  these  ideas  concerning  the  valence  of  the 
elements  there  has  been  a  great  deal  of  work  carried  on  with  the  object 
of  determining  whether  the  valence  of  an  element  is  constant  or  whether 
it  may  vary;  the  majority  of  chemists  are  now  convinced  that  it  may 
vary. 

The  valence  of  nitrogen  may  be  three  or  five.  The  valence  of 
hydrogen,  oxygen,  and  carbon,  on  the  other  hand,  have,  until  recently, 
been  assumed  always  to  remain  constant,  i.e.,  one,  two,  and  four, 
respectively. 

Since  the  complexity,  and  the  very  great  variety  and  number  of  exist- 
ing compounds  containing  carbon  are  unquestionably  to  be  attributed 
to  the  peculiar  nature  of  the  forces  inherent  in  the  carbon  atom,  let  us 
consider  a  little  more  in  detail  what  hypotheses  we  make  in  our  present 


336  THEORIES  OF  ORGANIC  CHEMISTRY 

system  of  carbon  chemistry  concerning  this  element.  We  assume  first 
that  the  valence  of  the  carbon  atom  is  always  four;  second,  that  the 
four  valences  or  affinity  units  of  the  carbon  atom  are  equivalent;  third, 
that  they  are  distributed  in  space  in  three  dimensions  and  act  in  the 
direction  of  the  axes  of  a  tetrahedron;  fourth,  that  the  carbon  atoms 
can  unite  with  one  another  by  means  of  one,  two,  or  three  affinity 
units  to  form  what  we  usually  call  chains.  These  chains  may  be  open, 
or  closed  rings  or  cycles.  The  number  of  carbon  atoms  thus  bound  to 
one  another  may  be  exceedingly  large.  The  closed  chains  usually  con- 
tain three,  four,  five,  six,  or  seven  carbon  atoms  in  the  ring.  We  may 
have  in  these  chains,  whether  open  or  closed,  some  of  the  carbon  atoms 
replaced  by  oxygen,  nitrogen,  sulphur,  or  other  elements.  If  now  we 
unite  the  extra  valences  of  each  carbon  or  other  atom — i.e.,  those 
affinity  units  which  are  not  necessary  for  binding  the  atoms  together  in 
chains — with  other  atoms  or  radicals,  it  is  at  once  evident  that  we  can 
represent,  theoretically,  by  so-called  graphical  formulas,  molecules  of 
great  complexity. 

It  is  also  at  once  obvious  that  with  a  small  number  of  atoms  it 
musfc  be  possible  to  construct  a  relatively  large  number  of  aggregates 
which  differ  from  one  another  simply  in  the  way  the  atoms  are  bound 
together.  In  1884,  for  instance,  fifty-five  totally  different  substances 
of  the  empirical  formula  CgHioOs  were  actually  known.  We  call  them 
isomers. 

One  of  the  chief  problems  of  organic  chemistry,  since  1858,  has  been 
to  determine  on  the  basis  of  these  ideas  of  valence  the  "  constitution  " 
of  the  carbon  compounds.  We  determine  by  methods  which  are  called 
synthetic,  as  well  as  by  an  exhaustive  study  of  the  reactions  of  a  given 
compound,  what  may  be  called  the  "  architecture  "  of  its  molecule, 
i.e.,  we  determine  how  the  various  atoms  of  carbon,  nitrogen,  oxygen, 
hydrogen,  etc.,  of  which  the  substance  may  be  composed  are  joined 
together  by  virtue  of  of  their  affinity  units.  How  much  has  been 
accomplished  on  the  basis  of  these  ideas  during  the  past  forty-six  years 
and  how  beautifully  and  simply  all  the  facts  known  with  regard  to  the 
almost  countless  carbon  compounds  are  thus  explained,  only  those  can 
fully  appreciate  who  have  a  detailed  knowledge  of  the  subject.  Not- 
withstanding the  large  number  of  workers  in  the  field,  it  has  often 
required  more  than  a  decade  of  work  to  determine  the  molecular  archi- 
tecture of  one  single  carbon  compound,  and  the  question  at  times  seri- 
ously presents  itself  whether  we  must  not  reach  our  limitations  in  this 
respect. 

In  any  case,  one  point  is  deserving  of  especial  emphasis:  this  idea 
of  structure  which  has  been  applied  chiefly  to  molecules  containing  the 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF         337 

element  carbon  attributes  to  them  a  rigidity  which  is  improbable  from 
a  purely  dynamic  standpoint. 

The  present  system  of  organic  chemistry  is  thus  founded  upon  the 
assumption  that  the  valence  of  all  the  atoms  of  carbon,  wherever  found, 
remains  invariably  four.  In  the  earlier  part  of  the  last  century  many 
attempts  were  made  to  isolate  the  hydrocarbon  methylene,  CH2, 
which  must  contain  bivalent  carbon.  Dumas  and  Peligot  tried  to 
obtain  this  substance  from  methyl  alcohol,  H-CH2-OH,  by  loss  of 
water.  Perrot  tried  to  isolate  it  from  methyl  chloride,  H  •  CH2  •  Cl,  by 
dissociation  into  methylene  and  hydrogen  chloride  at  a  high  tempera- 
ture. Berthelot,  Butlerow,  Wurtz,  and  Kolbe  also  made  many  fruitless 
attempts  in  this  direction.  As  a  final  result  of  these  repeated  and 
negative  efforts  chemists  finally  became  convinced  that  compounds 
containing  bivalent  carbon  could  not  be  isolated  and  the  conclusion, 
therefore,  that  carbon  was  one  of  the  few  elements  possessing  a  constant 
valence  became  very  general. 

There  has,  however,  long  existed  one  very  simple  compound  of 
carbon  which  does  not  adjust  itself  to  this  system — namely,  the  inactive 
and  poisonous  carbon  monoxide.  If  we  assume  the  valence  of  oxygen 
as  two,  then  we  have  here  simply  a  derivative  of  methylene  in  which 
the  two  hydrogen  atoms  are  substituted  by  oxygen,  C=0.  To  be  sure, 
there  were  many  chemists  who  preferred  to  consider  the  valence  of  car- 
bon in  carbon  monoxide  as  four,  thus  making  the  valence  of  oxygen 
four,  C=0,  and  when  we  bear  in  mind  that  the  other  members  of  the 
oxygen  group,  sulphur,  selenium,  and  tellurium,  exist  as  di-,  tetra-, 
and  hexavalent  atoms  there  is  some  justification  for  this  interpretation. 
To  me,  personally,  however,  it  seems  in  the  highest  degree  improbable 
that  two  atoms  should  be  thus  bound  to  each  other  by  four  affinity 
units. 

About  fourteen  years  ago  a  series  of  systematic  experiments  was 
undertaken  with  the  object  of  ascertaining  whether  carbon  can  exist  in 
a  bivalent  condition.  The  experiments  have  established  this  point  in  a 
most  decisive  manner;  we  have  now  quite  an  array  of  substances  which 
contain  bivalent  carbon. 

Furthermore,  it  has  been  possible  to  prove,  from  the  experience 
gained  in  their  study,  that  methylene  chemistry  plays  an  important  role 
in  many  of  the  simplest  reactions  of  organic  chemistry,  reactions  which 
have  hitherto  been  explained  on  the  basis  of  substitution. 

At  the  time  when  these  experiments  were  undertaken  there  existed, 
besides  carbon  monoxide,  several  substances  which  might  contain 
bivalent  carbon — namely,  prussic  acid  and  its  salts,  the  cyanides, 
HN=C  and  MN=C.  Also  the  so-called  carbylamines,  RN=C, 


338  THEORIES  OF  ORGANIC  CHEMISTRY 

discovered  in  1866  by  Gautier.  These  substances  were,  therefore, 
exhaustively  studied  in  order  to  establish  rigidity,  by  experiment, 
whether  bivalent  carbon  was  present  or  absent.  The  presence  of  dyad 
carbon  in  these  compounds  having  been  established  and  its  properties 
thus  being  known,  the  problem  then  presenting  itself  was  the  isolation 
of  methylene  and  its  homologues. 

You  are  probably  all  aware  that  Gay-Lussac  established,  in  1815, 
the  existence  of  a  radical,  composed  of  one  atom  of  carbon  and  one  of 
nitrogen,  in  prussic  acid  and  the  cyanides.  This  radical,  cyanogen, 
plays  in  its  compounds  a  role  similar  to  that  of  the  elements  of  the 
halogen  group.  In  1832  Pelouse  discovered  the  alkylcyanides, 
RC=N,  by  treating  potassium  cyanide  with  alkyliodides  or  with 
alkyl  potassium  sulphates; 

KCN+RI  or  ROS02OK  ->  R-C=N+KI  or  KOS02OK, 

an  apparent  double  decomposition  reaction  by  which  we  obtain  a 
compound  m  which  the  radical  R(C»H2n+i)  is  joined  to  the  cyanogen 
group  by  means  of  carbon.  The  alkylcyanides  thus  obtained  are  neu- 
tral, pleasant  smelling,  harmless  liquids  resembling  ether,  chloroform, 
and  the  alkylhaloids,  RC1,  RBr,  and  RI. 

In  1866  Gautier  discovered  a  new  class  of  organic  compounds  by 
treating  cyanide  of  silver  with  alkyliodide, 

RI+Ag(NC)  ->  AgI+RN=C, 

a  reaction  which  is  apparently  a  double  decomposition  reaction.  They 
are  isomeric,  not  identical,  with  the  alkylcyanides  of  Pelouse.  He 
called  them  the  carbylamines  or  isonitriles  and  proved  that  the  alkyl 
group  is  bound  to  the  cyanogen  radical  by  means  of  nitrogen,  RN=C 
or  RN=iC. 

It  thus  became  evident  that  we  must  distinguish  between  two 
cyanogen  radicals — namely,  one  which  in  its  compounds,  is  bound 
to  alkyl-hydrogen  or  metal  by  means  of  carbon,  RC=N,  HC=N, 
MC=N,  and  another  which  is  joined  to  these  elements,  or  groups 
by  means  of  nitrogen,  RN=C,  HN=C,  MN=C.  We  may  call  the 
former  radical  cyanogen,  — C=N,  and  the  latter  isocyanogen,  — N=C 
or  — N=C ;  these  radicals  may,  obviously,  combine  with  each  other  to 
form  three  isomers  of  the  empirical  formula,  C2N2.  The  substances 
discovered  by  Gautier,  the  alkylisocyanides,  RN=C  or  RN=C,  have 
properties  strikingly  different  from  those  of  their  isomers — the  alkyl- 
cyanides, RC=N,  of  Pelouse. 

They  are  poisonous,  nauseating  compounds  which  affect  the  throat 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF          339 

like  prussic  acid  and  color  the  blood  intensely  red;  they  produce  violent 
headaches  and  vomiting.  Their  odor  is  most  pronounced  and  persistent. 
Hofmann,  who,  in  1868,  discovered  another  method  for  making  them 
from  primary  amines,  chloroform,  and  caustic  potash, 

RNH2+3KOH+CHC13  -*  RN=C+3KC1+3H2O, 

found  it  impossible  to  work  with  them  except  for  very  short  periods. 

An  exhaustive  study  of  the  reactions  of  these  alkyl  isocyanides,  car- 
ried out  in  1891-2,  led  to  the  definite  conclusion  that  they  contain  a 
dyad  carbon  atom — i.e.,  they  possess  the  constitution  represented  by  the 
formula  RN=C;  the  other  possible  formula  with  quadrivalent  carbon 
and  quinquivalent  nitrogen,  RN=C,  is  excluded  by  the  facts. 

The  alkylisocyanides  belong  to  the  vast  category  of  unsaturated 
compounds  whose  chemistry  will  be  briefly  discussed  from  a  perfectly 
general  standpoint  below;  they  manifest  their  great  chemical  activity 
especially  by  absorbing  other  substances,  forming  new  molecules  in 
which  the  valence  of  carbon  has  changed  from  two  to  four.  Such 
reactions  we  call  additive.  Two  molecules  simply  unite  to  form  one 
new  molecule — the  addition  product. 

A  molecule  containing  an  unsaturated  carbon  atom — i.e.,  one  with 
two  of  its  valences  latent  or  polarized,  RN=C,  cannot,  per  se,  show 
any  chemical  activity  whatever. 

This  is  also  true  of  a  system  containing  a  pair  of  doubly  or  triply 
bound  carbon  atoms,  ethylene,  CH2=CH2,  and  acetylene,  HC=CH; 
and  finally  of  a  saturated  system  which  we  may  represent  by  a  paraffine, 
CwH2«+2,  for  instance,  marsh  gas,  CEU. 

All  these  substances  manifest  chemical  activity  simply  because  they 
are,  to  a  greater  or  less  degree,  in  a  dissociated,  or  what  may  be  called 
an  active  condition. 

A  given  quantity  of  alkyl  isocyanide  contains  an  extremely  small 
per  cent  of  molecules  with  two  free  affinity  units,  RN=C  < ;  these  are 
in  dynamic  equilibrium  with  the  absolutely  inert  molecules,  RN=C. 
That  this  percentage  varies  with  the  nature  and  mass  of  R  is  shown 
by  the  fact  that  various  alkylated  and  arylated  isocyanides  manifest 
different  degrees  of  chemical  activity.  Carbon  monoxide  possesses 
relatively  a  smaller  number  of  such  active  particles,  O=C<,  and 
consequently  is  a  comparatively  inert  substance,  since  the  speed  of 
addition  reactions  shown  by  unsaturated  compounds  must  naturally 
be  directly  in  proportion  to  the  per  cent  of  active  molecules  present. 
A  similar  conception  obviously  explains  the  relative  differences  in 
reactivity  shown  by  the  various  members  of  the  olefine  and  acetylene 


340  THEORIES  OF  ORGANIC  CHEMISTRY 

series.     Marsh  gas,  a  saturated  system,  reacts  with  other  substances 
because  it  is  partially  dissociated  as  follows: 

CH4<=*CH3 H    and    CH^-^H 

From  this  point  of  view  chemical  action  depends  entirely  upon 
dissociation  processes.  The  reactions  often  proceed  with  very  great 
slowness  because  the  percentage  of  dissociation  is  extremely  low,  possibly 
0.1  to  0.001  per  cent,  or  even  less. 

Turning  now  to  a  consideration  of  the  reactions  of  alkyl  isocyanides, 
the  substances  which  are  absorbed  by  the  unsaturated  carbon  atom 
present  in  the  isonitriles  are  the  following : 

I.  "  Halogens  "  (chlorine,  bromine,  iodine;  speed  of  reaction  in  the 
order  named), 

X  X 

RN=C<+X=X    ->    RN=C/||     ->    RN=€<"' 
||  \X  \X 

The  reactions,  expecially  those  with  chlorine  and  bromine,  take  place 
with  great  evolution  of  heat  at  —20°. 

II.  "Acid  Chlorides,"  such  as  RCOC1,  C1OC2H5,  C1COC1,   C1CN, 
C1COOR  form  the  addition  products. 

/Cl 

JCl       RN=C< 

RN=C<  >C  :  O,  etc. 

XCOR,  RN=C< 

Na 

A  hyphen  denotes  the  point  where  the  compounds  are  partially  dis- 
sociated and  consequently  absorbed.  These  reactions,  especially  those 
with  phosgene  and  ethyl  hypochlorite,  take  place  with  great  violence 
at  -20°. 

III.  "  Oxygen  and  Sulphur,"  form  isocyanates  and  mustard  oils, 
RN=C=O  and  RN=C=S.     Methylisocyanide  unites  directly  at  its 
boiling-point,  58°,  with  the  oxygen  of  the  air.     The  dry  oxides  of  silver 
and  mercury  are  reduced  to  metals  with  violence  at  40°,  alkyl  isocyanate 
being  first  formed.     This  shows  the  great  affinity  of  bivalent  carbon  for 
oxygen. 

IV.  "  Primary  Amines  and  Hydroxylamine," 

/H 
RN=C<+H— NHR    or    H— NHOH    -»     RN  :  C< 

XNHR 

M 

or  RN=C< 

\NHOH 
give  amidines  and  oxyamidines. 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF          341 

V.  "  Alcohols,"  in  the  presence  of  an  alkali,  are  absorbed,  giving 
imido  ethers, 

/H 

RN=C< 

X)R 

VI.  h  Hydrogen,    Sulphide  and  Mercaptans,"  give  readily  at  100° 
the  addition  products 

/H  7K 

RNH—  C/     and    RN=C 


VII.  "  Acids."     Aqueous  mineral  acids  act  with  great  violence  on 
the  isonitriles,  giving  primary  amines  and  formic  acid, 

RN=C<+2H20    ->    RNH2+H-COOH 

In  the  absence  of  water  and  on  diluting  the  alkylisocyanides  with 
absolute  ether,  perfectly  dry  halogen  hydride  causes  the  separation  of 
white  hygroscopic  salt-like  substances  of  the  empirical  formula 
2RNC,  3HX(X=C1,  Br,  or  I).  For  this  reason  Gautier,  as  weU  as 
Hofmann,  supposed  the  isonitriles  to  be  basic  compounds  —  i.e.,  sub- 
stances behaving  like  ammonia  —  hence,  the  name  carbylamine  was 
given  them  by  Gautier.  Further  study  has  shown,  however,  that  this 
conclusion  was  erroneous.  The  isonitriles  are  entirely  devoid  of  basic 
properties;  the  great  violence  with  which  they  react  with  halogen 
hydrides  is  due  to  the  presence  of  unsaturated  carbon.  The  reaction 
probably  takes  place  as  follows  : 

M 

I.  RN=C<+H—  X    -»    RN  =  C< 

\X 

H  X 

II.  RN=C<+X—  C:NR    -»    RN  :  C—  CH  :  NR 

X 

HI.     RN=C—  CH=NR+2H—  X    ->    RNH—  C—  CHX—  NHR 

II 
X2 

Reversibilty    of    the    Reactions.  —  The  most  striking  property  cf 

/X 
these  addition  products  of  the  isonitriles,  RN  :  C\     ,  is  their  low  point 

XY 

of  dissociation  —  i.e.,  the  carbon  atom  which  has  absorbed  the  X  —  Y 
thus  becoming  quadrivalent  is  unable  to  hold  X  —  Y  above  certain 


342  THEORIES  OF  ORGANIC  CHEMISTRY 

temperature  limits.  There  is  consequently  in  every  case  a  temperature 
varying  with  the  nature  and  mass  of  X  and  Y,  as  well  as  with  the 
nature  and  mass  of  the  groups  bound  to  the  other  two  affinity  units  of 
carbon,  at  which  the  carbon  atom  becomes  spontaneously  dyad  and  is 
unable  to  remain  in  a  quadrivalent  condition;  it  was  subsequently 
possible  to  prove  that  this  is  a  perfectly  general  property  of  this  atom. 
All  the  addition  products  under  discussion  are  partially  dissociated, 

/^ 

RN  :  C<         <=>    RN=C  <  +X-Y 
\Y 

the  dissociation  increasing  as  the  temperature  is  raised  —  in  other  words, 
the  valence  of  carbon  at  temperatures  below  the  dissociation  point  is  an 
equilibrium  phenomenon;  dynamic  equilibrium  exists  between  bivalent 
and  quadrivalent  carbon. 

The  point  of  complete  dissociation  of  the  various  addition  products 
of  the  isonitriles  has  not  yet  been  accurately  determined  in  every  case. 
The  following  data  with  reference  to  the  dissociation  points  of  carbon 
monoxide  addition  products  are  of  interest  and  therefore  used  for 
illustration  in  this  connection: 

Dissociation  Point 

Formaldehyde,  O  :  C<H2  600° 

/H 

Formamide,  O=C<  250°  (about) 

XNH2 

M 

Formic  acid,  0=C<  169° 

X)H 

/H 

Formhydroxamic  acid,     0=C<(  85° 

XNHOH 

H 

Formylchloride,  O  :  C<(  -20°  (below) 


Since  these  substances  containing  quadrivalent  carbon  decompose 
spontaneously  into  carbon  monoxide,  i.e.,  cannot  exist  in  the  quadriva- 
lent condition  at  temperatures  above  those  indicated,  it  is  self-evident 
that  at  lower  temperatures  the  addition  products  must  be  partially 
dissociated  and  that  in  the  future  we  must  be  able  to  determine,  in  each 
case  with  absolute  accuracy,  the  percentage  of  dissociation  at  any  tem- 
perature. A  striking  experiment  with  formhydroxamic  acid,  with  a 
dissociation  point  of  85°,  proves  the  correctness  of  this  conclusion; 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF          343 


on  allowing  this  crystalline  substance  to  stand  at  20°  in  acetone  solution 
the  following  reaction  takes  place  quantitatively : 

HONH. 

\C=0         <=±        H— NHOH  +  ::::  CO     O~ C    I    (CH3)2 


\C=NOH+H— OH+   ,CO    - 
CH«/ 

H   v 

(CH3)2  :  C=NOH  +         >C  :  O 


>C  : 
HO/ 


In  a  similar  manner  we  can  prove  that  the  isonitrile  addition  products, 
many  of  which  have  definite  boiling-points  and  are  quite  stable,  are 
partially  dissociated  at  ordinary  temperatures.  Thus  the  addition 

/x 

products  with  halogens,  RN=C\     ,  are  all  converted  back  quanti- 

XX 

tatively  into  the  alkylisocyanides  by  treatment  with  finely  divided 
metals,  zinc  dust,  or  sodium,  which  simply  abstract  the  free  halogen. 

Many  of  the  acylhalide  addition  products  dissociate  spontaneously 
into  the  components  on  distillation;  these  phenomena  are  perfectly 
analogous  to  the  dissociation  of  dry  ammonium  chloride, 


c 


NH3+HC1 


For  this  reason  the  majority  of  the  addition  products  of  the  isoni- 
triles  can  be  kept  only  for  a  short  time;  this  property  rendered  futile 
many  attempts  to  isolate  definite  addition  products.  The  continual 
dissociation  of  such  products  sets  free  active  or  dissociated  alkylisccy- 
anide  particles,  and  these  slowly  condense  with  one  another, 

RN=C<     -»     (RN=C)x 

giving  rise  to  the  so-called  alkylisocyanide  resins  (non-reversible)  or 
products  whose  molecular  weight  has  not  yet  been  determined  and 
which  are  perfectly  analogous  to  azulmic  or  polymerized  prussic  acid. 
Consequently,  in  carrying  out  an  addition  reaction  with  an  isonitrile, 
especially  if  it  requires  much  time  or  a  temperature  above  20°,  large 
quantities  of  these  resinous  polymers  are  formed  from  which  it  is  pos- 
sible to  isolate  the  addition  product  only  with  great  difficult}'.  Many 
of  the  isonitriles  themselves,  even  when  perfectly  pure,  undergo  rapid 
polymerization  to  resins  so  that  they  can  be  kept  only  for  a  very  short 
time.  Phenylisocyanide,  C6HsN=C,  is  the  most  striking  instance,  as 


344  THEORIES  OF  ORGANIC  CHEMISTRY 

it  changes  in  a  few  minutes  from  a  colorless  to  a  dark-blue  liquid  and 
in  a  few  days  condenses  to  a  dark-brown  resin. 

Have  we  not  here  a  possible  explanation  of  the  fact  that  it  is  impos- 
sible to  isolate  methylene  and  a  large  number  of  its  derivatives,  although 
marsh  gas,  methyl  alcohol,  and  chloride  of  methyl, 

/K  /OH  /H 

HaC^     ,       H2C<^      ,       H2C<^ 

XI  XI  \j\. 


each  contain  a  relatively  small  percentage  of  active  methylene  particles 
at  ordinary  temperatures? 

The  presence  of  bivalent  carbon  in  the  alkylisocyanides  having  been 
established,  the  next  question  presenting  itself  was  whether  prussic 
acid  and  its  salts  contain  the  cyanogen  or  the  isocyanogen  radical.  In 
the  latter  case,  HN  :  C,  MN  :  C,  these  substances  must  be  analogous 
to  Gautier's  isonitriles.  It  had  hitherto  been  considered  as  established, 
but  without  sufficient  evidence,  that  prussic  acid  and  the  cyanides  were 
cyanogen  compounds  analogous  to  the  nitriles  of  Pelouse. 

When  one  considers  the  physical  and  physiological  properties  of 
prussic  acid  (boiling-point,  25°;  sp.  gr.,  0.70;  a  violent  poison)  and 
contrasts  these  with  the  corresponding  properties  of  methylcyanide 
(boiling-point,  81°;  sp.  gr.,  0.81;  sweet-smelling,  harmless  oil)  and  of 
methylisocyanide  (boiling-point,  58°;  sp.  gr.,  1.75;  a  poison)  one  at 
once  comes  to  the  conclusion  that  prussic  acid,  as  well  as  its  salts,  must 
belong  to  the  isocyanogen  compounds  and  consequently  must  contain 
bivalent  carbon. 

An  exhaustive  study  of  prussic  acid  and  the  cyanides  establishes 
this  sharply,  especially  in  the  case  of  the  salts,  from  a  chemical  stand- 
point. The  relation  of  fulminic  acid  to  prussic  acid  corroborated  the 
evidence. 

You  are  all  familiar  with  fulminate  of  mercury — a  substance  which 
is  made  on  a  commercial  scale  and  used  for  explosives.  It  was  dis- 
covered in  1800  by  Howard,  and  analyzed  in  1824  by  Liebig  in  Gay- 
Lussac's  laboratory.  We  obtain  it  by  dissolving  mercury  in  con- 
centrated nitric  acid  and  adding  the  resulting  solution  to  ordinary  alco- 
hol. It  has  the  empirical  formula  HgC2N2O2,  and,  being  obtained  from 
ethyl  alcohol,  CHsCH2OH,  fulminic  acid  was  supposed  to  have  two 
carbon  atoms  in  its  molecule,  H2C2N202.  The  constitution  of  this 
substance  was,  for  a  long  time,  a  great  puzzle  to  chemists.  That  we 
have  here  a  substance  very  closely  related  to  prussic  acid  was  dis- 
covered by  accident.  In  working  with  the  mercury  salt  of  isonitro- 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF          345 

methane  it  was  found  that  this  compound  is  spontaneously  converted 
at  0°  into  fulminate  of  mercury,  according  to  the  equation, 

H  v 

H2C=NOhg    ->  >C==NOhg    ->    H20+C=NOhg 

HCK 


This  synthesis  led  directly  to  the  conclusion  that  fulminate  of 
mercury  possesses  a  constitution  entirely  analogous  to  cyanide  of 
mercury,  C=Nhg,  i.e.,  that  it  contains  the  isocyanogen  radical  with 
bivalent  carbon.  A  further  study  of  the  fulminate  established  this 
point  with  precision.  Especially  striking  is  the  behavior  of  fulminates 
towards  dilute  acids.  Liebig  and  Gay-Lussac  stated,  in  1824,  judging 
from  the  odor,  that  fulminate  of  silver  gives  prussic  acid  with  dilute 
hydrochloric  acid.  A  more  careful  study  of  this  reaction,  in  1894, 
proved  that  not  a  trace  of  prussic  acid,  but  a  substance,  formyl  chloride 

H\ 

oxime,         /C=NOH,  is  formed,  which  possesses  the  following  remark- 

CK 

able  properties:  long  needles,  clear  as  glass,  which  decompose  and 
explode  with  violence  at  20°;  extremely  volatile  even  at  0°  and 
having  an  odor  similar  to  prussic  acid,  which  is  obviously  due  to  a 
partial  dissociation  into  fulminic  acid.  Aqueous  silver  nitrate  converts 
it  quantitatively  into  chloride  and  fulminate  of  silver, 

H\ 

\C=NOH+2AgNO3    -»    AgON  :  C+AgCl+2HN03 

CK 

Up  to  1897  the  presence  of  bivalent  carbon  had  been  established  in 
the  following  compounds:  1,  carbon  monoxide,  C  :  0;  2,  the  alkyl 
and  aryl  isocyanides,  RN  :  C;  3,  prussic  acid  and  the  cyanides, 
NH  :  C,  MN=C;  4,  fulminic  acid  and  the  fulminates,  (HO)N  :  C, 
MON=C.  2,  3,  and  4  are  all  compounds  containing  the  isocyan- 
ogen radical.  In  1897  the  presence  of  bivalent  carbon  was  established 
in  a  series  of  nitrogen-free  carbon  compounds  obtained  from  acetylene. 
They  are  the  mono-  and  di-halogen  substituted  acetylidenes, 

X  X 

Nc=C    and       "\C==C(X==C1,  Br  or  I) 
H/  X/ 

The  corresponding  members  of  the  acetylene  series,  XC=CH  and 
XC=CX,  do  not  exist,  although  we  have  substances  like 


346  THEORIES  OF  ORGANIC  CHEMISTRY 


^C  —  X,  whose  properties  are  in  marked  contrast  to  those  of  the 
acetylidene  derivatives. 

Diiodacetylidene,  which  possesses  an  odor  deceptively  like  that  of 
the  isonitriles,  dissociates  at  100°  with  violence  into  iodine  and  diatomic 
carbon, 

I2C=C  ->  I2+C=C 

the  latter  cannot  be  isolated  as  such,  but  polymerizes  explosively  to 
graphite  and  amorphous  carbon.  The  mono-  and  dihalogen  substituted 
acetylidenes  are  all  poisonous  and  spontaneously  combustible  com- 
pounds, possessing,  therefore,  like  methylisocyanide,  a  marked  affinity 
for  oxygen. 

Up  to  the  present  time  it  has  not  been  possible  to  isolate  compounds 
containing  bivalent  carbon,  other  than  those  mentioned  above.  We 
are,  however,  now  in  a  position  to  explain  clearly  why  we  cannct  hope, 
by  methods  now  known,  to  isolate  methylene  and  its  homologues  as 
such,  although  these  substances  play  a  great  role  in  many  of  the  fun- 
damental reactions  of  organic  chemistry. 

In  order  to  approach  this  point  more  intelligently  let  us  first  consider 
the  properties  of  unsaturated  compounds  in  general,  their  possibility 
of  existence,  etc. 


II.  On  the  Unsaturated  Compounds 

The  unsaturated  compounds  may,  first  of  all,  be  divided  into  three 
categories,  namely:  I.  Those  in  which  two  atoms,  which  may  be  the 
same  or  different,  are  bound  doubly  or  triply  to  each  other  by  two  or 
three  affinity  units,  such  as  olefines;  acetylenes;  chlorine,  CfeCl; 

R\ 
oxygen,  O=0;   aldehydes,      V)  :  O;  alkylcyanides,  R— C=N;  nitric 

/&  ^O 

acid,  HON:f     ;   sulphur  trioxide,  0  =  S^   ,    etc.     II.  Those  in  which 

an  atom  itself  is  unsaturated — i.e.,  does  not  exert  its  maximum  valence 
capacity,  as,  for  instance,  amines,  R3=N;  thioethers,  R2=S;  methylene 
derivatives,  etc.  We  must  assume  that  the  remaining  affinity  units 
are  latent,  or,  what  is  far  more  probable,  especially  where  two  or  four 
affinity  units  are  available,  that  they  mutually  polarize  each  other  in  a 
manner  entirely  similar  to  unsaturated  compounds  containing  doubly 
or  triply  linked  atoms. 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF          347 

Finally  we  have  a  third  class  of  unsaturated  compounds:  III.  Those 

CH2 

containing  closed  atomic  chains,  such  as  trimethylene,  CH2 CH2; 

0 

A 

propylenoxide,  CH3CH CH2,  etc.,  which  show  apparently  a  sat- 
urated molecular  system  like  the  paraffines,  and  yet  react  in  a  manner 
perfectly  analogous  to  olefines  and  methylene  derivatives. 

Fundamentally  considered  these  three  classes  of  unsaturated  com- 
pounds manifest  their  chemical  activity  in  the  same  way;  they  absorb  a 
great  variety  of  other  molecules  and  thus  form  combinations,  called 
addition  products.  How  does  this  union  take  place?  An  unsaturated 
compound  with  its  affinities  polarized  represents,  in  reality,  a  saturated 
system;  it  cannot,  per  se,  show  chemical  activity.  This  is  also  true  of 
molecular  systems  in  which  the  atoms  are  bound  to  one  another  by 
single  affinity  units.  The  sole  basis  of  reactivity  in  either  case  is  the 
presence  of  a  relatively  greater  or  smaller  number  of  dissociated  particles. 
The  reactivity  of  any  unsaturated,  as  well  as  of  a  saturated  compound, 
must,  in  fact,  be  directly  proportional  to  the  ratio  of  such  active  particles 
present.  If  that  ratio  is  very  small,  the  substance  may  be  entirely 
inert;  if  it  is  greater,  absorption  of  reagents  proceeds  with  regularly 
increasing  speed. 

Experience  has  shown,  furthermore,  that  many  unsaturated  com- 
pounds cannot  be  isolated,  but  polymerize  spontaneously.  It  is  clear 
that  when  the  percentage  of  active  particles  present  in  an  unsaturated 
compound  becomes  relatively  great  the  possibility  of  their  uniting 
with  each  other  to  form  condensed  molecules  increases — in  fact,  we  may 
imagine  a  condition  in  which  the  active  molecules  simply  cannot  be 
prevented  from  combining  with  each  other.  This  shows  us  why  we 
cannot  isolate  and  keep  substances  like  formaldehyde,  H2C=O,  or 
alkylcyanates,  ROC=N,  in  the  monomolecular  form.  Similarly  in 
many  cases  where  attempts  were  made  to  isolate  methylene  derivatives, 
like  mono-  and  di-phenylmethylene,  benzoyl,  and  acetylmethylene,  and 


v 

cyanmethvlenecarboxylate,  >C,  a  spontaneous  polymerization  to 

COOR/ 
the  di-  or  tri-molecular  systems, 


348  THEORIES  OF  ORGANIC  CHEMISTRY 

took  place.  One  further  point  with  reference  to  unsaturated  com- 
pounds must  now  be  presented. 

"  Intramolecular  Rearrangement  Shown  by  Unsaturated  Systems  ": 
From  the  discussion  presented  above  it  is  obvious  that  trimethylene 
and  propylenoxide,  belonging  to  class  III,  must  contain  a  small  per- 
centage of  active  particles;  the  dissociation  of  the  triatomic  ring  in 
the  former  case  can  lead  to  only  one  form  of  active  molecule — namely, 
— CH2 — CH2 — CH2 — ;  whereas  propylenoxide  may  give  the  following 
three  active  molecules : 

O- 

CH3CH— CH2— ;    CH3CH— CH2— O— 

(a)  (6) 

and  CH3CH— 0— CH2— 

I         (c) 

Since  propylenoxide  absorbs  dry  ammonia  or  hydrogen  chloride,  as 
was  proved  by  especially  careful  and  exhaustive  experiments,  giving 
addition  products  of  the  general  formula: 

CH3CHOH— CH2X(X=C1  or  NH2) 

the  only  possible  conclusion  that  can  be  reached  is  that  propylenoxide 
contains  relatively  more  active  (a)  than  active  (b)  or  (c)  molecules; 
consequently  the  absorption  reactions  proceed  by  preference  in  only 
one  of  three  theoretically  possible  directions. 

When  trimethylene  or  propylenoxide  is  heated  or  placed  in  contact 
with  various  catalytic  agents,  the  percentage  of  active  particles  must 
naturally  increase  and  when  a  definite  limit  has  been  reached  a  spon- 
taneous transformation  of  trimethylene  into  propylene  and  of  propylen- 
oxide into  propionaldehyde  (f)  and  acetone  (J)  takes  place;  both 
reactions  are  non-reversible. 

These  results  can  be  explained  only  in  the  following  manner:  aside 
from  the  increase  in  active  particles,  dissociation  in  other  parts  of  the 
molecule  and  especially  of  hydrogen  from  carbon  must  also  take  place. 
Consequently  the  following  intramolecular  addition  reactions  finally 
occur  spontaneously: 

CH2— CH— CH2    -*    CH3CH— CH2    <=>    CH3CH=CH2 

1  A  ' 

Active  trimethylene  particles.  Propylene 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF         349 


/•»•• 


H  9 


CH3-C— CH2    -*    CH3C-CH3    ±+    CH3-C-CH3(J) 
O—  O— 

Active  propylenoxide,  particles  (a)  Acetone 

CH3CH— CH— O »  CH3CH2— CH— 0—  ±?  CH3CH2CH=O(f) 

H 

Active  propylenoxide,  particles  (&)  Propionaldehyde 

It  is  interesting  to  note  that  the  active  (6)  propylenoxide  molecules 
which  are  present  in  smaller  ratio  suffer  rearrangement  more  readily 
than  the  active  (a)  molecules.  The  active  (c)  molecules,  on  the  other 
hand,  must  be  present  in  far  smaller  amount  and  certainly  no  trans- 
formation of  propylenoxide  to  vinylmethyloxide,  CH2=CH — OCH3, 
takes  place. 

It  is  important  to  realize  that  propylenoxide,  acetone,  and  propional- 
dehyde  are  isomers,  but  do  not  stand  in  a  tautomeric  relation  to  one 
another.  This  is  also  true  of  trimethylene  and  propylene,  as  well  as  of 
a-  and  /3-amylene  and  isoamylene,  etc. 

Similarly  it  can  be  rigidly  shown  by  experiment  that  a-  and 
j8-propylidene,  CH3CH2CH=  and  (CH2)2C=,  which  are  spontaneously 
combustible  substances  not  capable  of  isolation  as  such,  transform 
themselves  by  intramolecular  addition, 

CH3CH— CH=  ->  CHa-CH— CH2  <=>  CH3CH=CH2 


CH3— C— CH2— H  -»  CH3-CH— CH2  «=*  CH3CH=CH2 

l\  I          I 

into  propylene  (non-reversible). 

There  is  not  the  slightest  doubt  that  such  intramolecular  addition 
reactions  are  the  basis  of  the  majority  of  our  synthetic  methods  for 
making  cyclic  compounds.  The  cycloparaffines  in  Russian  petroleum 
are  probably  formed  from  ordinary  paraffines  by  dissociation  into 
hydrogen  and  methylene  derivatives  and  the  latter  then  spontaneously 
transform  themselves,  by  intramolecular  addition,  into  penta-  and 
hexa-methylene  rings. 

"  On  the  Reactions  of  Paraffines  and  Benzene  Derivatives  ": 
The  reactions  of  paraffines  and  benzene  derivatives  towards  halo- 
gens,  nitric  and  sulphuric  acids,   whereby  substitution  products  arc 
formed  are  still  interpreted  in  the  text-books  from  the  standpoint  of 


350  THEORIES  OF  ORGANIC  CHEMISTRY 

metalepsis  or  substitution,  although  a  vast  amount  of  evidence  has 
accumulated  which  makes  this  axiomatic  assumption  improbable. 

The  fact  that  ethane  and  benzene,  for  instance,  decompose  into 
hydrogen  and  into  ethylene  and  diphenyl  at  800°  and  600°,  respectively, 
proves  that  an  extremely  small  percentage  of  these  molecules  must 
exist  at  ordinary  temperatures  in  an  active  or  dissociated  condition, 

CH3CH3     <=»    CH3CH2— +H- 
and 

CH3CH3  <=>  C2H4+2H—     or    C6H6  «=*  C6H5 hH— 

The  same  is  true  of  ammonia, 
H3N  <=±  — NH2+H—     and     2H h=NH     and     =N+3H- 

and  of  a  great  variety  of  other  non-ionizable  hydrogen  compounds. 
Consequently,  when  chlorine  or  nitric  acid  reacts  with  benzene  or 
ethane  to  give  the  monochlor  or  mononitro  substitution  products  we 
have  these  reagents  in  the  active  molecular  condition,  simply  uniting 
by  addition  with  the  ethane  or  benzene  particles, 

C1=C1+H— C2H5    -»    C1=C1 


or 

0  O 


HO— N— O+H— C6H5    ->    HO— N— OH 


C6H. 


the  resulting  addition  products  then  lose  hydrogen  chloride  and  water, 
respectively,  and  thus  give  the  monochlor  or  nitro  substitution  product 
of  the  mother  substance.  From  this  point  of  view  all  so-called  substitu- 
tion reactions  belong  to  the  category  of  addition  reactions. 

What  is  now  especially  needed  in  order  to  place  the  reactions  of 
organic  chemistry  on  an  exact  mathematical  basis  is  a  precise  method  of 
determining  the  ratio  of  active  particles  present  at  various  temperatures 
in  the  case  of  the  unsaturated,  as  well  as  the  saturated  compounds. 

As  the  substances  under  discussion  are  almost  exclusively  non- 
electrolytes,  the  sole  methods  that  suggest  themselves  for  this  purpose 
are  determinations  of  the  speed  of  decomposition,  as  well  as  of  addition 
reactions. 

The  above  discussion  makes  it  evident  that  all  unsaturated  com- 
pounds belonging  to  classes  I  and  III  contain  a  small  and  relatively 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF         351 

varying  percentage  of  active  particles  with  one  or  more  carbon  atoms 
temporarily  in  an  active  or  trivalent  condition;  the  same  is  true  of 
compounds  containing  hydrogen  bound  to  carbon — paraffines, 
CnH2n+i — H,  benzene  derivatives,  etc.  The  isolation  of  compounds 
containing  trivalent  carbon  as  such,  I  believe,  however,  to  be  an  impossi- 
bility. Gomberg's  triphenylmethyl,  for  instance,  has  recently  been 
proved  by  him  to  be  a  bimolecular  aggregate,  C3gH3o — not  identical 
with  hexaphenylethane — which,  however,  like  the  above-mentioned 
compounds,  contains  a  very  small  percentage  of  active  triphenylmethyl, 
(Cells) 3=C — ,  particles  in  dynamic  equilibrium  with  the  bimolecular 
aggregate;  as  soon  as  the  percentage  of  triphenylmethyl  particles  is 
increased  by  heat  or  by  means  of  catalytic  agents  a  spontaneous  poly- 
merization to  the  real  hexaphenylethane  (non-reversible)  takes  place. 

We  are  now  in  a  position  to  consider  the  evidence  showing  that 
methylene  and  its  homologues  play  a  great  role  in  many  of  the  funda- 
mental reactions  of  organic  chemistry  which  have  hitherto  been  explained 
on  the  basis  of  substitution. 


HI.  On  the  Reactions  of  the  Monatomic  Alcohols  and  the 

Alkylhaloids 

The  experiments  which  first  suggest  themselves  as  a  means  of 
isolating  methylene  and  its  homologues  are:  I.  dissociation  of  defines 
as  ethylene: 

CH2=CH2     <=±    2CK2 
CH3CH=CH— CH3     <=±    2CH3CH=,  etc. 

Since  ethylene  gives  hydrogen  and  acetylene  by  heat  and  the  higher 
olefines  also  decompose  with  evolution  of  hydrogen,  there  was  little 
prospect  of  success  by  experiments  in  this  direction.  2.  dehydration 
of  the  mono-atomic  alcohols,  CnH2n+iOH,  or  removal  of  halogen  hydride 
from  the  alkylhalides,  CnH2«+iX;  naturally  only  primary  and  second- 

R\ 
ary  derivatives,  RCH2X  and      >CHX(X=OH,  Cl,  Br,  or  I),  and  not 

W 

tertiary  compounds,  R3=C — X,  can  yield  methylene  and  its  homo- 
logues. Furthermore,  since  many  of  the  alcohols  and  alkylhalides 
containing  more  than  one  carbon  atom  in  the  molecule  are  known  to  give 
olefines  by  dissociation,  dehydration,  or  treatment  with  alcoholic  potash, 
respectively,  the  conclusion  might  naturally  at  first  be  drawn  that  only 


352  THEORIES  OF  ORGANIC  CHEMISTRY 

a  direct  olefine  dissociation  existed  in  these  cases.  From  a  purely 
theoretical  standpoint,  however,  it  is  clear  that  a  primary  or  secondary 
alkylhalide  or  a  corresponding  alcohol  with  more  than  one  carbon  or 
hydrogen  atom  in  the  molecule  may  dissociate  with  loss  of  halogen 
hydride  or  water  in  two  possible  ways:  it  may  undergo  (1)  methylene 
dissociation,  as 

/X 

R-CH2-CH<         <=>    RCH2CH=+HX 
\H 

and 

/H 

(RR')C<         <=±     (RR')0=+HX 

\X 
or  (2)  olefine  dissociation,  as 

CH3CH2\ 

RCH2— CH2X    <=»    RCH— CH2+HX  and  >CHX    <± 

CH3/ 


CH3— CH— CH— CH3     or    CH3CH2CH— CH2+HX 

or  both  kinds  of  dissociation  may  take  place  simultaneously. 

A  third  kind  of  dissociation  where  the  hydrogen  atom  does  not  come 
from  the  atom  containing  the  X  or  from  a  carbon  atom  adjacent  to  it  is 
also  possible  and  at  times  important,  but  it  need  not  be  considered  in 
this  connection. 

An  exhaustive  study  of  the  primary  and  secondary  alcohols  and 
alkylhalides,  covering  a  period  of  nine  years,  has  proved  very  conclu- 
sively that  these  substances  undergo  methylene  dissociation  only. 

Preliminary  experiments  with  alcohols  and  alkylhalides  where  no 

/H 
olefine  dissociation  is  possible,  i.e.,  in  the  methane,  CH2<      ,  toluene, 


C6H5CH^     ,    diphenylmethane,    (C6H5)2C<      ,    acetone    and   aceto- 

/H  /H 

:H3COCH<^ 

ester  series 

(COOR)2C<f 

\X  COOR/     \X 


phenone,  CH3COCH<     ,  C6H5COCH<      ,  and  malonic  and  cyanacetic 
XX  \X 

M  CNX      /H 

(COOR)2C<         and  >C< 

\X  COOT?/       V 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF          353 

have  proved  that  all  these  compounds  have  very  low  dissociation 
points — never  above  300°  in  the  aromatic  nor,  with  few  exceptions,  in 
the  aliphatic  series.  Nevertheless  it  was  found  impossible  to  isolate 
the  methylene  derivative  as  such  in  any  case;  there  was  either  a  spon- 
taneous conversion  to  a  di-  or  trimolecular  polymer,  an  olefine  or  a 
trimethylene  derivative,  or  a  conversion  to  resinous  polymers  analogous 
to  azulmic  acid  and  the  alkylisocyanide  resins.  Most  important  was  the 

Z\ 
discovery  that  these  nascent  or  active  methylene  residues        /C=, 

y/ 

are  always  spontaneously  combustible,  burning  often  with  marvelous 

Z\ 
evolution  of  heat  to  the  corresponding  oxides,       >C=O:  this  was  not 

Y/ 

surprising  in  view  of  the  properties  of  the  methylene  derivatives  described 
above.  Furthermore,  the  affinity  of  unsaturated  carbon  for  oxygen 
is  strikingly  shown  by  the  fact  that  these  residues  have  the  power  of 
decomposing  water, 

\rj 
C=-J-0=H2    -»      '\C=0+2H— 
Y/  y/ 

with  evolution  of  hydrogen. 

A  subsequent  investigation  of  the  primary  and  secondary  alcohols 
and  alkylhalides  containing  more  than  one  carbon  atom  proved,  first  of 
all,  that  all  these  substances  have  comparatively  low  points  of  dissocia- 
tion. In  no  case  was  the  decomposition  point  found  to  be  higher  than 
700°;  it  was  often  as  low  as  160°  to  300°.  The  products  of  dissociation 
are  water  or  halogen  hydride  and  CnH2n,  respectively;  and  the  latter, 
as  emphasized  above,  is  invariably  methylene  or  a  homologue  and 
never  an  olefine.  This  naturally  means  that  all  these  compounds  are 
partially  dissociated  in  this  way  at  ordinary  temperatures,  and 


H 

+HX 
R'/         X 


Rx 

>C= 
R'/ 


relatively  the  more  the  lower  the  actual  decomposition  point. 

It  is,  therefore,  possible  that  in  all  the  interactions  of  the  primary 
and  secondary  alkylhalides  with  other  substances,  such  as  salts,  am- 
monia, metals,  benzene,  etc.,  they  do  not  act  as  such,  but  by  virtue  of 
being  partially  dissociated.  An  enormous  amount  of  evidence  has 
accumulated  in  favor  of  this  conclusion.  Let  us  consider  chiefly  the 
results  obtained  in  the  ethyl  series,  including  ethyl  alcohol  and  its 


354 


THEORIES  OF  ORGANIC  CHEMISTRY 


derivatives.  The  dissociation  or  decomposition  point  of  the  following 
compounds  containing  ethyl  has  been  determined  with  a  fair  degree 
of  accuracy: 


Ethane, 


Ethyl  alcohol, 


CH3CH< 


vH 

CH3CH< 


Sodium  and  potassium  ethylate,    CH3CH 
Ethyl  ether, 


Ethyl  chloride, 
Ethyl  bromide, 
Ethyl  iodide, 
Diethyl  sulphate, 


Monoethyl  sulphate, 


Ethyl  potassium  sulphate, 


Ethyl  nitrate, 


OM 

H 

H 


CH3CH/ 

X) 

CH3CH— H 
CH3CH< 


CH3CH 


CH3CH 


H 

Br 

H 


/H 
CH3CH< 

X0— S02 


CH3CH 
CH3CH 
CH3CH 


H 
H 

\OSO2OH 
H 

OSO2OK 


800C 


650C 


250C 


550C 


600C 


500( 


400( 


200C 


160C 


250° 


/H 

CH3CH<  200°  (?) 

X)N02 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF         355 

Ethane,  ethyl  chloride,  and  bromide,  when  heated  to  the  tempera- 
tures named,  give  ethylene  and  hydrogen  and  halogen  hydride,  respect- 
ively, and  on  cooling  these  products  do  not  again  recombine.  We  can, 
therefore,  obtain  ethylene  quantitatively  from  chloride  or  bromide  of 
ethyl  by  simply  passing  their  vapors  through  tubes  heated  to  the 
decomposition  point.  Nevertheless  it  is  impossible  to  obtain  more 
than  very  small  amounts  of  ethylene  from  the  ethylhalides  by  means  of 
alcoholic  potash,  caustic  potash,  or  quicklime;  in  these  cases  ethyl 
halide  is  passed  over  quicklime  in  tubes  to  from  300°  to  500°. 
Furthermore,  the  percentage  of  ethylene  obtained  varies  remark- 
ably with  the  temperature,  the  concentration,  and  with  the  nature  of 
the  halogen  in  the  alkylhalide  used. 

The  conclusions  finally  reached  from  these  data  and  also  from  an 
exhaustive  study  of  the  behavior  of  the  various  alkylhalides,  nitrates, 
sulphates,  alkylpotassium  sulphates  towards  heat,  sodium  ethylate, 
caustic  potash,  quicklime,  and  other  salts  are  that  ethylene  cannot 
possibly  be  a  primary  product  of  dissociation  of  the  ethylhalides,  sul- 
phates, and  nitrates,  and  of  free  ethyl  alcohol. 

The  ethylene,  when  obtained,  is  formed  from  ethylidene  by  an 
intramolecular  addition  reaction, 


H 
H2— CH=    ->    CH2— CH2     +±    CH2=CH2 


J.  A 

i 


which  is  not  reversible.  A  similar  intramolecular  change  always,  in 
fact,  takes  place  whenever  an  olefine  is  formed,  whether  from  a  primary 
or  secondary  alcohol,  or  from  a  corresponding  alkyl  halide  sulphate  or 
nitrate.  This  transformation  is  perfectly  analogous  to  the  conversion, 
discussed  above,  of  trimethylene  and  of  propylenoxide  into  propylene, 
propionaldehyde,  and  acetone. 

When  ethyl  alcohol  or  ethyl  ether  is  heated  to  its  dissociation  point 
the  ethylidene  interacts  at  once  in  great  part  with  the  other  dissociation 
product,  water,  to  give  hydrogen  and  acetaldehyde, 

CH3CH==  +  0=H2  ->  CH3CH  :  O+2H- 

In  the  case  of  ether,  since  there  are  two  ethylidene  molecules  to 
one  of  water,  the  atomic  hydrogen  is,  in  part,  absorbed  by  ethylidene  to 
give  ethane.  Finally,  a  portion  of  ethylidene,  20  and  37  per  cent, 
respectively,  is  transformed,  by  intramolecular  addition,  into  ethylene. 
The  most  striking  proof  that  ether  is  dissociated  into  water  and  2C2H4 
particles  is  the  following:  on  passing  ether  vapor  over  phosphorus 


356  THEORIES  OF  ORGANIC  CHEMISTRY 

pentoxide  at  temperatures  varying  from  200°  to  400°  ethylene  is  formed 
quantitatively . 

The  primary  and  secondary  alcohols  and  their  corresponding  ethers 
being  in  a  state  of  very  slight  dissociation  at  ordinary  temperatures,  we 
are  able  to  understand  perfectly  their  behavior  towards  oxidizing  agents. 
The  alkylidenes  are  all  spontaneously  combustible  substances  possessing 
a  great  affinity  for  oxygen.  Absolutely  pure  dry  ethyl  ether,  dissocia- 
tion point  550°,  contains  a  sufficient  percentage  of  ethylidene  particles 
at  ordinary  temperatures  to  burn  very  slowly  in  dry  oxygen;  sodium 
ethylate,  dissociation  point  250°,  on  the  other  hand,  being  dissociated 
to  a  far  greater  extent,  burns  with  great  violence  in  dry  air.  Ethyl 
alcohol,  dissociation  point  650°,  is  not  capable  of  burning  in  the  air; 
if,  however,  we  increase  the  percentage  of  ethylidene  particles  by  means 
of  catalytic  agents,  enzymes,  platinum  sponge,  etc.,  it,  too,  oxidizes 
readily,  with  incandescence  with  platinum  sponge,  giving  acetic  acid. 

The  aldehydes,  RCH  :  0,  as  has  long  been  known,  reduce  Fehling's 
solution  and  silver  solutions  with  great  ease.  This  is  due  to  the  presence 


of  oxyalkylidene  particles,          /C=,  which  burn  at  the  expense  of  the 

HCK 

oxygen  in  the  water. 

The  discovery  that  all  primary  and  secondary  alcohols  reduce  silver 
oxide  to  metallic  silver  in  aqueous  solution  in  the  presence  of  caustic 
alkalies  has  only  very  recently  been  made.  The  function  of  the  alkali 
is  obviously  to  form  first  the  metallic  alcoholate, 

XR\       /H 
+MOH    <=>        >C<  +H20 

H  R/    \)M 

which,  having  a  far  lower  dissociation  point  than  the  free  alcohol,  causes 
a  great  increase  in  the  percentage  of  alkylidene  particles  present; 
consequently,  the  following  reaction  can  take  place: 

R  TJ 

=+2H— OH+Ag20    r?   "    Nc=(OH)2+Ag2+H20,etc., 

R'/ 

giving,  as  the  end  result,  a  fatty  acid  in  the  case  of  primary  alcohols. 

The  most  striking  proof  that  ethyl  alcohol  is  dissociated  only  into 
ethylidene  and  water, 

>H 

CH3CH<  <±    CH3CH=+H2O 

X)H 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF         357 

i.e.,  contains  no  ethylene  particles,  is  the  following:  ethyl  alcohol, 
containing  one  molecule  of  aqueous  sodium  hydroxide,  gives,  in  the 
cold,  with  potassium  permanganate  solution,  practically  acetic  acid 
only.  If  any  active  ethylene  particles  were  present, 

CH3CH2OH    *±    CH2— CH2+H2O 

these  must  necessarily,  in  view  of  the  work  of  Wagner  with  olefines  and 
permanganate,  be  first  converted  by  oxidation  to  ethylene  glycol, 

3CH2— CH2+6H— OH+2KMnO4    -> 


3CH2  -  CH2 + 2MnO2 + 2KOH + 2H2O 
>H 


>Vx>AA^  WJ 

OH      01 


Analogous  results  would  naturally  be  expected  in  the  case  of  all 
the  homologous  primary  and  secondary  alcohols.  Now  a  primary 
alcohol  invariably  first  gives,  by  oxidation  with  potassium  permanganate 
or  other  oxidizing  agents,  the  corresponding  fatty  acid.  Glycols  or 
their  oxidation  products  have  never  been  observed  in  such  cases. 

The  fact  that  ethyl  alcohol  gives  glyoxal,  glyoxylic,  and  oxalic  acids 
with  nitric  acid  is  no  exception  to  this  rule,  because  these  substances 
result  from  the  hydrolysis  and  oxidation  of  isonitrosoacetaldehyde, 
which  is  formed  by  the  action  of  nitrous  acid  on  acetaldehyde  as  follows : 

H— CH2CHO+0— NOH    ->    HO— N— OH 

CH2-CH  :  O 

HON 

||  +H20 

CH— CH  :  O 

The  behavior  of  aldehydes  and  of  primary  alcohols  towards  aqueous 
or  solid  caustic  potash  also  leads  to  the  conclusion  that  only  alkylidene 
dissociation  occurs.  Ethyl  alcohol  gives,  at  250°,  with  an  excess  of 
caustic  potash,  hydrogen  and  potassium  acetate  quantitatively: 

CH3CH/       +2KO— H    -»    CH3CH=+3KO— H    -* 
X)K 


3v 
CH3CH(OK)2+2H+KOH    -»          >C=+2H—  OK+H2 


CH3\ 

>C(OK)2+2H+H2     ->    CH3C(OK)2+2H2 

KO/ 


358  THEORIES  OF  ORGANIC  CHEMISTRY 

If  any  of  the  potassium  ethylate,  which  is  first  formed,  were  dis- 
sociated into  ethylene  and  caustic  potash, 

CH3CH2OK    <=*    CH2=CH2+HOK 
the  olefine  must  naturally  give,  besides  hydrogen,  ethyleneglygol, 

CH2— CH2+2H— OK    ->     CH2— CH2+H2 

OK      OK 

or  its  decomposition  products;  these  are,  however,  not  formed.  The 
reaction  with  potash-lime  and  primary  alcohols  is  so  delicate  and 
accurate  that  it  has  been  suggested  by  Hell  as  a  means  of  determining 
the  molecular  weight  of  an  unknown  primary  alcohol. 

As  mentioned  above,  ethyl  ether  is  the  chief  product  when  ethyl 
halides  are  treated  with  alcoholic  potash  or  with  dry  sodium  ethylate; 
this  is  also  true  when  dry  silver  oxide  and  ethyl-halides  are  used. 

These  reactions,  which  have  been  interpreted  by  Williamson  and 
others  on  the  basis  of  double  decomposition  or  of  minute  ionization, 
must  obviously  be  attributed  to  the  absorption  by  the  ethylidene  of 
alcohol  or  of  water,  which  is  set  free  by  the  action  of  the  halogen  hydride 
particles  on  the  sodium  ethylate  or  silver  oxide,  respectively, 

CH3CH=+H— OC2H5     -»    CH3CH2OC2H5 

or 

M 

2CH3CH=+H2=O     ->    CH3CH< 

>O 


CH3CH/ 
\ 


We  are  now  able  to  consider  an  entirely  new  explanation  of  the 
function  of  sulphuric  acid,  or  of  phenyl  sulphonic  acid,  in  converting 
ethyl  alcohol  into  ether.  Sulphuric  acid  acts,  first  of  all,  with  alcohol 
at  ordinary  temperatures  to  give  both  mono-  and  diethyl  sulphate; 
the  first  stage  in  the  reaction  cannot  be  ascribed  to  the  union  of  ethyl- 
idene, formed  by  dissociation  of  alcohol,  with  free  sulphuric  acid  since 
ethyl  ether,  which  is  relatively  more  dissociated  than  alcohol,  reacts 
only  very  slowly  with  concentrated  sulphuric  acid  at  ordinary  tempera- 
tures to  give  monoethyl  sulphate.  Furthermore,  since  sulphuric  acid 
itself  is  completely  dissociated  into  its  components  sulphur  trioxide  and 
water  at  400°,  it  is  extremely  probable  that  monoethyl  sulphate  is 


THEORETICAL  SPECULATIONS  OF  JOHN  ULRIC  NEF         359 

formed  by  the  union  of  ethylalcohol  and  sulphur  trioxide  formed  by 
dissociation  as  is  expressed  below: 

I.  H2S04     <=»    S03+H2O 

/OH 
II.  O2S— O+H— OC2H5    ->     O2S< 

I       I  XOC2H5 

Now  it  is  well  known  that  ether  formation  in  a  mixture  of  sulphuric 
acid  and  alcohol  begins  perceptibly  only  at  95°  and  proceeds  very 
slowly  at  that  temperature.  The  favorable  temperature  for  ether  manu- 
facture is  140°.  This  is  self-evident  in  view  of  the  following  considera- 
tions: primary  and  secondary  ethyl  sulphate  possess  the  dissociation 
points  of  160°  and  200°,  respectively;  consequently,  these  substances 
must  be  dissociated  to  a  very  great  extent  into  sulphuric  acid  and  one 
or  two  molecules  of  ethylidene,  respectively.  Addition  of  alcohol  at 
140°,  therefore,  simply  necessitates  a  combination  with  the  ethylidene 
particles, 

CH3CH=  +  H— OC2H5  ->  CH3CH2OC2H5 

to  give  ether,  and  this  process  can  naturally  go  on  indefinitely. 

When  ethyl  alcohol  is  mixed  with  an  excess  of  concentrated  sulphuric 
acid  and  heated  to  160°  no  ether,  but  some  ethylene,  is  formed;  in 
fact,  this  method  is  still  suggested  and  used  as  the  best  means  of  pre- 
paring ethylene. 

The  yield  of  olefine,  however,  can  never  be  raised  above  20  per  cent 
of  the  theory  and  the  operation  is  extremely  tedious  because  carboniza- 
tion and  formation  of  sulphur  dioxide  takes  place  to  a  very  marked 
extent.  These  results  are  now  easily  understood.  The  ethylidene 
molecules,  formed  by  dissociation  of  ethylated  sulphuric  acid,  burn 
chiefly  at  the  expense  of  the  oxygen  present  in  sulphuric  acid, 

CH3CH=+0=S02    -»    CH3CH  :  O+SO2 

and  the  resulting  acetaldehyde  is  then  at  once  charred  by  the  vitriol 
present.  Only  20  per  cent,  at  the  utmost,  of  the  ethylidene  particles 
escape  this  oxidation  by  intramolecular  conversion  to  ethylene. 

Finally  we  may  summarize  the  conclusions  reached  hi  the  above 
discussion  as  follows: 

The  valence  of  carbon  is  not  a  constant.  At  definite  temperatures, 
which  vary  remarkably  with  the  nature  of  the  groups  bound  to  it.  :i 
carbon  atom  becomes  spontaneously  dyad.  Below  these  limits  there  is 
dynamic  equilibrium  between  bivalent  and  quadrivalent  carbon.  The 
existence  of  carbon  compounds  containing  bivalent  carbon  has  been 


360  THEORIES  OF  ORGANIC  CHEMISTRY 

definitely  established;   methylene  chemistry  plays  a  great  role  in  many 
of  the  fundamental  reactions  of  organic  chemistry. 

The  conception  of  substitution  or  metalepsis,  which  has  been  our 
guide  in  interpreting  the  reactions  of  carbon  chemistry  since  1833,  is  no 
longer  tenable.  It  must  be  replaced  by  the  conception  of  dissociation  in 
its  broadest  sense.  Fundamentally  speaking,  there  are  but  two  classes 
of  carbon  compounds — the  saturated  and  the  unsaturated.  Excluding 
reactions  called  ionic,  a  chemical  reaction  between  two  substances  always 
first  takes  place  by  their  union  to  form  an  addition  product.  The  one 
molecule  being  unsaturated  and  partially  in  an  active  molecular  condi- 
tion absorbs  the  second  molecule  because  it  is  partially  split  or  dis- 
sociated into  two  active  portions.  The  resulting  addition  product  then 
often  dissociates  spontaneously,  giving  two  new  molecules.  The 
similarity  of  such  reactions  to  those  called  ionic  is  at  once  apparent, 
but  their  relationship  cannot,  in  the  present  state  of  our  knowledge, 
be  clearly  understood. 


CHAPTER  XV 

CONCEPTIONS  IN  REGARD  TO  THE  INDEPENDENT  EXIST- 
ENCE  OF  FREE   ORGANIC  RADICALS 

NEF'S  theoretical  conceptions  are  based  upon  the  assumption  of 
the  changing  valency  of  carbon  with  sharp  emphasis  upon  the  impor- 
tance of  considering  addition  processes  in  all  reactions  in  organic  chem- 
istry. These  conceptions  were  by  no  means  original  since  von  Baeyer 
in  1894  had  explicitly  stated  that  "  carbon  is  as  a  rule  tetravalent " 
and  had  even  suggested  that  the  properties  of  cyanogen  compounds 
might  be  explained  by  assuming  the  presence  of  bivalent  carbon.  The 
significance  of  addition  processes  in  organic  reactions  had  also  been 
pointed  out  by  the  American  investigator,  Arthur  Michael.  Nefs 
contribution  consisted  in  extending  the  scope  of  these  conceptions 
and  in  establishing  them  more  firmly  on  the  basis  of  experimental 
evidence.  By  supplying  the  additional  assumption  of  molecular  dis- 
sociation he  was  able  to  combine  several  sets  of  more  or  less  isolated 
ideas  into  a  theoretical  system  which  was  capable  of  including  a  variety 
of  chemical  processes  and  which  was  also  plastic  enough  to  lend  itself 
to  the  interpretation  of  new  reactions. 

In  summing  up  Nefs  particular  achievements  it  may  be  said  that 
he  was  able  to  demonstrate  the  presence  of  bivalent  carbon  in  several 
groups  of  organic  compounds,  among  which  may  be  mentioned  the 
alkyl-  and  aryl-isocyanides,  RN=C,  and  the  mono-  and  di-substituted 
acetylenes.  The  reactions  of  these  and  other  similar  classes  of  sub- 
stances were  interpreted  by  assuming  that  the  relative  reactivity  of 
a  given  compound  depends  primarily  upon  dissociation  phenomena. 
All  of  the  reactions  of  methane,  for  example,  are  supposed  to  be  preceded 
by  a  preliminary  dissociation  of  the  substance  in  either  of  two  ways: 

I.  CH4     *±    CH3+H 

II.  CH4    +±    CH2+2H 

Dissociation  is  supposed  to  take  place  even  at  ordinary  temperatures 
and  the  fact  that  it  has  been  impossible  as  yet  to  isolate  free  methylene 
is  explained  by  assuming  that  free  radicals  instantly  polymerize.  It 

361 


362  THEORIES  OF  ORGANIC  CHEMISTRY 

may  be  mentioned  in  this  connection  that  recent  experiments  which 
have  aimed  to  prepare  free  diary Ime  thy lene  by  splitting  off  nitrogen 
from  aryl  substituted  aliphatic  diazo  compounds  have  not  achieved 
the  desired  result.1 

Nef  undoubtedly  revived  a  belief  in  the  possibility  of  the  independ- 
ent existence  of  organic  radicals.  It  may  be  remembered  that  this 
possibility  had  engaged  the  attention  of  chemists  for  many  years  but 
that  attempts  to  isolate  free  radicals  had  ultimately  been  abandoned. 
While  Nef  did  not  himself  succeed  in  actually  demonstrating  the  exist- 
ence of  such  a  substance,  this  accomplishment  may  nevertheless  be 
accredited  to  a  member  of  his  school. 

In  1900  M.  Gomberg2  discovered  triphenylmethyl  as  the  result 
of  an  investigation  which  was  undertaken  with  a  view  to  establish 
definitely  the  constitution  of  tetraphenylmethane.  Since  the  problem 
involved  the  preparation  of  hexaphenylethane,  (CoR^aC^,  and  since 
the  Fittig  synthesis  and  other  of  the  usual  methods  had  failed  to  give 
this  hydrocarbon,  Gomberg  tried  to  prepare  it  by  treating  triphenyl 
chlormethane  and  triphenyl  brommethane  with  finely  divided  metals 
such  as  Hg,  Ag,  Zn,  Cu,  etc.  Under  ordinary  conditions  the  only 
product  of  this  reaction  is  a  substance  which  contains  oxygen.  When 
however,  the  experiment  was  conducted  in  an  atmosphere  of  carbon 
dioxide,  an  orange-colored  solution  was  formed,  and  this,  when  evapo- 
rated in  the  absence  of  oxygen,  gave  crystals  of  what  appeared  to  be 
hexaphenylethane.  But  although  the  results  of  the  analysis  pointed 
to  this  conclusion,  the  properties  of  the  substance  were  so  remarkable 
as  to  leave  the  matter  in  some  doubt.  For  example,  it  forms  colorless 
crystals  which  gradually  change  to  yellow  in  the  air  and  which,  when 
dissolved  in  a  number  of  different  solvents,  give  yellow  solutions.  But 
above  all  it  is  chemically  so  reactive  as  to  appear  to  be  strongly  unsatu- 
rated.  Such  a  condition  of  seeming  unsaturation  was  explained  by 
Gomberg  as  due  to  the  presence  of  trivalent  carbon,  and  the  substance 
was  regarded  as  an  example  of  a  triaryl  substituted  free  methyl  radical : 

(C6H5)3C- 

Many  other  formulas  have  been  suggested  for  this  very  interesting 
substance  as  the  result  of  a  discussion  which  immediately  arose  in  regard 
to  its  constitution.  This  discussion  has  continued  up  to  the  present 
time  and  will  be  reviewed  more  fully  later  in  the  chapter  on  the  Relation 
of  Color  to  Chemical  Constitution.  At  the  present  time  it  need  only 

1  Staudinger,  Ber.,  49,  1923  (1916). 

2Ber.,  33,  3150  (1900);  also  Schmidlin,  "Das  Triphenylmethyl,"  Stuttgart, 
1914. 


INDEPENDENT  EXISTENCE  OF  FREE  ORGANIC  RADICALS  363 

be  said  that  the  reactions  of  this  extraordinary  substance  are  most 
readily  understood  on  the  basis  of  the  triphenylmethyl  formula  of 
Gomberg.  For  example,  the  substance  is  readily  oxidized  to  a  peroxide 
by  the  oxygen  of  the  air: 

2(C6H5)3C—  +02=(C6H5)3CO  •  O  •  C(C6H5)3 

The  product  is  a  colorless  compound  which  crystallizes  readily,  is  only 
slightly  soluble,  and  possesses  the  properties  of  a  saturated  substance. 
Triphenylmethyl  also  reacts  readily  with  halogen,  especially  iodine, 
according  to  the  equation: 

2(C6H5)3C—  +I2  =  2(C6H5)3CI 

This  product,  in  the  presence  of  an  excess  of  halogen,  reacts  to  give  a 
perhalide  of  the  formula  (C6H5)3CI  •  I5  and,  .in  the  presence  of  metallic 
halides  such  as  Aids,  SnCU,  SbCl3,  etc.,  it  reacts  to  give  organo- 
metallic  compounds  having  the  general  character  of  double  salts. 
Triphenylmethyl  also  readily  adds  benzene,  ether,  esters,  etc.,  to  give 
compounds  having  the  formulas: 


(C6H5)3CX     XC2H5          (C6H5)3CX     /C 

>0<  >O<  ,  etc. 

(C6H5)3CX      XC2H5          (C6H5)3CX      XCOCH3 

If  nitric  oxide  is  mixed  with  carbon  dioxide  and  conducted  into  an 
ethereal  solution  of  triphenylmethyl,  a  reaction  takes  place  which  is 
accompanied  by  the  momentary  appearance  of  a  bluish  green  color, 
due  to  the  formation  of  nitrosotriphenylmethyl.  The  latter  poly- 
merizes immediately  to  give  a  binitrosyl  compound  which  is  colorless: 


(C6H5)3C+NO    ->     (C6H5)3C.NO 

Bluish  Green.  Colorless. 

Nitrogen  dioxide  under  the  same  conditions  gives  a  mixture  of  a  nitro- 
derivative  and  an  ester  of  nitrous  acid  which  can  be  separated: 

2(C6H5)3C+2N02=(C6H5)3C  -  NO2+  (C6H5)3C  •  ONO 

Unsaturated  organic  radicals  add  triphenylmethyl  in  much  the  same 
way.  For  example  methylene,  which  is  formed  as  an  intermediate 
product  in  the  decomposition  of  diazomethane,  reacts  to  give  hexa- 
phenylpropane  : 

/N      C(C6H6)3 

CH2|  +  =N2+CH2 

XN      C(C6H5)3 


364  THEORIES  OF  ORGANIC  CHEMISTRY 

Diphenylnitride  likewise  reacts  to  give  completely  substituted  deriva- 
tives of  metlrylamine: 


These  and  a  great  variety  of  other  reactions  which  need  not  be  reviewed 
at  this  time,  serve  to  demonstrate  the  preeminently  unsaturated  proper- 
ties of  the  substance.  Attempts  to  confirm  the  above  formula  for 
triphenylmethyl  by  means  of  molecular  weight  determinations  were 
then  made. 

When  the  molecular  weight  of  the  substance  was  determined  in 
fused  naphthalene  by  means  of  the  freezing-point  method,  values  of 
330  and  370  were  obtained.  These  are  both  considerably  higher  than 
243,  which  represents  the  value  calculated  on  the  basis  of  the  simplest 
molecular  formula  of  the  substance.  These  values  are,  however,  also 
considerably  lower  than  the  molecular  weight  of  hexaphenylethane, 
which  equals  486.  In  other  solvents  such  as  nitrobenzene,  dimethyl 
aniline,  para-bromtoluene,  and  phenol,  a  value  averaging  477  was 
obtained  and  it  must  therefore  be  assumed  that  hexaphenylethane  is 
present  in  these  solutions.  Gomberg  explains  the  cryoscopic  behavior 
of  solutions  in  naphthalene  very  simply  by  supposing  that  hexaphenyl- 
ethane dissociates  in  this  solvent  according  to  the  equation 

(C6H5)3C.C(C6H5)3     **    2(C6H5)3C- 

Since  the  condition  of  equilibrium  in  such  a  system  as  this  would  depend 
upon  temperature,  concentration,  and  the  nature  of  the  solvent,  it 
is  easy  to  see  why  the  behavior  of  the  substance  should  be  entirely 
different  when  dissolved  in  other  solvents.  This  explanation,  which  was 
later  endorsed  by  Wieland,1  has  been  the  subject  of  much  discussion, 
but  has  finally  come  to  be  generally  accepted  as  a  result  of  the  researches 
of  Schlenk  and  Mair,  J.  Piccard  and  others. 

Schlenk  and  Mair  2  determined  the  molecular  weight  of  hexaphenyl- 
ethane in  benzene  and  calculated  from  their  results  that  the  degree  of 
dissociation  into  triphenylmethyl  is  29.9  per  cent.  J.  Piccard  3  has  also 
succeeded  in  demonstrating  by  means  of  colorimetric  observations  that 
a  condition  of  equilibrium  exists  in  solutions  of  hexaphenylethane. 
He  discovered  for  example  that  the  product  which  is  obtained  as  a  result 
of  the  action  of  metals  upon  triphenylmethyl  chloride  shows  a  molec- 
ular weight  which  corresponds  to  hexaphenyl  ethane  when  examined 
in  concentrated  solutions  but  that  this  value  decreases  in  proportion 

*Ber.,  42,  3029(1909). 

2Annalen  der  Chemie,  394,  179  (1912). 

3  Annalen  der  Chemie,  381,  349  (1911);  also  Ber.,  48,  1097  (1915). 


INDEPENDENT  EXISTENCE  OF  FREE  ORGANIC  RADICALS  365 

to  dilution.  This  change  is  accompanied  by  a  marked  change  in  the 
color  of  the  solution.  For  example,  when  a  five  per  cent  solution  of 
the  substance  in  ether  is  diluted  with  ether  which  is  free  from  air,  in 
an  atmosphere  of  carbon  dioxide,  it  was  observed  that  the  solution, 
which  was  originally  light  yellow,  became  more  and  more  intensely 
colored  until  finally  it  was  changed  to  a  reddish  orange.  This  process 
is  reversible  and  on  evaporation  of  the  solvent  the  strong  color  gradually 
disappears  and  at  the  same  time  the  values  representing  the  molecular 
weight  of  the  dissolved  substance  gradually  increase. 

In  spite  of  these  discoveries  weak  points  remained  in  the  chain  of 
evidence  supporting  the  assumption  of  the  existence  of  a  free  methyl 
radical,  but  these  were  finally  strengthened  as  a  result  of  the  researches 
of  Schlenk,  who  succeeded  in  preparing  other  substances  with  properties 
similar  to  those  which  have  been  noted  in  the  case  of  triphenylmethyl. 
These  substances  were  obtained  by  treating  solutions  of 


v  e^v  ese-iv 

—  -?C  Cl  CeHsCeEU—  -^C  Cl  CeHsCeH^—  C  Cl 

CcH5-C6H4/  CeHsCoKU/  CeHsCaH*/ 
I                                           II  III 

with  active  copper  and  are  analogous  to  triphenylmethyl  in  every  way 
except  that  while  the  latter  dissolves  to  give  pale  yellow  solutions, 
I  gives  orange,  II  gives  deep  red,  and  III  gives  deep  violet  colorations. 
Of  these  substances  the  first  and  second  are  colorless  in  solid  form, 
while  the  third  has  a  violet  color  and  is  very  unstable.  The  molecular 
weights  of  these  respective  substances  have  been  determined  and  results 
show  that  the  two  colorless  compounds  resemble  hexaphenylethane 
in  being  bimolecular,  while  the  third  is  monomolecular  and  must  there- 
fore possess  the  formula  (CeHs  •  CeH^sC  —  .  This  free  organic  radical 
when  perfectly  pure  and  dry  consists  of  a  gray-green  crystalline  powder 
which  melts  at  186°  and  is  extremely  sensitive  to  oxygen  whether  in 
the  solid  state  or  in  solution.  Schlenk,1  in  co-operation  with  his  students, 
has  been  able  to  obtain  other  free  organic  radicals,  as  for  example, 

C6H5\ 

CeHs  •  CeH4—  ->C  — 
CioH.7/ 

Phenyl-p-biphenyl-a-naphthyl  methyl 

and  has  found  that  they  are  all  highly  unsaturated,  exceedingly  sensi- 
tive to  oxygen,  and  intensely  colored.  The  property  of  color  will  be 
referred  to  in  greater  detail  in  the  next  chapter. 

'Annalen  der  Chemie,  394,  195  (1912). 


366 


THEORIES  OF  ORGANIC  CHEMISTRY 


It  has  been  noted  that  substances  which  correspond  to  I  and  II 
give  highly  colored  solutions.  This  is  explained  by  supposing  that 
such  colored  solutions  contain  equilibrium  mixtures  of  the  bi-  and 
mono-molecular  modifications  : 


Ar\ 
Ar-^ 
AT/ 


/Ar 
f-Ar 
\Ar 


A 
A 
AT/ 


and  that  a  relatively  higher  percentage  of  the  latter  is  present  than  in 
the  corresponding  solutions  of  hexaphenylethane.  It  has  been  observed, 
for  example,  that  while  hexaphenylethane  is  dissociated  to  the  extent 
of  10  per  cent  in  a  given  solvent,  the  dissociation  of  I  is  equal  to  15  per 
cent,  that  of  II  is  equal  to  80  per  cent,  and  III  is  completely  dissociated 
into  the  free  radical. 

The  naphthyl  group  has  an  even  stronger  influence  upon  dissocia- 
tion than  the  biphenyl  group.  Thus  for  example  while  tetraphenyl- 
dibiphenylethane 


_ 

Cells  •  CeH4          CeH4  • 

is  dissociated  only  to  the  extent  of  15  per  cent,  tetraphenyl  dinaphthyl- 
e  thane 

(Cells)  2^  _  Q  (CeHs)2 

J 


is  dissociated  under  the  same  conditions  to  the  extent  of  60  per  cent. 

It  would  seem  to  follow  that  derivatives  of  benzene  which  possess 
still  denser  groupings  of  the  substituents  as,  for  example  : 


/eHs  HsCe 

Dibiphenylene-dibiphenylethane 

would  dissociate  even  more  completely  than  those  which  have  just 
been  mentioned,  but  this  is  not  the  case.     Both  substances  are  undis- 


INDEPENDENT  EXISTENCE  OF  FREE  ORGANIC  RADICALS  367 


sociated  in  low  boiling  solvents  and  only  slightly  dissociated  at  high 
temperatures.     Of  the  following  substances  I,  II  and  III: 


\C6H5  H5C6/ 


Diphenyl  dianthronylethane 


\C6H5  H5C6/ 


Diphenyl  dixanthylethane 
II 


Diphenyl  dithioxanthylethane 
III 

the  first  dissociates  to  give  33  per  cent,  the  second  to  give  82  per  cent, 
and  the  third  to  give  14  per  cent  of  the  free  organic  radical.  It  may 
be  added  that  hexanitroethane 

(N02)3C— C(N02)3 

is  a  colorless  fairly  stable  substance  which  melts  vat  142°  and  which 
appears  to  be  undissociated  in  its  solutions. 

In  summing  up  the  present  status  of  this  investigation  it  may  be 
said  that  the  presence  of  trivalent  carbon  in  the  above  series  of  com- 
pounds is  generally  conceded,  and  that  free  organic  radicals  are  assumed 
to  exist  not  only  in  solution,  but  in  certain  instances  even  in  the  solid 
state.  The  further  question  as  to  whether  such  radicals  are  ionic  in 
character,  as  indicated  by  the  equation 

(C6H5)3COH+HC1    ->    (C6H5)3C'+C1'+H20 

has  been  answered  in  the  negative  by  K.  H.  Meyer  and  H.  Wieland.1 
A  systematic  study  of  the  absorption  spectra  of  these  substances  shows 

'Ber.,  44,  2557(1911). 


368  THEORIES  OF  ORGANIC  CHEMISTRY 

that  triphenylmethyl  and  all  of  its  triaryl  substitution  products  possess 
very  characteristic  band  spectra,  and  that  even  at  great  dilutions 
sharply  differentiated  bands  are  plainly  visible.  Ionized  solutions  of 
carbinol  salts,  on  the  other  hand,  all  show  continuous  absorption  which 
shades  off  much  sooner  in  the  region  of  the  short  wave  lengths.  This 
seems  to  demonstrate  that  triphenylmethyl,  (CeHs^C — ,  together  with 
its  derivatives,  is  distinctly  different  from  the  triphenylmethyl  ion 
(C6H5)3C-. 

The  mechanism  of  the  formation  of  triphenylmethyl  has  been 
explained  by  supposing  that  six  such  voluminous  substituents  as  phenyl 
are  unable  to  accommodate  themselves  within  the  limits  of  the  ethane 
molecule.  Assuming  that  the  formation  of  hexaphenylethane  is  pos- 
sible, the  crowding  produced  by  the  presence  of  such  dense  groupings 
might  easily  give  rise  to  a  condition  of  strain,  which  in  certain  cases 
might  lead  to  the  decomposition  of  the  substance  and  in  other  cases 
might  even  be  so  great  as  to  prevent  its  initial  formation.  If  these 
assumptions  are  correct  it  should  follow  that  tetraphenylmethane 
would  have  the  same  properties  which  have  been  noted  in  the  case  of 
hexaphenylethane  since  there  is  still  greater  crowding  within  its  mole- 
cule. Such  a  conclusion  is,  however,  not  in  harmony  with  the  fact  that 
tetraphenylmethane  has  been  observed  to  be  a  very  stable  substance 
which  distills  without  decomposition  under  atmospheric  pressure  and 
does  not  resemble  hexaphenylethane  in  any  respect.  It  would,  there- 
fore, seem  that  steric  influences  do  not  play  the  decisive  role  in  determin- 
ing the  relative  stability  of  the  substances  in  question. 

The  most  plausible  explanation  of  the  phenomena  which  has  been 
offered  up  to  the  present  time  is  that  which  was  advanced  by  Thiele 
in  1901 l  and  which  supposes  that  in  addition  to  steric  influences,  other 
influences  are  operative  in  the  form  of  small  residual  affinities  present 
on  the  carbon  atoms  of  the  phenyl  groups.  In  triphenylmethyl  three 
sets  of  such  affinities  act  upon  the  carbon  atom  of  the  methyl  group 
with  the  result  that  practically  the  total  affinity  of  this  atom  is  engaged 
and  little  remains  with  which  to  hold  a  fourth  atom  or  group  within 
the  molecule.  This  accounts  for  the  ease  with  which  such  a  radical 
dissociates  and  also  for  the  fact  that  under  certain  circumstances  the 
trivalent  condition  represents  the  only  possible  arrangement.2 

In  1891  E.  Beckmann  and  T.  Paul 3  discovered  that  ketones  such 
as  benzophenone,  phenyl-a-naphthylketone,  and  others  react  with 
sodium  to  give  intensely  colored  addition  products  which  consist  of 

!Annalen  der  Chemie,  319,  134  (1901). 

2  Compare  Pummerer  and  Frankfurter,  Ber.,  47,  1472,  2957  (1914). 

3  Annalen  der  Chemie,  266,  1  (1891). 


INDEPENDENT  EXISTENCE  OF  FREE  ORGANIC  RADICALS    369 

one  atom  of  sodium  to  one  molecule  of  ketone.  Twenty  years  later 
W.  Schlenk  and  co-workers  1  repeated  this  work  because  they  suspected 
that  the  phenomenon  might  be  associated  with  the  presence  of  trivalent 
carbon.  They  studied  the  behavior  of  the  alkali  metals,  especially 
K,  Na  and  Li  in  perfectly  dry,  air-free  solvents,  such  as  ether  and 
benzene,  and  found  that  no  trace  of  hydrogen  was  evolved  during  the 
course  of  the  reaction.  The  resulting  compounds  belong  to  a  class 
known  as  metal-ketyls  and  resemble  the  class  of  triarylmethyls  in  that 
they  are  intensely  colored  and  extremely  sensitive  to  the  oxidizing 
action  of  the  air.  That  they  are  monomolecular  has  been  demonstrated 
in  the  case  of  the  addition  product  which  is  formed  by  the  action  of 
potassium  upon  phenylbiphenyl  ketone,  CeHsCOCe^  •  CeHs.2  The 
analogy  which  exists  between  such  compounds  and  derivatives  of  tri- 
phenylmethyl  is  indicated  by  the  formula : 

Arx  Arv       /OM 

>C— OM    or  >C<  (M  =  metal) 

Ar/  Ar/ 

in  which  the  metal  is  represented  as  in  union  with  oxygen.  It  is  assumed 
that  the  saturation  of  the  groups  ONa,  OK,  etc.,  requires  such  a  large 
fraction  of  the  total  affinity  of  the  central  carbon  atom  that  the  fourth 
valence  of  this  atom  is  reduced  to  the  value  of  a  residual  valence.  The 
presence  of  free  affinity  in  the  molecule  is  shown  by  the  ease  with 
which  the  substance  adds  a  second  atom  of  sodium  to  form  reddish 
colored  di-sodium  salts  of  the  formula: 

Ar  ONa 

Ar 

Sodium  benzophenone  may  be  prepared  in  other  ways  than  by  the 
action  of  this  metal  upon  the  ketone.  It  is  formed  for  example  when 
benzophenone  is  treated  with  sodium  amalgam  or  sodium  alcoholate 
in  dry  ether.  Schlenk  explains  the  latter  action  by  supposing  that  the 
sodium  salt  of  benzopinacone  is  formed  as  a  primary  product  and 
that  this  then  breaks  down  to  give  two  free  radicals: 

(C6H5)2  :  C— OH     NaOC2H5  (C6H5)2  :  CONa 

I  +  -* 

(C6H6)2  :  C— OH     NaOC2H5  (C6H5)2  :  CONa 

/ONa 
2(C6H5)2O(         +2C2H5OH 

1  Ber.,  44,  1182  (1911);  46,  2840  (1913);  47,  486  (1914). 
2Ber.,  46,  2840(1913). 


370  THEORIES  OF  ORGANIC  CHEMISTRY 

The  metal-ketyls  resemble  the  triarylmethyls  in  being  very  sensi- 
tive to  the  action  of  oxygen.  It  has  not  as  yet  been  demonstrated, 
however,  that  in  this  case  organic  peroxides  are  formed,  although  it 
is  possible  to  assume  that  intermediate  products  of  this  character  are 
present  in  the  reaction  mixture  and  that  they  decompose  immediately 
according  to  the  equation : 

(Ar)2C— OjNa 

(Ar)2CONa  !  O 

+O2    ->  -*    Na2O2+2Ar2  :  C  :  O 

(Ar)2CONa  !  O 


(Ar2)2C— OiNa 

The  action  of  water  upon  sodium  benzophenone  results  primarily  in 
the  formation  of  the  radical 

/OH 
(Ar)2C<( 

which  either  polymerizes  to  give  a  pinacone 

(Ar)2  :  C— OH 

I 
(Ar)2  :  C— OH 

or  rearranges  to  give  a  mixture  of  ketone  and  carbinol: 
2Ar2C-OH     -»    Ar2-CO+Ar2CHOH 

Iodine  reacts  smoothly  with  potassium  diphenylketone  with  the  result 
that  the  free  ketone  is  quantitatively  regenerated.  The  action  of 
carbon  dioxide  takes  place  according  to  the  equation 


N   /°K  CeHsCeH^0"000^ 

2  C6H5CeH/C\         +^°^  *    "^    ^OCO2K 


and  the  resulting  compound  is  decomposed  by  water  to  give  a  mixture 
consisting  of  the  original  ketone,  acid  potassium  carbonate,  and  phenyl- 
biphenyl  glycollic  acid: 


INDEPENDENT  EXISTENCE  OF  FREE  ORGANIC  RADICALS  371 

;o2K 

+HOH=KHC03  + 
OC02K 

C6H5v  C6H6\      /OH 

^>CO+  \C/ 

The  potassium  derivative  of  phenylbiphenyl  ketone  also  reacts  with 
the  corresponding  halogen  derivative  of  methane  to  give  tribiphenyl- 
methyl: 

K  CeH5\ 

i  TT  \  _  \r<n 

y6Al5J3==  >UU 


+KC1+  .  .  . 

The  fact  that  ketones  are  able  to  combine  not  only  with  one  but 
also  with  two  atoms  of  sodium  led  Schlenk  to  assume  that  the  reaction 
might  possibly  belong  to  the  ordinary  type  of  simple  addition  to  unsat- 
urated  double  bonds  and  he,  therefore,  investigated  the  question  from 
this  point  of  view.  As  the  result  of  a  series  of  experiments  in  regard  to 
the  behavior  of  sodium  upon  compounds  which  contain  unsaturated 
linkages  other  than  carbonyl,  Schlenk  made  the  surprising  discovery 
that  under  favorable  conditions  unsaturated  compounds  with  the  atomic 
groupings,  C=C,  C=C,  C=N  and  N=N  will  add  sodium  and  that  this 
action  is  accompanied  by  a  decrease  in  the  unsaturated  condition  of 
the  substance.  For  example,  stilbene  reacts  to  give  a  violet  brown 
derivative  : 

—  C6H5 


Na     Na 

which  reacts  with  water  to  give  diphenylethane  and  sodium  hydroxide 
(a)  and  which  adds  carbon  dioxide  to  give  the  sodium  salt  of  diphenyl- 
succinic  acid  (6) 

C6H5-CHNa      HOH     C6H5-CH2 
(a)  +  +2NaOH, 

C6H5-CHNa      HOH 


C6H5-CHNa      CO2     C6H5  •  CH  •  COONa 

I  +  I 

C6H5  CHNa      C02     C6H5  •  CH  •  COONa 


372  THEORIES  OF  ORGANIC  CHEMISTRY 

Oxygen  decomposes  the  sodium  derivatives  of  stilbene  according  to  the 
equation 

C6H5-CHNa      0     C6H5— CH 

I          +  ||  =  ||      H-Na202 

C6H5  CHNa      0     C6H5— CH 

Asymmetrical  diphenylethylene  also  reacts  with  sodium  in  ethereal 
solution  but  the  product  in  this  case  is  a  brick  red  di-sodium  derivative  of 
tetraphenylbutane : 


2(C6H5)2C=CH2+2Na=2  /(C6H5)2C  -  Na  x-*(C6H5)2C.Na  Na-  C(C6H5)2 

-CH2 


/652.a\->652 
\  CH2.../  H2C 


I  II 

The  carbon  atom  in  diphenylethylene  to  which  the  two  phenyl  groups 
are  attached  is  much  more  reactive  towards  metals  than  the  other 
ethylene  carbon  atom  and  Schlenk  therefore  assumes  that  the  first  atom 
of  sodium  adds  in  this  position  to  give  the  above  unstable  intermediate 
product  (I).  He  also  assumes  that  the  rate  of  reaction  with  which  two 
such  radicals  polymerize  is  greater  than  that  with  which  a  second 
sodium  atom  can  add  so  that  the  final  product  is  as  indicated  (II). 
Addition  of  carbon  dioxide  takes  place  readily  as  in  the  preceding  cases : 


(C6H5)2C-CH2CH2C(C6H5)2+C02   - 

Na  Na  COONa       COONa 


OONa       COO 


Benzophenone-phenylimide,  (C6H5)2C=N-C6H5,  reacts  with  sodium 
to  give 

(C6H5)2C— NC6H5 

Na  Na 

and  this  substance  shows  the  same  general  properties  as  others  of  its 
class.  It  reacts  with  water  to  give  a  product  in  which  the  two  atoms  of 
sodium  have  been  replaced  by  hydrogen, 


and  adds  carbon  dioxide  in  the  usual  way  to  give  the  sodium  salt  of  the 
following  dicarboxylic  acid : 

(C6H5)2C—     -N— C6H5 
CO2Na 


INDEPENDENT  EXISTENCE  OF  FREE  ORGANIC  RADICALS    373 

Azobenzene  also  reacts  with  metallic  potassium  to  give  a  product  which 
is  analogous  in  all  respects  to  those  which  have  just  been  described. 
In  the  case  of  the  di-sodium  derivatives  of  the  ketones,  as  for  example 

/ONa 

c 

\Na 

it  should  be  noted  that  the  two  sodium  atoms  are  different  in  their 
properties,  in  that  the  one  which  is  in  union  with  carbon  is  more  reactive 
than  the  other.  Careful  treatment  of  such  a  substance  with  either  oxygen 
or  iodine  might  very  easily  result  in  the  formation  of  compounds  of  the 
metal-ketyl  type,  although  the  latter  are  so  unstable  that  they  would 
immediately  rearrange  to  give  the  free  ketone  and  sodium  peroxide,  in 
the  presence  of  an  excess  of  the  reagent  : 


ONa  C6H5x       /ONa 

+O2=2  >C<          +Na202 

Na 


C6H5x      /ONa  C6H5\ 

2  >C<  +02=2  >C:0+Na202 

CeHsCeH^  P.TT.P.TT  ./ 


There  are,  of  course,  other  interpretations  of  the  action  of  sodium 
upon  ketones  than  that  which  assumes  the  formation  of  compounds 
which  contain  trivalent  carbon.  Schmidlin1  supposes,  for  example 
that  metal-ketyls  are  analogous  in  constitution  to  the  highly  colored 
molecular  compounds  which  are  formed  by  the  action  of  metallic 
chlorides  upon  ketones,  and  this  possible  solution  of  the  problem  will  be 
considered  again  in  some  detail  in  the  chapter  on  Color  and  Consti- 
tution. 

The  question  of  the  existence  of  free  organic  radicals  has  received 
further  elucidation  as  a  result  of  the  recent  investigation  of  W.  Schlenk 
and  his  students  who  have  prepared  and  studied  a  large  number  of  labile 
compounds  which  contain  sodium  in  union  with  organic  radicals. 
Schlenk  and  Ochs2  have,  for  example,  succeeded  in  separating  sodium  tri- 
phenylmethyl  (CeHs^CNa  and  have  found  that  it  very  closely  resembles 
the  alkyl  magnesium  compounds  discovered  by  Grignard,  but  possesses 
even  greater  chemical  reactivity.  Schlenk  and  Holtz3  have  prepared 
the  following  organo-metallic  compounds:  sodium  methyl,  lithium 
methyl,  sodium  ethyl,  lithium  ethyl,  sodium  propyl,  sodium  n-octyl, 

1  "Das  Triphenylmethyl,"  p.  186,  Stuttgart,  1914. 
2Ber.,  49,  608  (1916). 
*Ber.,  60,  262  (1917). 


374  THEORIES  OF  ORGANIC  CHEMISTRY 

sodium  phenyl,  lithium  phenyl  and  sodium  benzyl.  With  the  excep- 
tion of  the  latter: 

NaCH2C6H5 

which  is  intensely  red,  these  compounds  are  all  colorless  and  are  either 
insoluble  and  amorphous  or  else  soluble  in  benzene  and  crystalline.  All 
decompose  without  melting  when  heated  and  are  so  unstable  that  they 
are  spontaneously  inflammable  in  the  air.  The  inflammability  decreases, 
however,  in  proportion  as  the  alkyl  radical  increases  in  size.  The  ques- 
tion as  to  whether  the  metal  is  in  the  same  form  of  combination  in  the 
colored  as  in  the  colorless  compounds  must  be  left  for  detailed  consider- 
ation in  the  next  chapter,  and  the  present  discussion  will  be  confined  to  a 
review  of  certain  properties  of  these  substances  which  are  of  immediate 
interest. 

It  is  well  known  that  nitrogen  does  not  usually  exercise  all  of  its  five 
valencies  in  attracting  similar  atoms  or  groups  of  atoms.  For  example, 
compounds  which  contain  pentavalent  nitrogen  are  stable  only  when 
one  or  even  two  of  the  nitrogen  valencies  are  exercised  in  holding 
groups  which  are  chemically  different  from  the  others,  as  is  the  case  in 
(CH3)4NC1,  (CHa^N-NOs,1  etc.,  while  compounds  such  as  (CH3)5N 
have  not  as  yet  been  obtained  although  various  attempts  have  been 
made  to  prepare  them.  Quite  recently,  however,  Schlenk  and  Holtz  2 
have  succeeded  in  synthesizing  a  substance  in  which  nitrogen  appears 
to  be  in  union  with  five  carbon  residues.  This  has  been  accomplished 
by  the  ingenious  device  of  treating  sodium  triphenylmethyl  with  tetra- 
methylammonium  chloride : 

(C6H5)3CNaH-Cl  •  N(CH3)4=NaCl+  (C6H5)3C  •  N(CH3)4 

The  reaction,  which  seems  to  be  one  of  double  decomposition,  takes 
place  smoothly  and  the  product  consists  of  glistening  red  crystals  which 
have  a  blue  metallic  luster.  A  similar  substance  which  consists  of  a  fine 
red  powder  is  obtained  when  sodium  benzyl  is  treated  with  tetramethyl 
ammonium  chloride : 3 

C6H5  •  CH2  •  Na+ClN(CH3)4=NaCl+C5H6  •  CH2  -  N(CH3)4 

Both  substances  are  extremely  sensitive  to  the  oxygen  of  the  air  and 
react  with  water  and  carbon  dioxide  according  to  the  equations: 
(C6H5)3C  •  N(CH3)4     +HOH=(C6H5)3CH+HON(CH3)4 
CGH5  •  CH2  •  N(CH3)4  +HOH=C6H5  •  CH3 +HON(CH3)4 
(C6H5)3C  •  N(CH3)4    +C02=(C6H5)3  •  C  •  C02  -  N(CH3)4 
Compare  A.  Lachman,  Ber.,  33,  1035  (1900). 
2Ber.,  49,  603(1916). 
3  Ber.,  60,  274  (1917). 


INDEPENDENT  EXISTENCE  OF  FREE  ORGANIC  RADICALS  375 

It  will  be  noted  in  the  last  equation  that  carbon  dioxide  enters  the  mole- 
cule and  assumes  a  position  between  the  triphenylmethyl  radical  and 
the  nitrogen.  This  is  exactly  analogous  to  what  takes  place  in  the  case 
of  sodium  triphenylmethyl,  viz., 


so  that  it  seems  reasonable  to  conclude  that  the  substituted  ammonium 
radical  plays  the  part  of  the  metal  in  triarylmethyl  tetramethylammo- 
nium  compounds.  Taken  as  a  whole  the  phenomenon  serves  to  empha- 
size the  truth  of  a  statement  which  is  very  frequently  made  in  chemistry, 
namely  that  the  most  important  factor  in  determining  the  character 
of  a  chemical  compound  is  not  the  nature  of  the  elements  which  com- 
pose it  but  the  arrangements  of  these  elements  as  shown  in  the  chemical 
constitution  of  the  molecule.1 

The  chemistry  of  free  organic  radicals  has  recently  been  supple- 
mented by  a  study  of  organic  radicals  which  contain  nitrogen.  Devel- 
opments in  this  field  are  due  in  large  measure  to  the  researches  of  H. 
Wieland  who  in  the  course  of  preparing  bi-tertiary  aromatic  hydrazines 
discovered  that  the  stability  of  the  union  between  the  two  nitrogen 
atoms  is  very  greatly  influenced  by  the  character  of  the  aromatic  sub- 
stituents.  For  example,  colorless  tetraphenyl-hydrazine  is  so  unstable 
that  it  readily  dissociates  into  radicals  which  contain  bivalent  nitrogen 
and  which  may  be  distinguished  by  the  fact  that  they  possess  a  yellow 
color. 

(C6H5)2N.N(C6H5)2  -»  2(C6H5)2N 

Decompositions  of  this  type  may  be  retarded  by  the  substitution  of  such 
groups  as  CeHs  and  NO2  or  accelerated  by  the  substitution  of  CHa, 
OCHs,  etc.2  In  the  following  series  of  compounds  for  example  the 
tendency  to  dissociation  increases  in  passing  from  the  first  to  the  last 
member: 


v  /es  esx.  /Cells 

>N—  N<  >N—  N<; 

O2NC6H4/  XC6H4NO2  Cells/  XC6H4NO2 

p-Dinitrotetraphenyl  hydrazine  p-Mononitrotetraphenyl  hydrazine 

I  II 

C6H5-C6H4v  /C6H4  -Cells 

>N-N< 
C6H5  •  C6H4/          XC6H4  •  C6H5  (C6H5)2N  -  N(C6H5)2 

p-Tetrabiphenyl  hydrazine  Tetraphenyl  hydrazine 

III  IV 

1  Compare  Kehrmann,  Ber.,  47,  3055  (1914). 

2  H.  Wieland,  "Die  Hydrazine/'  p.  72,  Stuttgart,  1913;  also  Ber.,  48,  1078,  1098, 
1112  (1915). 


376  THEORIES  OF  ORGANIC  CHEMISTRY 


C6H5\     /C6H5       H3CC6H4x     /C6 

-N 

6H4-CH3 


N-N 


/C6H4-CH 
< 
XC6H4-CH 


p-Ditolyldiphenyl  hydrazine  p-Tetratolyl  hydrazine 

V  VI 


\  /Cells 

>N-N<  >N-N 

/         XC6H4OCH3  H3C.C6H4/          XC6H4CH3 

p-Dianisyl  diphenylhydrazine  p-Tetratolyl  hydrazine 

VII  VIII 

H3COC6H4x            /C6H40CH3  [(CH3)2N  -  C6H4]2  :  N  •  N  : 

>N-N<  [C6H4N(CH3)2]2 
/          XC6H4OCH3 

p-Tetraanisyl  hydrazine  Tetra-  [p-dimethylaminophenyl]  hydrazine 


Since  the  hydrazines  are  colorless  while  the  bivalent  radicals  into 
which  they  dissociate  are  yellow  the  change  may  be  followed  colori- 
metrically.  The  first  two  members  show  no  trace  of  dissociation  but 
succeeding  members  give  evidence  of  an  ever-increasing  dissociation,  if 
comparisons  are  made  at  90°.  Since  the  radicals  which  form  in  this  way 
polymerize  readily  and  since,  therefore,  the  phenomenon  of  color  is  only 
transitory,  dissociation  may  be  detected  more  readily  by  subjecting  the 
solid  hydrazine  to  the  action  of  cathode  rays.  Tetraphenyl-hydrazine, 
for  example,  becomes  intensely  green  under  the  influence  of  cathode 
rays  but  loses  this  color  at  once  when  the  emanation  is  withdrawn.1 
Solutions  of  the  last  three  hydrazines,  on  the  other  hand,  may  be  studied 
spectroscopically  since  it  has  been  observed  that  even  at  low  tempera- 
tures these  three  solutions  contain  considerable  amounts  of  the  free 
radicals.  J.  Piccard2  has,  for  example,  investigated  the  absorption 
spectra  of  tetraanisyl-hydrazine  in  solutions  of  different  concentrations 
and  demonstrated  the  presence  of  a  free  nitrogen  radical  which  is  green  in 
color.  Molecular  weight  determinations,  on  the  other  hand,  have  been 
used  to  follow  the  dissociation  of  tetra-[p-dimethylaminophenyl]- 
hydrazine  and  it  has  been  demonstrated  with  certainty  that  it  is  disso- 
ciated into  its  free  radical  to  the  extent  of  10  per  cent  in  benzene  and  of 
21  per  cent  in  nitrobenzene  solution.3 

Free  diaryl-nitrogen  radicals  may  be  said  to  resemble  the  inorganic 
radical  NO,  although  they  are  much  less  stable.  They  are  less  capable 

1  Hexaphenylethane  behaves  in  the  same  way. 

2Annalen  der  Chemie,  381,  347  (1911);  also  Wieland,  "Die  Hydrazine,"  pp.  75 
and  76. 

3Ber.,  48,  1078  (1915). 


INDEPENDENT  EXISTENCE  OF  FREE  ORGANIC  RADICALS    377 

of  maintaining  an  independent  existence  than  triphenylmethyl,  since 
even  in  solution  they  are  extremely  sensitive  to  changes  in  temperature 
and  rearrange  spontaneously  to  give  a  mixture  of  a  reduction  and  a 
polymerization  product  (perazine) : 


l\ 


H3CO— C6H4x  H3COC6H4s 

4  >N    ->    2  >NH 

H3CO— C6H4/  H3COC6H4/ 

C6H4OCH3 


,-OCH3 

H3CO— ' 

N 

;H4OCH3 

In  boiling  benzene  this  transformation  requires  from  ten  to  thirty  min- 
utes. 

Nitric  oxide  is  the  characteristic  reagent  which  is  used  to  detect 
the  presence  of  radicals  of  the  diary  1-nitrogen  type  just  as  oxygen  is  a 
characteristic  reagent  for  detecting  triarylmethyl  radicals.  Addition 
takes  place  smoothly  according  to  the  equation: 

Ar2N+ NO=Ar2N  •  NO 

and,  in  the  case  of  tetraanisyl-hydrazine  and  tetra-(p-dimethylamino- 
phenyl)-hydrazine,  proceeds  rapidly  even  at  ordinary  temperatures. 
In  the  case  of  those  hydrazines  which  dissociate  only  upon  heating  a 
temperature  of  from  80°  to  100°  is  necessary  for  the  reaction. 

Addition  reactions  between  diary  1-nitrogen  radicals  and  triarylmethyl 
have  been  observed 


Ar2N+C(C6H5)3=Ar2N 

and  the  resulting  compounds  have  been  found  to  be  relatively  very  stable, 
showing  no  tendency  to  decompose  below  temperatures  of  130-140°. 

According  to  the  investigations  of  Wieland  and  Reverdy *  and 
Wieland  and  Offenbacher  radicals  also  exist  which  contain  univalent 
and  tetravalent  nitrogen.  Wieland  with  M.  Offenbacher  2  and  K.  Roth  3 
has  recently  succeeded  in  obtaining  diaryl  derivatives  of  nitrogen  perox- 
ide by  processes  which  involve  the  elimination  of  hydrogen  from  diphenyl 

*Ber.,  48,  1112  (1915). 
*Ber.,  47,  2111  (1914). 
*Ber.,  63,  210(1920). 


378  THEORIES  OF  ORGANIC  CHEMISTRY 

hydroxylamine  and  also  from  certain  of  its  derivatives  in  which  the 
hydrogen  of  the  benzene  ring  has  been  substituted : 

(C6H5)2NOH-(H)  ->  (C6H5)2N=0 

Diphenyl  nitrogen  oxide  itself  is  a  substance  which  crystallizes  well  and 
which  resembles  NO2  in  possessing  a  deep  red  color  and  a  similar  char- 
acteristic band  absorption.  Like  nitrogen  peroxide  this  substance 
behaves  as  if  it  were  a  free  radical.  For  example,  it  combines  readily 
with  nitrogen  peroxide : 

O 

(C6H5)2NO+NO2    ->    (C6H5)2N- 

O 

-»    (N02C6H4)(C6H5)N=0    ->     (02NC6H4)(C6H5)N.N02 
-»    (02NC6H4)2NOH    -»    (02NC6H4)2N=0 

The  product  which  is  obtained  in  this  way  is  capable  of  reacting  with 
certain  other  similar  unsaturated  substances  such  as  triphenylmethyl 
to  give  corresponding  addition  products : 

O 

(02NC6H4)2N=0+C(C6H5)3     ->     (02NC6H4)2N.C(C6H5)3 

Another  radical  containing  tetravalent  nitrogen  is  supposed  to  be 
formed  as  an  intermediate  product  when  quaternary  alkyl  derivatives 
of  pyridine  are  treated  with  sodium  amalgam.  The  final  product  of 
this  reaction  is  N,N'-dialkyl-7,  7r-tetrahydro-7,  7r-dipyridyl: 

/CH=CH     /CH=CH 
R-N      \CH-CH     \NR 


\CH=CH     \CH=CH 

The  same  substance  is  formed  as  a  result  of  the  electrolysis  of  quater- 
nary alkyl  pyridine  salts.  According  to  Bruno  Emmert 1  the  mechan- 
ism of  this  reaction  consists  in  the  formation  of  a  primary  radical  I, 
which  then  rearranges  to  give  II  as  is  expressed  below, 

,CH— CH 


\CH=CH 
II 

.,  53,  370(1920). 


INDEPENDENT  EXISTENCE  OF  FREE  ORGANIC  RADICALS  379 

and  the  latter  then  polymerizes  to  form  the  dipyridyl  derivative.  The 
product  dissociates  again  into  the  free  radical  when  dissolved  in  certain 
solvents  such  as  alcohol  for  example.  These  solutions  possess  a  char- 
acteristic deep  blue  color,  which  disappears  under  the  action  of  iodine: 

2RNC5H5+I2=2I  •  (R)  •  NC5H5 


Sulphur  radicals  have  been  made  the  subject  of  an  investigation 
by  H.  Lecher,1  who  has  observed  that  colorless  diphenyl  disulphide, 
CeHsS-SCeHs,  dissolves  in  all  indifferent  solvents  to  give  light-yellow 
solutions  which  become  deeply  colored  upon  heating.  Moreover  the 
substance  melts  to  a  yellow  liquid  which  becomes  colorless  again  upon 
solidification.  Similar  phenomena  have  been  observed  in  the  case  of 
other  sulphur  compounds,  and  the  question  therefore  arises  as  to  whether 
this  change  in  color  is  not  due  to  the  dissociation  of  the  different  sub- 
stances into  their  corresponding  free  radicals.  The  fact  that  disulphides 
show  a  certain  analogy  in  constitution  to  the  hexaaryl-e  thanes  and  the 
tetraaryl-hydrazines  makes  it  seem  probable  that  such  is  the  case. 
This  reaction  cannot,  however,  be  followed  colorimetrically  and  the 
chemical  relationships  do  no  more  than  point  to  a  weakening  of  the  link- 
ages between  the  sulphur  atoms  under  conditions  which  might  in 
themselves  be  sufficient  to  account  for  a  change  in  color.  For  example, 
diphenylsulphide  and  p-dimethylanilino-disulphide  combine  with  metals 
according  to  the  equations  : 

C6H5  -  S  •  S  -  C6H5+2Na  =  2C6H5  •  SNa 
(H3C)2N  •  C6H4  •  S  •  S  •  C6H4N(CH3)2  +2Na  =  2(H3C)2NC6H4  •  SNa 


The  second  of  these  two  substances  also  adds  triphenylmethyl  to  give 
l-dimethylaminophenyl-4  triphenylmethyl  sulphide  : 

(H3C)2N  •  C6H4  -  S  -  S  -  C6H4  •  N(CH3)2  +2(C6H5)3C= 

2(H3C)2NC6H4  •  S  •  C(C6H5)3 

In  conclusion  it  may  be  pointed  out  that  the  conception  of  the  inde- 
pendent existence  of  free  organic  radicals  is  a  conception  which  has 
been  completely  abandoned  by  chemists  for  almost  a  century.  It  may 
be  recalled  that  in  the  time  of  Liebig  the  conception  of  a  radical  included 
the  possibility  of  its  actual  existence,  but  that  when  all  attempts  to 
isolate  free  radicals  failed,  the  word  came  to  signify  nothing  more  than 
an  atomic  complex  which  remained  unchanged  during  the  course  of 

1  Ber.,  48,  524,  1425  (1915);  also  "  Untersuchungen  iiber  aromatische  Disulfide," 
Hans  Lecher.  Munich,  1920. 


380  THEORIES  OF  ORGANIC  CHEMISTRY 

chemical  reaction.  It  seems  obvious  at  the  present  time  that  those 
chemists  who  sensed  the  truth  most  clearly  were  those  who  believed 
that  radicals  were  not  only  actually  capable  of  maintaining  an  inde- 
pendent existence  but  that  they  also  possessed  great  chemical  reactivity. 
On  these  grounds  it  is  easy  to  understand  why  so  few  radicals  have  as 
yet  been  isolated  since  they  must  be  imagined  as  forced  by  their  very 
nature  into  combinations  with  other  atoms  or  groups  of  atoms  or  even 
into  combinations  with  each  other. 

H.  Wieland  limits  the  conception  of  free  radicals  to  "free  unsat- 
urated  complexes  which  are  atomic  in  character  and  in  which  at  least 
one  component  exhibits  an  abnormal  valency."  Radicals  differ  from 
ions  since  the  latter  conception  carries  with  it  the  idea  of  an  elec- 
tric charge.  While  at  the  present  time  only  a  few  chemists  assume 
the  existence  of  free  radicals  in  interpreting  chemical  change  it  seems 
probable  that  this  conception  must  in  the  future  come  into  more  and 
more  general  use  and  open  the  way  to  a  freer,  but  at  the  same  time  more 
exact  exposition  of  the  mechanism  of  chemical  transformations.  While 
it  may  be  said  that  explanations  of  this  kind  have  been  applied  fre- 
quently in  the  past  in  interpreting  the  most  varied  chemical  processes, 
an  important  difference  between  the  old  and  the  new  method  is  to  be 
found  in  the  fact  that  in  the  past  such  explanations  have  not  seemed 
capable  of  experimental  verification.  Such  experimental  verification  is 
obviously  essential  to  correct  thinking  since  even  in  cases  where  the 
formulation  of  a  reaction  appears  to  point  most  decisively  to  the  forma- 
tion of  free  radicals  as  intermediate  products,  the  greatest  caution  must 
be  exercised  before  such  a  conclusion  can  be  accepted.  It  must  be 
remembered,  too,  in  this  connection  that  the  actual  existence  of  free 
radicals  is  at  best  very  difficult  to  demonstrate  experimentally  since  new 
methods  must  be  worked  out  in  each  individual  case.  According  to 
H.  Wieland  l  explanations  of  this  kind  are  open  to  many  sources  of  error. 
For  example,  the  synthesis  of  ethane  from  methyl  bromide  by  the  action 
of  sodium,  and  the  formation  of  diphenyl  from  benzene  by  the  action  of 
heat,  would  seem  at  first  to  depend  upon  the  polymerization  of  the  free 
radicals  CHs  and  CeHs  which  might  be  supposed  to  form  as  intermediate 
products  during  the  course  of  these  respective  reactions.  There  are, 
however,  other  explanations  of  these  phenomena  which  are  equally 
plausible.  For  instance  benzene  itself  might  polymerize  to  dihydrodi- 
phenyl: 


L2 

^er.,  48,  1098  (1915). 


INDEPENDENT  EXISTENCE  OF  FREE  ORGANIC  RADICALS    381 

and  the  splitting  off  of  hydrogen  might  then  follow  as  a  secondary  reac- 
tion. And  again  in  the  synthesis  of  ethane  it  might  be  supposed  that 
instead  of  a  free  methyl  radical,  a  very  reactive  sodium  compound  is 
formed  as  the  intermediate  product  in  the  reaction.1  Good  reasons  for 
rejecting  these  and  other  possible  interpretations  of  such  reactions  must 
obviously  be  found  before  an  explanation  which  is  based  upon  the 
assumption  of  free  radicals  can  unreservedly  be  accepted. 

It  may  be  added  that  in  the  case  of  the  benzidine  rearrangement 
and  of  the  analogous  Fischer-Hepp  rearrangements  of  diarylnitrosamines 
into  p-nitrosodiarylamines,  Wieland 2  would  exclude  any  explanation 
which  presupposes  a  dissociation  of  the  respective  substances  into  the 
free  radicals  CeHsNH —  and  (CcHs^N.  As  an  argument  against  such 
an  assumption  in  the  case  of  the  first  type  of  transformation  Wieland 
reasons  that  if  tetraphenyl-hydrazine,  which  undergoes  the  benzidine 
rearrangement  as  readily  as  hydrazobenzene,  actually  dissociated  into 
the  free  radical  (Cells) 2N,  no  diphenylbenzidine  could  possibly  be 
formed.  In  the  second  case  he  argues  simply  that  — NO  and 
cannot  be  made  to  combine  to  give  p-nitrosodiphenylamine. 

1  Am.  Chem.  Jour.,  29,  588  (1903);  Ber.,  48,  1100  (1915). 
*Ber.,  48,  1100(1915). 


CHAPTER  XVI 


THE  RELATIONSHIP  BETWEEN   COLOR  AND   CHEMICAL 
CONSTITUTION 

WHEN  white  light  passes  through  a  prism  it  is  broken  up  into  waves 
of  different  lengths  corresponding  to  the  colors  red,  orange,  yellow, 
green,  blue,  indigo,  violet.  This  is  due  to  the  fact  that  light  of  different 
wave  lengths  suffers  different  degrees  of  refraction  in  passing  from 
a  less  into  a  more  dense  medium.  For  example  light  of  the  Frauenhofer 
line  A  in  the  red,  which  has  a  wave  length  equal  to  0.00076  mm.,  suffers 
least  refraction,  while  that  of  H  in  the  violet  which  has  a  wave  length 
equal  to  0.000393  mm.  suffers  the  greatest  refraction.  All  of  the  dif- 
ferent kinds  of  light  are  transmitted  through  the  air  at  approximately 
the  same  velocity  because  of  wave  motions  in  the  ether. 

If  during  the  transmission  of  light  by  transverse  wave  motion  a 
given  particle,  such  as  an  electron,  an  ion,  etc.,  moves  from  o  to  a, 
back  again  to  o,  from  o  to  b  and  back  to  o,  it  is  said  to  have  performed  a 
complete  oscillation.  The  time  required  for  a  complete  oscillation  is 
spoken  of  as  a  period  (T)  while  the  distance  traversed  by  the  wave  during 
this  time  is  referred  to  as  a  wave  length  X.  The  reciprocal  of  T  (or  T) 
represents  the  number  of  oscillations  and  the  frequency  v,  the  number  of 
oscillations  in  2ir  seconds.  This  may  be  expressed  by  the  equation: 

27T 

v  =  — 

T 

The  amplitude  of  an  oscillation  is  measured  by  the  distance  between 
the  position  of  equilibrium  of  a  given  particle  and  the  position  furthest 
from  this  which  is  assumed  by  the  particle  in  any  given  swing.  This  is 
represented  by  the  line  cd  in  the  accompanying  diagram. 

d 


x 
382 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      383 

Different  colors  correspond  to  light  of  different  wave  lengths  which 
are  usually  measured  in  terms  of  micro-millimeters,  w. 

1  M/x=0.000001    mm.=10-6  mm. 

Light  waves  are  also  measured  in  terms  of  Angstrom  units  —  : 
1  A=10-7  mm.=10-8  cm. 

The  reciprocal  of  this  value,  which  equals  the  number  of  oscillations  to 
1  mm.,  is  used  in  graphic  representations.  For  example,  the  D  line  for 
sodium  light  has  a  wave  length  of  0.0005892  mm.  =  589.2  MM  =  5892A. 

107 
The  number  of  oscillations  is  r^rz  =  1697  to  1  mm.     The  wave  length 


of  Frauenhofer's  line  A  in  the  red  equals  0.00076  mm.,  or  760  w,  and 
that  of  the  line  H  in  the  violet  0.000393  mm.,  or  393  MM,—  these  values 
marking  the  limits  of  the  visible  spectrum.  White  light  is,  however, 
made  up  of  other  rays  in  addition  to  these  which,  although  they  cannot 
be  perceived  by  the  eye,  may  be  measured  indirectly.  These  rays  lie 
beyond  the  regions  of  the  red  and  the  violet  and  are,  therefore,  referred 
to  as  the  infra-red  and  the  ultra-violet  respectively.  Regions  of  the 
spectrum  which  correspond  to  the  following  series  of  wave  lengths: 


infra-red  rays 
=  0.06  -0.00076  mm. 

visible  rays 
=  0.00076  -0.0004  mm. 

ultra-violet  rays 
=  0.0004-0.  0001  mm. 

have  been  more  or  less  carefully  investigated.  A  consideration  of  these 
figures  clearly  demonstrates  that  visible  rays  constitute  only  a  relatively 
small  fraction  of  the  total  spectrum.  This  fact  has  not  always  been 
fully  recognized  in  the  past  and  as  a  result  somewhat  misleading  con- 
ceptions in  regard  to  absorption  phenomena  have  been  current. 

If  ordinary  white  light  is  allowed  to  pass  through  colored  bodies  or 
colored  solutions,  certain  rays  are  absorbed  while  those  which  pass 
through  unchanged  give  a  characteristic  color  to  the  body  or  solution. 
If,  for  example,  red  is  absorbed  the  substance  will  appear  blue-green. 
In  other  words,  red  and  blue-green  are  complimentary  colors  and  when 
mixed  together  give  the  effect  of  white  light.  If,  on  the  other  hand, 
light  which  has  passed  through  a  colored  substance  is  resolved  into 
its  component  colors  by  means  of  a  prism,  it  will  be  found  to  lack  certain 
colors.  The  spectrum  which  is  obtained  in  this  way  is  called  an  absorp- 
tion spectrum,  the  absorption  being  referred  to  as  unilateral  if  it  takes 
place  in  only  the  blue  or  the  red  respectively,  and  bilateral  if  it  occurs 


384  THEORIES  OF  ORGANIC  CHEMISTRY 

in  both  regions.  An  absorption  spectrum  is  said  to  be  banded  if  dark 
bands  are  formed  as  the  result  of  absorption. 

Many  substances  which  appear  to  be  colorless  have  been  found  to 
possess  absorption  spectra.  In  such  cases  absorption  takes  place  in  the 
region  of  the  infra-red  or  of  the  ultra-violet  and  not  in  the  region  of 
the  visible  spectrum.  Benzene,  for  example,  although  it  is  apparently 
an  absolutely  colorless  substance,  possesses  a  very  complicated  absorp- 
tion spectrum  which  may  be  readily  detected  by  means  of  the  photo- 
graphic plate.  In  a  strictly  scientific  sense  benzene  should,  therefore, 
be  regarded  not  as  a  colorless  but  as  a  definitely  colored  compound. 

Absorption  relationships  in  ultra-violet  have  been  very  carefully 
studied  by  W.  N.  Hartley.1  These  investigations  have  been  of  value 
because  they  have  served  to  throw  new  light  upon  many  of  the  problems 
of  organic  chemistry.  Hartley  found,  for  example,  that  aromatic 
compounds  such  as  benzene  and  its  derivatives,  pyridine,  pyrazine,  etc., 
exhibit  selective  absorption  while  derivatives  of  the  fatty  hydrocarbons, 
on  the  other  hand,  usually  show  continuous  absorption.  Certain  excep- 
tions to  the  latter  statement  are  to  be  found,  however,  among  the 
ketones,  diketones  and  similar  substances,  all  of  which  possess  strongly 
banded  absorption  spectra. 

While  benzene  absorbs  in  ultra-violet  and  appears  colorless,  nitro- 
benzene and  aniline  absorb  within  the  limits  of  the  visible  spectrum 
and  appear  light  yellow  in  color.  This  seems  to  indicate  that  the  sub- 
stitution of  the  hydrogen  of  benzene  by  NO2  or  NH2  respectively  tends 
to  shift  the  absorption  bands  of  the  substance  from  the  invisible  region 
into  the  visible  region  of  the  spectrum.  If  such  an  assumption  is  correct 
it  should  be  possible  to  reverse  this  process  and  to  shift  absorption  in  the 
opposite  direction,  viz.,  from  infra-red  — >  red  — >  violet  — >  ultra-violet. 
This  can  in  fact  be  accomplished. 

Within  the  limits  of  the  visible  spectrum  changes  in  color  from 
yellow  — >  orange  — >  red  — >  blue  — >  violet  are  referred  to  as  a  "  deepening 
color  "  and  are  supposed  to  be  brought  about  by  the  action  of  so-called 
bathochrome  groups,  while  changes  in  the  opposite  direction  are  said 
to  "  lighten  the  color  "  and  are  brought  about  by  so-called  hypso- 
chrome  groups.2 

In  order  to  explain  the  emission  and  absorption  of  light  Angstrom 
made  the  assumption  in  1855  that  a  definite  analogy  existed  between 
the  phenomena  of  light  and  sound.  He  supposed  that  the  same  theory 
which  interpreted  resonance  as  due  to  the  reinforcement  of  a  particular 

1  Jour.  Chem.  Soc.,  53,  641  (1888);  73,  695  (1898);  77,  846  (1900). 

2  H.  T.  Bucherer,  "Lehrbuch  der  Farbenchemie,"  Leipzig,  1914,  p.  245;  H.  Kauff- 
mann,  "Valenzlehre,"  1911,  p.  433;  Urbain,  Compt.  rend.,  167,  594  (1913). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      385 

sound,  might  be  applied  to  explain  absorption.  In  the  latter  case  a 
ray  of  given  wave  length  might  be  assumed  to  be  neutralized  by  the 
movements  of  molecules  or  atoms  which  possessed  the  same  period  of 
vibration. 

According  to  the  electro-magnetic  theory  of  light  the  relation 
between  emission  and  absorption  is  explained  by  supposing  that  light 
waves  possess  the  power  of  exciting  the  electrons  of  the  atoms  to  move- 
ments which  are  electrical  in  character.  These  oscillations  of  electrons 
may  be  such  that  energy  is  radiated  in  all  directions,  in  which  case  gen- 
eral absorption  takes  place.  In  order  to  account  for  selective  absorption 
the  further  assumption  is  made  that  in  such  cases  the  electrons  are  more 
readily  affected  by  certain  vibrations  than  by  others.  In  this  way 
waves  of  light  which  have  approximately  the  same  periods  of  oscillation 
as  the  electrons  are  damped  or  absorbed. 

J.  Stark  assumes  that  valence  fields  are  present  on  the  atoms  and 
that  these  oscillate  about  positions  of  equilibrium  which  have  their 
centers  at  points  somewhere  between  the  negative  electrons  and  the 
positive  zones  on  the  surface  of  the  atoms.  The  vibrations  of  these 
valence  fields  depend  upon  simultaneous  movements  of  the  electrons 
which  may  or  may  not  be  accompanied  by  oscillations  of  the  atoms 
themselves.  The  vibrations  of  the  valence  fields  of  the  chemical  atoms 
are  assumed  to  be  electro-magnetic  in  character,  and  may  be  accom- 
panied by  either  the  emission  or  absorption  of  light.  It  follows  that 
only  those  rays  of  light  which  possess  the  same  frequencies  as  the  valence 
fields  of  the  chemical  atoms  will  be  absorbed  and  that  the  measurement 
of  the  absorption  of  a  given  compound  therefore  affords  a  means  for 
determining  the  character  of  the  valence  fields  which  are  present  on  its 
atoms. 

Stark  makes  the  further  assumption  that  the  valence  electrons  which 
are  present  on  or  above  the  surface  of  the  atom  represent  the  centers 
of  the  so-called  band  spectra.  This  assumption  is  based  upon  a  sys- 
tematic study  of  the  oscillation  of  particular  valence  fields  in  the  case  of 
individual  atoms  (monatomic  gases)  and  also  in  the  case  of  molecules, 
within  and  without  the  influence  of  intermolecular  unions.  As  a  result  of 
this  investigation  it  was  possible  to  draw  certain  fairly  definite  conclu- 
sions in  regard  to  the  character  of  the  vibrations  of  the  valence  fields  of 
electrons  alone  or  of  atoms  and  groups  of  atoms  alone,  or  of  valence 
electrons  coupled  with  atoms,  etc.  The  reader  must  be  referred  to 
Stark's  original  presentation  of  the  subject  for  a  full  description  of  the 
manner  in  which  banded  absorption  may  be  brought  about  in  the  infra- 
red, in  the  ultra-violet  and  in  the  visible  regions  of  the  spectrum.  For 
purposes  of  the  present  discussion  the  general  statement  must  suffice 


386  THEORIES  OF  ORGANIC  CHEMISTRY 

that  under  favorable  conditions  the  characteristic  behavior  of  every 
vibrating  particle  in  a  given  molecule  may  be  determined,  and  that  as  a 
result  it  has  been  discovered  that  the  absorption  spectrum  of  any  chem- 
ical individual  is  compounded  of  a  great  number  of  separate  parts  which 
represent  respectively  the  absorption  of  the  different  units  present  in 
the  complex.  According  to  Lifschitz,  for  example,  the  broad  bands 
which  are  so  frequently  observed  in  connection  with  the  spectra  of  organic 
compounds,  actually  consist  of  a  series  of  fine  single  bands  which  follow 
each  other  so  closely  that  they  appear  to  merge.  They  may  be  regarded 
as  corresponding  to  a  series  of  individual  resonators  possessing  frequencies 
which  are  approximately  the  same. 

Since  liquids  and  solids  do  not  possess  sharply  defined  absorption 
bands,  it  was  impossible  in  the  case  of  organic  compounds,  with  very 
few  exceptions,  to  study  directly  the  influence  of  individual  atoms  upon 
the  absorption  spectrum  of  the  molecule.  Insight  into  atomic  relation- 
ships in  organic  chemistry  was,  nevertheless,  obtained  indirectly  from 
a  study  of  changes  in  absorption  which  result  from  substitutions  in  given 
molecules.  Here  interest  at  first  centered  in  the  behavior  of  so-called 
chromophore  and  auxochrome  groups  because  it  was  generally  believed 
that  the  presence  of  such  groups  in  a  given  molecule  was  sufficient 
to  account  for  the  phenomenon  of  color.  Later  the  presence  of  certain 
characteristic  bands  in  the  absorption  spectra  of  different  substances 
came  to  be  associated  with  a  particular  structure  within  the  molecule 
and  in  this  way  it  became  possible  to  detect  the  presence  of  the  benzene 
ring,  the  dicarbonyl  grouping,  the  enol-keto  grouping,  etc.  These 
observations  served  to  establish  a  tentative  relationship  between  the 
optical  properties  of  a  particular  substance  and  the  chemical  constitu- 
tion of  its  molecule,  and  marked  the  beginning  of  very  important  devel- 
opments in  the  field  of  physical  chemistry. 

This  branch  of  chemical  science  has  gradually  grown  until  at  the 
present  time  the  absorption  spectra  of  organic  compounds  afford  a  much 
more  accurate  and  minutely  differentiated  representation  of  the  relation- 
ships which  exist  between  the  different  atoms  in  the  molecule  than  is 
afforded  by  structural  or  constitutional  formulas.  Indeed  the  means 
for  expressing  interatomic  relationships  have  been  so  enlarged  and 
enriched  through  a  study  of  absorption  phenomena  that  a  direct  measure 
of  the  strength  of  different  forms  of  union  between  the  atoms  of  a  mole- 
cule is  now  possible.  It  must  be  added,  however,  that  up  to  the  present 
time  the  results  which  have  been  obtained  by  the  application  of  optical 
measurements  lack  uniformity.  This  is  due  to  the  fact  that  no  single 
method  for  the  determination  and  graphic  representation  of  absorption 
spectra  has  been  found  acceptable  by  all  investigators.  It  follows  that 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      387 

any  general  comparison  of  the  absorption  spectra  of  different  compounds 
is  obviously  open  to  error  and  that  this  situation  will  continue  until  a 
greater  uniformity  of  method  prevails.  Various  ways  for  determining 
the  absorption  spectra  of  organic  compounds  have  been  suggested  by 
Hartley,  Baly,  J.  Stark,  Henri,  Weigert,  K.  Schaefer  and  others  and 
these  may  now  be  considered  briefly. 

If  light  of  a  definite  intensity  penetrates  a  solid  body  a  fraction  of  its 
intensity  is  absorbed  and  the  total  intensity  is  thereby  weakened. 
Intensity  of  light  is  also  weakened  in  direct  proportion  to  the  depth  to 
which  it  penetrates  a  given  body.  If  during  absorption  all  fields  of  light 
absorb  more  or  less  equally,  absorption  is  said  to  be  "  continuous  " 
but  if  certain  regions  are  markedly  weakened  as  compared  with  other 
adjacent  regions,  absorption  is  said  to  be  "  selective." 

According  to  Lambert  the  intensity  of  light  of  definite  wave  lengths 
depends  upon  the  relative  thickness  of  the  layers  through  which  it 
passes  viz., 

J  =  JQe~Kd 

where  Jo  represents  the  intensity  of  light  entering  a  body;  J  the  inten- 
sity of  light  leaving  the  body;  d,  the  thickness  of  the  layer  through 
which  the  light  passes;  e  the  base  of  the  natural  logarithms  (2.71828); 
and  K,  a  constant  which  depends  upon  the  nature  of  the  absorbing 
substance  and  upon  the  respective  wave  length  of  light. 

It  follows  that  the  intensity  may  be  kept  constant  if  d  varies  inversely 
as  the  so-called  coefficient  of  absorption,  K.  The  expression  may  be 
rearranged  to  read 

ln^  =  Kd    or    logJ-^  =  kdl 
J  J 

Moreover  if  -7-  =77:,  it  follows  that  k  =  ,. 
J  Q     1U  a 

In  accordance  with  these  deductions  Bunsen  and  Roscoe  have  called  K 
the  coefficient  of  extinction  and,  in  the  case  of  solids,  defined  it  as  the 
reciprocal  of  the  value  represented  by  a  layer  of  such  thickness  that  the 
intensity  of  light  would  be  weakened  by  one-tenth  in  passing  through  it. 
In  other  words  the  coefficient  of  extinction  is  a  measure  of  the  strength  of 
absorption,  and  may  be  represented  by 


In  the  case  of  solutions  2  the  same  general  rule  holds,  assuming  of 
course  that  the  solvent  itself  does  not  absorb  so  that  given  a  solution  of 

1  A'  =2.3026  k. 

2  Ley,  "Beziehungen  zwischen  Farbe  und  Konstitution,"  p.  59. 


388  THEORIES  OF  ORGANIC  CHEMISTRY 

definite  concentration  absorption  will  depend  upon  the  thickness  of 
the  layer  through  which  light  passes.  In  the  case  of  solutions  of  dif- 
ferent concentrations,  on  the  other  hand,  Beer  has  discovered  that 
absorption  is  proportional  to  the  concentration,  or 

e  =  log^  =  k.  c.d. 
J 

where  c  equals  the  concentration  of  a  given  solution.  It  follows  that  the 
same  effect  may  be  produced  by  changing  the  thickness  of  the  layer  and 
keeping  the  concentration  constant,  as  by  changing  the  concentration 
and  keeping  the  thickness  of  the  layer  constant.  In  other  words,  the 
absorption  will  remain  the  same  if  the  thickness  of  the  layer  through 
which  light  passes  is  inversely  proportional  to  the  concentration  of  the 
solution.  Thus  a  layer  of  a  given  concentration  is  equal  to  another  of 
double  the  thickness  and  one-half  the  concentration.  This  is  known 
as  Beer's  law  and  methods  of  colorimetry  and  spec  tro-analy  sis  are 
founded  upon  it. 

While  Beer's  law  holds  in  a  great  number  of  instances,  certain  impor- 
tant exceptions  have  been  observed  which  are  of  interest  because  they 
help  to  elucidate  the  chemical  nature  of  solution.  It  seems  probable 
that  Beer's  law  holds  in  every  case  where  the  composition  of  the  sub- 
stance is  unchanged  by  solution.1  If,  however,  polymerization  occurs 
as  of  An  +±  nA  or  dissociation  as  of  A  B  +±  A  '+B',  or  if  the  particular 
solvent  affects  the  condition  of  equilibrium  in  a  chemical  system  so  that 
changes  in  mass  take  place,  variations  from  Beer's  law  must  result.2 

J.  Piccard  3  has  investigated  the  phenomena  of  dissociation  in  solu- 
tion from  an  experimental  and  also  from  a  purely  theoretical  point  of 
view  and  has  formulated  a  so-called  colorimetric  law  of  dilution  which 
combines  the  law  of  mass  action  and  Beer's  law.  If  a  substance  A  dis- 
sociates in  solution  to  form  a\,  0,2,  .  .  .  an  so  that 
A  <± 


and  if  the  molecular  concentrations  of  these  substances  are  equal  to 
C,  ci,  c2  ......  cn  respectively,  it  follows  that 

C 

—  =  Const. 

Cn 

If  it  now  happens  that  dilution  favors  the  formation  of  an,  which  is 
colored,  at  the  expense  of  A  which  is  colorless,  it  is  obvious  that  the 
effect  of  dilution  will  be  to  decrease  C  more  rapidly  than  cn  and  the 

1  A.  Hantzsch,  Ber.,  50,  1414  (1917). 

2  Ber.,  45,  554  (1912);   Ley,  "Farbe  und  Konstitution  bei  Organischen  Verbind- 
ungen,"  p.  61,  Leipzig,  1911. 

3  Annalen  der  chemie,  381,  347  (1911);  also  Hantzsch,  Annalen  der  chemie  384, 
135  (1911). 


RELATION  BETWEEN  COLO&  AND  CHEMICAL  CONSTITUTION      389 

observed  phenomena  will  therefore  appear  to  deviate  from  Beer's  law. 
It  has  been  demonstrated  experimentally  by  Piccard  in  the  case  of 
ethereal  solutions  of  hexaphenylethane  that  the  color  of  the  solution 
becomes  more  and  not  less  intense  with  successive  dilutions  until  at  very 
great  dilutions  it  is  orange-yellow.  In  this  case  dilution  obviously 
favors  the  formation  of  triphenylmethyl  : 


^    2(C6H5)3C 

Colorless  Yellow 

Wieland  1  has  been  able  to  point  out  quite  recently  that  in  the  case  of 
hexaphenylethane  in  solution  in  benzene  it  may  be  demonstrated  experi- 
mentally that  the  substance  on  dilution  obeys  both  the  law  of  mass 
action  and  Beer's  law.  Supposing  that  the  experiment  is  conducted 
with  great  care  under  the  proper  conditions,  it  will  be  observed  that  in 
the  first  second  after  dilution  the  color  of  the  solution  becomes  paler 
and  almost  seems  to  disappear.  It  thus  obeys  Beer's  law  and  suffers  a 
loss  of  color  proportional  to  dilution.  Immediately  following  this, 
however,  a  deepening  of  color  takes  place  and  the  solution  becomes 
bright  yellow.  This  change  is  due  to  the  establishment  of  a  new  condi- 
tion of  equilibrium  and  indicates  the  presence  of  a  mixture  in  which  the 
concentration  of  the  colored  component  (cn)  is  greater  than  that  of  the 
colorless  component  (C). 

It  is  obviously  possible  to  apply  Beer's  law  with  a  view  to  deter- 
mining whether  a  colored  substance  suffers  a  change  in  constitution  upon 
solution.  If,  for  example,  a  substance  is  dissolved  in  a  given  solvent 
and  the  intensity  of  its  absorption  in  that  solvent  is  determined  before 
and  after  dilution,  it  follows  that  if  it  is  found  to  obey  Beer's  law  exactly 
no  change  in  its  constitution  can  have  occurred.  Such  determinations 
are  valuable  because  by  means  of  them  it  is  possible  to  follow  the  effect 
of  solution,  temperature,2  etc.,  upon  the  chemical  constitution  of  col- 
ored compounds. 

In  order  to  define  the  color  of  a  substance  exactly  it  is  necessary  to 
know  what  fraction  of  light  of  a  specific  wave  length  it  absorbs  under 
a  given  set  of  conditions.  After  this  has  been  determined  in  as  many 
different  fields  of  the  spectrum  as  possible  and  the  relation  of  the  sub- 
stance to  light  of  different  wave  lengths  established,  an  exact  conception 
of  its  power  of  absorption  is  obtained. 

Two  general  methods  for  the  measurement  of  absorption  are  in  use. 
The  first  consists  in  determining  the  extinction  coefficient,  —  or,  in 
the  case  of  solutions,  the  molecular  extinction,3  —  in  as  extended  a  field 

'Ber.,  48,  1097(1915). 

2  Compare  K.  Schaefer,  Zeitschr.  angew.  Chemie,  33,  26  and  following  (1920). 

Coefficient  of  Extinction 

3  Molecular  Extinction  =  -  —  -  . 

Concentration 


390 


THEORIES  OF  ORGANIC  CHEMISTRY 


of  the  visible  spectrum  as  possible.  This  method  is  very  exact  but 
is  limited  in  its  application  to  substances  absorbing  within  the  field  of 
the  visible  spectrum.  The  second  method  is  less  exact  but  much  more 
general  in  its  scope.  It  has  been  developed  by  Hartley 1  and  is  in  current 
use  in  the  case  of  all  determinations  which  involve  solutions.  Hartley's 
procedure  is  to  dissolve  the  molecular  weight  of  the  substance  expressed 
in  milligrams  in  a  known  volume  of  solvent  and  to  photograph  the 
spectrum  in  layers  of  decreasing  thickness  at  definite  intervals,  say  of 
1  mm.  When  a  final  thickness  of  1  mm.  has  been  reached  the  solution 
is  diluted  to  a  definite  strength,  and  the  process  repeated  until  the 
solution  reaches  a  state  of  such  dilution  as  to  be  non-absorbent.  The 
results  are  set  forth  by  plotting  the  oscillation  frequencies  of  the  limits 
of  the  absorption  bands  against  the  logarithms  of  the  respective  thick- 
nesses of  the  layers.2  When  the  points  obtained  in  this  way  are  joined, 
the  absorption  curve  or  a  curve  showing  the  molecular  vibrations,  of  the 
substance  is  formed. 

The  following  illustration  shows  the  appearance  of  such  a  curve :  3 


Number  of  Oscillations 


-3*10  ram 


10mm  N/100 


10  mm   N/1000 


10  mm.  N/10  000 


1.0 


10-mm.  N/100  000 


1  Jour.  Chem.  Soc.,  47,  685  (1885). 

2  Jour.  Chem.  Soc.,  85,  1029  (1904). 
3Ber.,  43,  1189  (1910). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     391 

By  the  application  of  these  methods  the  absorption  of  a  great  many 
substances  has  been  measured  and  as  a  result  the  optical  effect  of 
the  presence  of  different  groups  in  the  molecule  has  been  determined 
with  great  exactness.1  Moreover  a  comparative  study  of  the  absorp- 
tion spectra  of  different  substances  aids  in  the  interpretation  of  their 
structural  relationships.  For  example,  Hantzsch  recently  discovered  a 
new  absorption  spectrum  for  nitro-compounds,  with  the  result  that  it  is 
now  thought  that  nitro-groups  may  be  present  in  organic  molecules  in 
any  of  three  different  forms  of  combination,  each  of  which  possesses 
a  characteristic  absorption  spectrum.  These  are  differentiated  as  true 
nitro-groups  (NCfe),  aci-nitro  groups  (0=N-O  —  ),  and  the  new  so-called 
"  conjugated  aci-nitro  groups."2 

The  results  which  are  obtained  by  use  of  the  Hartley-Baly  method 
are,  however,  frequently  very  misleading,  as  J.  Bielecki  and  V.  Henri 
and  also  F.  Weigert  have  recently  demonstrated,3  and  it  has  therefore 
been  urged  that  this  method  be  abandoned  in  favor  of  one  which  is  more 
exact.4  The  substitute  which  has  been  suggested  by  J.  Bielecki  and 
Henri  consists  in  the  quantitative  determination  of  absorption  cf  ultra- 
violet by  the  photometric  comparison  of  the  intensity  of  different  lines 
of  definite  wave  lengths  after  these  lines  have  passed  through  layers  of 
different  thickness  of  the  solution  to  be  investigated.  For  exact  details 
in  regard  to  procedure  the  reader  is  referred  to  the  original  paper.5  The 
curves  which  serve  as  a  graphic  representation  of  the  phenomena  are 
plotted  upon  abscissas  which  represent  either  the  wave  length  (X)  or 
the  oscillation  frequencies  per  second  (?;X10~12),  and  on  ordinates  which 
represent  molecular  absorptions  e,  as  calculated  on  the  basis  of 


where  c  equals  the  molecular  concentration  and  d  the  thickness  of  the 
layer  in  centimeters.  In  some  of  the  more  recent  work  log  e  has  been 
substituted  for  e.6 

A  somewhat  simpler  method,  which  is  applicable  to  the  determina- 
tion of  absorption  in  the  visible  spectrum  has  been  suggested  by  F. 
Weigert.  It  depends  upon  a  comparison  of  the  absorption  of  the  sub- 

1  Stobbe,  Annalen  der  Chemie,  349,  364  (1906)  ;  370,  93(1909);    Ber.,  39,  293, 
761    (1906);     also  Martens   and   Grunbaum,    Drudes   Annalen,     12,    984    (1903); 
Hantzsch,  Ber.,  39,  4153  (1906). 

2  Ber.,  46,  85  and  553  (1912). 

3  Ber.,  46,  3628  (1913);  49,  1496  (1916). 

4  Ber.,  46,  2819  (1912);  46,  1304,  2596,  3627,  3650  (1913);  49,  1496  (1916). 
*  Physikal.  Zeitschr.,  14,  151  (1913);  Ber.,  46,  1306  (1913). 

6  Ber.,  46,  3631  (1913). 


392 


THEORIES  OF  ORGANIC  CHEMISTRY 


stance  whose  structure  is  in  doubt  with  a  substance  of  known  constitu- 
tion (the  so-called  normal  substance),  supposing  of  course  that  both 
contain  certain  lines  of  the  same  wave  length.  For  the  practical  carrying 
out  of  this  experiment  the  reader  must  again  be  referred  to  the  literature 
on  the  subject.1  Weigert's  so-called  typical  color  curves  are  plotted  on 
abscissas  which  represent  either  the  wave  lengths  (X)  or  the  oscilla- 
tion frequencies  (v)  and  on  ordinates  which  represent  the  logarithms  of 
the  extinctions.  The  curves  shown  in  the  accompanying  figures  serve 
to  illustrate  how  much  more  apparent  variations  become  when  the 
successive  curves  are  plotted  on  log  e  instead  of  €. 


Curves  plotted  for  different  concentrations  possess  the  same  general 
form  and  this  is  also  true  if  the  layers  are  of  different  thickness  or  if 
different  units  are  used  in  representing  the  abscissas  and  the  ordinates. 
In  every  instance  curves  which  are  characteristic  of  the  absorbing  sub- 
stance are  obtained. 

The  disadvantages  and  misconceptions  connected  with  the  methods 
which  have  thus  far  been  considered  and  the  complications  which  arise 
because  of  the  absorption  of  the  solvent,  may  be  avoided  by  using  a 
method  which  has  recently  been  worked  out  by  K.  Schaeffer  and  A. 
Hardtmann.2  The  apparatus  is  relatively  simple  and  permits  the  deter- 

^er.,  49,  1530(1916). 

2Zeitschr.  angew.  Chemie,  33,  25  (1920);  also  compare  Zeitschr.  wiss.  Phot., 
8,  212  (1910);  Zeitschr.  anorg.  Chemie,  97,  285  (1916);  98,  70,  77  (1916);  Zeitschr. 
wiss.  Phot,,  17,  193  (1918);  Zeitschr.  anorg.  Chemie,  100,  249  (1917);  104,  212  (1918); 
Zeitschr.  physikal.  Chemie,  93,  312  (1919). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      393 

mination  of  comparatively  exact  extinction  measurements  even  in  the 
region  of  the  extreme  ultra-violet.  The  so-called  extinction  or  absorp- 
tion curve  is  drawn  by  plotting  either  the  wave  lengths  or  the  number  of 
oscillations  as  abscissas  against  the  logarithms  of  the  thickness  of  the 
layers  of  the  solution  as  ordinates.  If  curves  are  plotted  for  solutions  of 
different  concentrations,  these  curves  will  either  all  coincide,  in  which 
case  it  may  be  assumed  that  Beer's  law  holds  since  the  absorbing  com- 
plex is  unchanged  upon  dilution,  or,  the  position  of  the  curves  may  be 


Number  of  oscillations 


different  so  that  they  no  longer  coincide,  in  which  case  it  may  be  assumed 
that  dilution  is  accompanied  by  changes  in  equilibrium  in  the  chemical 
system,  as  for  example, 

A  (colored)  <=±  B  (colorless) . 

Under  these  circumstances  the  curves  plotted  for  different  concentra- 
tions of  solution  may  be  similar  in  form  but  suffer  a  vertical  displacement. 
The  accompanying  figure  'illustrates  such  a  condition,  the  degree  of 
vertical  displacement  serving  to  measure  the  change  in  the  concentra- 
tion of  A, 

Number  of  oscillations 


In  certain  instances  the  curves  plotted  for  different  concentrations 
show  a  horizontal  displacement  in  which  case  it  is  usually  assumed  that 
the  two  substances,  A  and  B,  are  structurally  identical  but  differ 


394 


THEORIES  OF  ORGANIC  CHEMISTRY 


in  the  strength  of  the  affinities  which  operate  between  the  different 
atoms.     Such  a  relationship  is  illustrated  in  the  following  figure: 

Kumber  of  Oscillations 


Log.  of  thickness  of 
layer  of  solution. 

,1 

\ 

\ 

—  ) 

\ 

\ 

r 

*\ 

r\ 

\ 

2 

/ 

Y 

i 

\ 

/ 

/\ 

\ 

\ 

v 

/ 

/ 

\ 

\ 

V. 

L> 

; 

\ 

Finally  in  cases  where  the  change  A  -*  B  involves  a  complete  change 
in  the  chemical  constitution  of  the  substances,  curves  plotted  for  dif- 
ferent concentrations  of  solution  will  exhibit  marked  differences  in  form 
as  well  as  is  position.  Such  a  relationship  is  shown  by  the  curves  in 
the  following  figure : 

Number  of  Oscillations 


Log,  of  thickness  of 
layer  of  solution. 

\ 

\ 

1 

\ 

\ 

g 

\ 

V 

/ 

\ 

i 

\ 

/ 

\ 

\ 

\ 

/ 

\ 

\ 

N 

\ 

v_ 

7 

\ 

\ 

\ 

\ 

\ 

This  method  may  be  applied  to  determine  the  extent  to  which  two  sub- 
stances interact.  For  this  purpose  two  curves  representing  the  two 
substances  respectively  are  plotted  one  behind  the  other,  and  this  figure 
is  then  compared  with  a  curve  plotted  for  a  mixture  of  the  two  substances. 
Such  a  comparison  will  readily  show  whether  or  not  a  reaction  between 
the  two  substances  has  taken  place.  Conclusions  may  also  be  drawn 
in  regard  to  the  relative  conditions  of  equilibrium  in  the  reaction  product. 
A  number  of  more  or  less  important  rules  have  been  formulated  in 
regard  to  the  relation  which  exists  between  the  chemical  constitution 
of  organic  compounds  and  their  absorption  curves.  For  example,  V. 
Henri  and  J.  Bielecki  have  constructed  absorption  curves  for  a  large 
number  of  alcohols,  acids,  esters,  aldehydes,  and  ketones  in  order  to 
ascertain  by  means  of  systematic  comparisons  the  way  in  which  absorp- 
tion depends  upon  the  differences  in  atomic  groupings  within  the  mole- 
cule. The  results  of  these  experiments  seem  to  indicate  that  minute 
differences  in  chemical  constitution  may  be  detected  by  means  of  a  com- 
parative study  of  absorption  curves  and  that  phenomena  in  this  field 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      395 


are  open  to  a  numerical  treatment  in  much  the  same  manner  as  in  the 
case  of  refractions.  While  the  problem  is  still  in  the  process  of  develop- 
ment and  is  not  always  treated  in  a  consistent  way  even  by  one  and  the 
same  investigator,  certain  more  or  less  definite  conclusions  have,  never- 
theless, been  arrived  at  which  are  of  interest  and  which  may  be  con- 
sidered briefly. 

A  comparative  study  of  the  absorption  curves  of  homologous  aliphatic 
alcohols,  acids,  esters,  etc.,  has  resulted  in  the  formulation  of  a  number  of 
important  rules  in  regard  to  the  effect  of  substitutions  upon  absorption 
spectra.  The  addition  of  CH2,  for  example,  is  always  accompanied  by 
(1)  a  shifting  of  the  absorption  band  of  the  substance  in  the  direction  of 
of  the  red,  and  (2)  an  increase  in  the  height  of  the  curve  at  the  point  of 
maximum  absorption.  In  the  case  of  substances  having  the  general 
formula  CnH2n+i-COOR,  where  R  represents  an  alkyl  group,  it  has 
been  found  that  absorption  depends  largely  upon  the  value  of  n  hi  the 
residue  CnH2n+i  and  is  only  very  slightly  influenced  by  the  nature  of  R. 
The  carboxyl  residue,  COO,  on  the  other  hand  seems  to  have  a  very 
marked  effect  upon  the  absorption  as  is  obvious  from  a  comparison 
of  the  absorption  curves  of  mono-,  di-,  and  tri-basic  acids,  such  as,  for 
example,  acetic,  succinic,  and  tricarballylic  acids: 

CH2 CH CH2 

and     | 

COOH    COOH  COOH 


COOH 


OOH 


E 

200 


150 


100 


50 


— 22?Succ 


Acet 


cacid 
nic-a'c 
Tricarballylic  acid 


/>3 
Uc|fe— 


X  =  2450  2400   2350   2300   2250   2200   2150  2100 
4- =4082  4168  4255   4348   4444   4545   46514762 


396  THEORIES  OF  ORGANIC  CHEMISTRY 

Of  these  acids  the  last  two  may  be  regarded  as  respectively  doubling  and 
trebling  the  CH2COOH  grouping  which  is  present  in  acetic  acid  and 
they  should  therefore  be  expected  to  show  a  proportional  increase  of 
absorption  from  left  to  right.  As  a  matter  of  fact  it  has  been  observed, 
however,  that  while  the  absorption  curves  of  these  three  acids  are  similar, 
the  relative  increase  in  absorption  is  not  proportional  to  the  number  of 
carboxyl  groups  but  is  so  much  stronger  as  to  suggest  a  definite  exalta- 
tion in  optical  properties.  Under  such  circumstances  absorption  cannot 
be  regarded  as  simply  additive  in  character,  as  is  the  case  in  refraction, 
but  it  must  be  clearly  recognized  as  composite  and  as  representing  not 
merely  the  individual  additive  effects  of  different  chromophore  groups 
but  also  the  increase  and  decrease  in  exaltations  which  result  from  the 
mutual  interactions  of  these  groups  upon  each  other.  This  conclusion 
is  very  important  in  connection  with  the  development  of  a  quantitative 
relation  between  absorption  and  constitution  and  will  be  considered 
again  in  some  detail  later  in  this  chapter. 

The  absorption  curves  of  saturated  mono-,  di-,  and  tri-basic  acids 
have  been  found  to  resemble  these  of  the  corresponding  hydroxy  acids 
very  closely.  In  fact  the  introduction  of  alcoholic  hydroxyl  into  a 
molecule  of  acid  seems  to  produce  only  a  very  slight  exaltation  in  the 
absorption  of  the  substance.  In  comparing  saturated  and  unsaturated 
acids  it  has  been  observed  that  the  latter  differ  from  the  former  in  that 
rfhey  possess  a  much  stronger  absorption  in  the  region  of  the  ultra-violet. 
It  may  be  said  in  general  that  the  presence  of  unsaturated  ethylene 
linkages  causes  relatively  greater  exaltations  in  absorption  than  is  pro- 
duced by  the  presence  of  carbonyl,  but  a  comparison  of  different  unsat- 
urated acids  shows  that  the  degree  of  exaltation  in  any  given  case  depends 
upon  the  relative  proximity  of  these  two  groups.  Acids  belonging  to  the 
acetylene  series  also  absorb  ultra-violet  rays  more  strongly  than  the  cor- 
responding saturated  acids,  and  here  again  the  exaltation  seems  to 
depend  upon  the  relative  proximity  of  the  triple  bond  to  the  carbonyl 
group. 

Stereoisomers  such  as  the  dextro  and  kevo  tartaric  acids,  fumaric 
and  maleic  acids,  mesaconic  and  citraconic  acids,  possess  different 
absorptions.  The  only  rule  which  can  be  formulated  in  regard  to  them 
is  that  in  the  case  of  geometrical  isomers  the  Zrans-modification  is  apt 
to  absorb  more  strongly  than  the  cis-modification. 

A  study  of  saturated  aldehydes  and  ketones  reveals  the  fact  that 
both  possess  characteristic  absorption  bands  between  X  =  2800  MM  and 
X  =  2700  juju.  In  the  case  of  the  simplest  aldehydes  the  height  of  the  curve 
at  maximum  absorption  is  relatively  low  but  it  is  observed  to  rise  regu- 
larly as  the  number  of  CH2  groups  is  increased.  Unsaturated  aldehydes 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     397 

and  ketones  show  exaltations  in  their  absorptions  as  compared  with  the 
corresponding  saturated  compounds  and,  as  in  the  case  of  unsaturated 
acids,  the  degree  of  exaltation  depends  upon  the  relative  proximity  of 
the  ethylene  and  carbonyl  groups.  The  absorption  curves  of  the 
majority  of  aldehydes  and  ketones  are  characterized  by  minimum 
absorptions  in  the  region  of  the  shorter  wave  lengths  immediately  to  the 
right  of  the  point  whose  relative  height  seems  to  depend  upon  the  rela- 
tive complexity  of  the  alkyl  residue  (being  highest  in  the  case  of  those 
substances  which  are  most  complex).  Acetone  represents  an  exception 
to  this  general  rule  since  it  does  not  show  the  characteristic  minimum 
absorption  in  the  shorter  wave  lengths  up  to  X  =  2144  up. 

A   comparison   of  the  absorption  curves  of  corresponding  acids, 
aldehydes,  and  ketones, 


.o  .o 

CH3-C  CH3-C<f 

XOH 


shows  that  they  apparently  bear  no  resemblance  to  each  other  and  it 
must,  therefore,  be  concluded  that  the  replacement  of  hydroxyl  by  hydro- 
gen or  by  alkyl  so  completely  alters  the  constitution  of  the  molecule  as 
to  produce  a  fundamental  change  in  the  general  absorption  of  the  sub- 
stance. This  observation  is  not  in  harmony  with  the  structural  for- 
mulas which  are  generally  accepted  as  representing  the  atomic  relation- 
ships of  these  three  classes  of  substances  and  which  assume  the  presence 
of  a  carbonyl  group  as  common  to  all.  If  carbonyl  is  actually  present 
in  acids  it  is  difficult  to  understand  why  these  substances  do  not  possess 
the  absorption  bands  which  are  so  characteristic  of  aldehydes  and  ketones 
and  which  have  been  interpreted  as  due  to  the  presence  of  a  carbonyl 
group  in  the  molecule.  The  substitution  of  hydrogen  or  of  alkyl  by 
hydroxyl  is  not  in  itself  sufficient  to  account  for  this,  since  in  the  case 
of  other  classes  of  compounds  substitutions  of  this  type  produce  rela- 
tively slight  changes  in  absorption.  It  would  therefore  seem  to  follow 
that  the  old  formulas  do  not  adequately  express  the  actual  chemical 
relationships  which  exist  in  the  case  of  these  three  classes  of  substances, 
and  Henri  and  Bielecki  propose  the  following  substitutes: 


.  . 

C<  CH3-Cf  CH3-Cf 

\H  \CH3  XO 


CH3 

H 


These  investigators  agree  with  Lowry  and  Southgate  in  believing  that 
acids  differ  radically  from  aldehydes  and  ketones  in  their  chemical  con- 
stitution and  they  suppose  that  these  differences  may  be  satisfactorily 


398  THEORIES  OF  ORGANIC  CHEMISTRY 

accounted  for  on  the  basis  of  differences  in  saturation.  In  the  case  of 
acids,  for  example,  the  partial  valencies  on  the  oxygen  of  the  carbonyl 
group  are  assumed  to  be  saturated  while  in  the  case  of  aldehydes  and 
ketones  they  remain  unsaturated.  These  assumptions  find  further 
confirmation  in  the  optical  properties  of  unsaturated  aldehydes  and 
ketones.  The  formulas  for  acrolein  and  a-crotonic  acid  for  example 
must  be  corrected  to 


•C/  and      CH3-CH=CH.C/:: 

XH  XOH 


since,  while  both  substances  have  been  observed  to  possess  strong  exalta- 
tions of  absorption,  only  the  former  shows  the  particular  bands  which 
characterize  the  presence  of  carbonyl. 

A  similar  explanation  applies  to  diacetyl,  which  exhibits  a  much 
weaker  absorption  than  might  be  expected  from  the  fact  that  it  contains 
two  carbonyl  groups  and  from  the  additional  fact  that  acetonylacetone, 

CH3  •  CO  •  CH2  •  CH2  •  CO  •  CH3 

possesses  an  absorption  constant  which  in  maximum  is  eight  times 
greater  than  that  of  acetone.  Such  abnormal  behavior  on  the  part  of 
diacetyl  is  intelligible  only  on  the  assumption  that  partial  valencies  on 
the  oxygen  atoms  of  the  two  adjacent  carbonyl  groups  mutually  saturate 
each  other: 

CH3-C— C-CH3 


A 4 


When  Henri  and  Bielecki l  discovered  that  the  mutual  interaction 
of  two  chromophore  groups  which  are  present  in  the  same  molecule 
corresponds  to  hypsochromic  and  hyperchromic  changes  in  absorption 
they  made  this  observation  the  basis  for  the  formulation  of  a  method 
by  which  it  is  now  possible  to  calculate  the  absorption  curve  of  a  com- 
pound of  known  structure.  The  particular  observation  was  to  the  effect 
that  if  two  chromophore  groups  occupy  conjugate  positions  with  refer- 
ence to  each  other  they  produce  a  weak  hyperchromic  and  a  strong 
hypsochromic  effect  while  if,  on  the  other  hand,  the  two  groups  are 
widely  separated  in  the  molecule  the  opposite  effect  is  observed.  In  the 
latter  instance  the  interaction  of  the  two  chromophore  groups  will  tend 
to  increase  the  height  of  the  absorption  curve  (hyperchromic  effect) 
while  the  shifting  of  absorption  in  the  direction  of  longer  wave  lengths 
(hypsochromic  effect)  will  be  negligible. 

'Ber.,  47,  1690  (1914). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      399 

According  to  this  observation  the  absorption  constant,  €,  must  be 
regarded  as  composite  and  must  be  assumed  to  represent  not  merely  the 
additive  effects  of  the  different  chromophores  but  also  the  effect  of 
their  interaction  upon  each  other  inside  the  molecule.  Henri  and 
Bielecki  1  were  then  able  to  demonstrate  that  the  absorption  curves  of 
acetone,  acetic  acid  and  other  monobasic  aliphatic  acids  may  be  repre- 
sented by  means  of  three  characteristic  constants  which  are  inter- 
dependent and  whose  relation  may  be  expressed  either  by  the  Ketteler- 
Helmholtz-Drude  equation  : 

q\2 


or  by  the  Henri-Bielecki  exponential  formula: 

vo)2 


In  the  first  equation  a,  g2  and  \m  represent  the  three  characteristic  con- 
stants and  in  the  case  of  carbonyl  for  example  have  the  value  a  =  2.57 
X106,  X  =  2721,  and  02  =  1.63X10-5.  In  the  second  equation  c  repre- 
sents the  absorption  constant  for  the  frequency  v,  while  a,  (3  and  j>o  rep- 
resent the  characteristic  constants.  As  their  name  implies  these  con- 
stants possess  different  values  in  the  case  of  every  chromophore.  It 
follows  that  if  the  characteristic  constants  are  known  the  value  €  for 
any  given  frequency  may  be  calculated  and  in  this  way  the  absorption 
curve  of  a  compound  of  known  structure  may  be  predicted. 

If  the  two  chromophore  groups  are  present  in  a  molecule  the  hypso- 
chromic  and  hyperchromic  effect  will  vary  depending  upon  their  relative 
positions.  Under  such  circumstances  the  absorption  curve  of  the 
substance  may  be  determined  by  means  of  the  following  equation  : 


—  *i—  *         n  •  a2  • 

in  which  viaijSi,  and  vwifa  represent  the  characteristic  constants  of 
the  two  chromophores;  n,  the  factor  of  hyperchromism;  and  Av  the 
factor  of  hypsochromism.  If  the  chromophores  are  present  in  conjugate 
systems  n  is  small  and  Ay  is  large,  while  if  the  two  groups  are  in  isolated 
positions  this  relation  is  reversed. 

In  summing  up  the  work  of  Henri  and  Bielecki  it  must  be  stated 
that  while  these  investigators  are  of  the  opinion  that  the  formulation 
of  the  above  relationships  has  made  a  numerical  treatment  of  absorp- 
tion possible  and  affords  a  means  for  calculating  the  values  for  absorp- 
tion constants  in  much  the  same  way  as  in  the  case  of  refractions,  the 
evidence  upon  which  their  conclusions  are  based  is  distinctly  contro- 
versial in  character.  The  objections  which  have  been  brought  forward 
lPhysikal.  Zeitschr,  14,  516  (1913). 


400  THEORIES  OF  ORGANIC  CHEMISTRY 

by  Lifschitz  l  while  they  cannot  be  reviewed  in  detail  in  the  present 
treatise  are,  nevertheless,  of  serious  importance. 

An  extended  study  of  absorption  phenomena  in  the  field  of  organic 
chemistry  has  led  A.  Hantzsch  2  to  recognize  that  the  interaction  of 
the  solvent  and  the  solute  may  "produce  one  or  the  other  of  the  following 
effects : 

I.  The  absorption  of  the  substance  may  be  radically  changed  in 
which  case  the  absorption  curve  assumes  an  entirely  different  form. 
Under  these  circumstances  alterations  in  the  chemical  constitution  of  the 
substance  may  be  supposed  to  have  taken  place.  Such  alterations  rep- 
resent changes  in  the  distribution  of  affinity  and  rray  involve  either  the 
principal  or  the  partial  valencies  present  on  the  atoms.  Reactions  which 
may  be  assumed  to  result  directly  from  the  action  of  the  solvent  are 
exemplified  in  rearrangements  from  keto  to  enol  modifications,  from 
nitro-  to  aci-compounds,  from  colorless  to  yellow  dioxyterephthalic 
esters,  from  colorless  to  polychromic  salts  of  the  oximidoketones,  from 
colorless  to  yellow  pyridonium  salts,  etc.  In  these  cases  the  change  is 
not  supposed  to  be  catalytic  in  character  but  to  depend  upon  differ- 
ences in  the  stability  of  the  solvates  which  are  formed  by  the  addition 
of  the  solvent  to  one  or  the  other  modification  of  the  solute.  II.  The 
absorption  of  the  substance  may  not  be  materially  changed  in  which 
case  the  form  of  the  absorption  curve  will  remain  the  same  while  its 
position  may  either  be  the  same  or  may  be  shifted  slightly.  Under 
these  circumstances  no  appreciable  chemical  change  is  assumed  to  have 
taken  place,  although  this  does  not  preclude  the  formation  of  unstable 
addition  products  between  molecules  of  the  solute  and  solvent.  Tri- 
nitrotriphenyl  carbinol,  (CeH^NC^aCOH,  for  example,  reacts  with 
almost  all  solvents  to  give  solid  heterogeneous  products  of  association. 
Of  these  (C6H4N02)3COH  .  .  .  CH3OH  and  (C6H4NO2)3COH  . .  . 
CHCls  possess  identical  absorption  curves  within  the  limits  of  experi- 
mental error. 

From  a  chemical  point  of  view  all  solvents  may  be  regarded  as 
liquids  which  are  able  to  form  either  relatively  stable  homogeneous 
association  products  (solvates)  or  heterogeneous  association  products 
with  the  dissolved  substance.  Solvents  have  the  power  of  inducing 
rearrangements  in  the  case  of  isomeric  and  tautomeric  compounds 
because  the  equilibrium  relationships  of  stable  and  meta-stable  modifica- 
tions are  frequently  different  from  those  of  the  corresponding  solvates. 
In  certain  instances  the  presence  of  even  a  trace  of  solvent  may  be  suf- 
ficient to  cause  the  isomerization  of  substances  in  the  solid  state. 

1  Zeitschr.  wiss.  Phot.,  16,  149  (1916). 
2Ber.,  60,  1413  (1917). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      401 

Solvents  also  have  the  power  of  entering  into  actual  chemical 
combinations  with  the  dissolved  substance.  Here  the  reaction  may 
result  from  the  saturation  of  either  partial  or  principal  valencies. 
An  example  of  the  latter  type  of  combination  is  to  be  found  in  the 
formation  of  certain  hydrates  such  as  : 

Ri\  Ri\      /OH 

\C=0+H20    -»         >C< 
n2'  Rj/      X)H 

In  these  cases  the  form  of  the  absorption  curve  is  radically  changed. 

As  the  result  of  a  very  exhaustive  study  of  absorption  A.  Hantzsch 
has  discovered  the  existence  of  very  fine  differences  in  the  properties  of 
organic  acids  and  their  derivatives.  On  the  one  hand  Hantzsch  com- 
pared the  absorption  of  salts  of  the  alkali  and  alkali  earth  metals  with 
the  absorption  of  the  corresponding  acids.  These  measurements  were 
made  in  the  ultra-violet  region  of  the  spectra  and  the  resulting  pairs 
of  curves  were  found  to  be  identical  in  all  respects.  When,  on  the  other 
hand,  the  absorption  of  the  esters  of  these  acids  was  determined  it  was 
found  that  although  different  esters  of  the  same  acid  gave  identical 
absorption  curves,  these  curves  were  markedly  different  from  the  curves 
of  the  corresponding  free  acid  or  of  its  salt.  In  general  it  was  observed 
that  the  esters  showed  a  much  stronger  absorption  than  the  correspond- 
ing salt.  While  absorption  in  every  case  has  been  found  to  depend 
both  upon  the  nature  of  the  acid  and  of  the  solvent,  it  may  be  said  in 
general  that  the  absorption  of  a  given  acid  stands  approximately  mid- 
way between  the  respective  absorptions  of  its  salts  and  esters.  Tri- 
chloracetic  acid,  for  example,  possesses  an  absorption  as  weak  as  that 
of  its  salts  when  examined  in  solutions  of  water  or  petroleum  ether, 
while  in  solutions  of  alcohol  and  ethyl  ether  it  absorbs  just  as  strongly 
as  its  esters.  Hantzsch  is  of  the  opinion  that  differences  in  optical 
properties  in  these  and  other  similar  instances  cannot  be  accounted  for  on 
the  basis  of  associations  or  dissociations  in  solution  and  that  they  may, 
therefore,  be  assumed  to  correspond  to  actual  differences  in  the  chemical 
constitution  of  the  substances.  In  other  words,  in  so  far  as  it  is  possible 
to  ascertain  under  the  conditions  of  Hantzsch's  experiments,  all  the 
evidence  tends  to  support  the  conclusion  that  the  differences  in  absorp- 
tion which  have  been  observed  in  the  case  of  acids,  esters  and  salts  are 
due  to  differences  in  the  structure  of  these  substances.  Moreover  a 
close  study  of  the  formula 


X)H 


402  THEORIES  OF  ORGANIC  CHEMISTRY 

shows  that  although  it  has  been  generally  accepted  as  representing  the 
constitution  of  organic  acids,  it  fails  to  account  for  the  distinctly  acid 
character  of  the  hydrogen  of  the  hydroxyl  group  as  compared  with  that  of 
the  hydrogen  which  is  present  in  the  hydroxyl  groups  of  alcohols.  The 
very  strong  tendency  towards  dissociation  which  particularly  distin- 
guishes acid  hydrogen  is  much  more  readily  accounted  for  on  the  basis 
of  Werner's  coordination  formula: 


0 
R-C 

O 


H 


since  this  represents  the  hydrogen  atom  as  occupying  a  position  in  the 
outer  or  dissociable  zone  with  respect  to  the  central  carbon  atom.  The 
dissociation  of  metallic  atoms  may  be  explained  by  means  of  analogous 
formulas 


0 


RC 


or  O 


H     and    RC 


M 


RC02  :  H      and    RC02  :  M  1 

Since  acids  lose  their  power  to  dissociate  hydrogen  when  dissolved 
in  certain  solvents  and  under  these  conditions  are  optically  different 
from  the  corresponding  dissociable  modification,  Hantzsch  assumes  that 
changes  in  constitution  have  occurred  which  involve  the  formation  of  a 
pseudo-acid.  Moreover  since  these  pseudo-acids  are  optically  identical 
with  the  corresponding  esters  Hantzsch  assumes  that  they  have  an 
analogous  structure  which  he  represents  by  the  use  of  the  older 
formulas  : 


and        RCf 
X)H  N5R 

According  to  this  conception  trichloracetic  acid  would  have  the 


formula  CClsGy         when  present  in  solution  in  alcohol  or  ether  and  the 

XOH 

formula  CClsCC^  :  H  when  dissolved  in  water  or  petroleum  ether.  Other 
carboxylic  acids  are  assumed  to  form  equilibrium  mixtures  of  these  two 
modifications  when  dissolved  in  water  or  petroleum  ether  : 


RC< 


•H 


Pseudo-acid  True  acid 

1M=metal. 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     403 

The  condition  of  equilibrium,  as  in  the  case  of  keto-enol  modifications, 
depends  upon  the  nature  of  the  solvent,  the  nature  of  the  substance,  the 
character  of  R,  etc.  The  isomerization  of  one  modification  into  the 
other  in  solution  may  be  explained  as  due  to  chemical  causes  as,  for 
example,  the  formation  of  solvates  or  association  products  which  possess 
different  degrees  of  stability,  viz., 


RC 


O 


/o 

H.(OH2)n       RC< 
X) 


H.(OC2H5), 


.0  •  (HOC2H5)n  ^O  •  (HOC2H5)n 

RC<  or     RCf 

XOH  \OH.(OC2H5)« 


The  influence  of  R  is  illustrated  by  the  fact  that  the  substitution  of 
chlorine  for  hydrogen  in  acetic  acid  results  in  a  condition  of  equilibrium 
which  favors  the  formation  of  the  true  acid.  The  progressive  changes 
in  the  condition  of  equilibrium  which  take  place  as  a  result  of  the  intro- 
duction of  one,  two  and  finally  three  chlorine  atoms,  correspond  to  similar 
changes  in  absorption.  Since,  moreover,  a  fully  ionized  acid  is  optically 
identical  with  its  salts  the  relative  absorption  of  a  given  solution  may  be 
regarded  as  a  direct  measure  of  its  condition  of  equilibrium.  In  the 
case  of  the  fatty  acids,  however,  because  of  their  relatively  weak  absorp- 
tion, optical  methods  are  much  less  exact  than  determinations  of  elec- 
trical conductivity  so  that  the  latter  are,  therefore,  generally  preferred. 
The  relation  between  color  and  constitution  was  first  developed  by 
O.  N.  Witt 1  following  the  fundamental  discovery  of  Graebe  and  Lieber- 
mann.2  Witt's  so-called  "  chromophore  theory "  attempted  so  to 
systematize  the  phenomena  of  color  as  to  make  it  possible  to  synthesize  a 
colored  substance  or  a  dye  from  its  essential  constituents.  The  appear- 
ance of  color  has  for  many  years  been  connected  with  the  introduction 
into  an  organic  molecule  of  certain  groups  as  for  example  NO2, — N=N — , 
etc.  When  these  groups  are  changed  in  any  way,  as  in  reduction  to 
NH2  and  — NH — NH —  respectively,  the  color  which  they  imparted 
to  a  given  substance  disappears,  but  reappears  again  on  oxidation  of  the 
so-called  colorless  "  leuco-compound."  Groups  which  like  these  by 
their  mere  presence  seem  to  have  the  power  to  produce  color  are  called 
chromophore  groups,  and  the  colored  compounds  which  contain  them  are 
called  chromogens.  In  order  that  a  chromogen  become  a  dye,  and  there- 
fore capable  of  entering  into  direct  or  indirect  relations  with  a  fabric,  it 
must  possess  in  addition  to  a  chromophore,  a  so-called  auxochrome  group. 

1  Ber.,  9,  522  (1876);  21,  325  (1888). 
2Ber.,  1,  106  (1868). 


404  THEORIES  OF  ORGANIC  CHEMISTRY 

The  latter  are  characterized  by  the  fact  that  they  are  capable  of  reacting 
with  either  acids  or  bases  respectively  to  form  salts.  Examples  of 
auxochromes  are  NEb,  OH,  etc.,  and  they  not  only  enter  into  direct 
combination  with  fabrics  such  as  silk  or  wool,  but  they  also  have  the 
effect  of  deepening  the  color  of  the  chromogen.  To  repeat,  a  chromogen 
is  a  substance  which  contains  a  chromophore  group,  while  a  dye,  on  the 
other  hand,  is  a  substance  which  contains  both  a  chromophore  and  an 
auxochrome  group.  This  in  brief  outlines  Witt's  original  theory.  It 
has  been  modified  and  very  greatly  extended  in  recent  years  and  will  now 
be  considered  in  some  of  its  ramifications  and  applications.  The  subject 
of  chromophores  and  auxochromes,  must,  however,  first  be  examined 
separately  and  in  detail.1 

The  most  important  chromophore  groups  are   >C=0;   >  C=S; 

O 

/\ 

>  C=N —  (including  azomethine) ;  — N — N —  (azoxy) ;  — NO  •  — N02 ; 

— N=N;  and  the  quinoid  groups2 : 


According  to  Werner's  theory  of  partial  valencies    groups    such    as 


Metal 

RC 


0 

Metal  salts  of  1-3  diketones  Dinitro-compounds 

must  also  be  regarded  as  chromophore  groups,  in  which  case  the 
phenomenon  of  color  is  assumed  to  be  due  to  the  mutual  saturation 
of  partial  valencies  inside  the  molecule  as  represented  by  the  dotted 
lines  in  the  formulas.  This  particular  phase  of  the  subject  cannot  be 
discussed  fully  at  this  point  but  will  be  considered  later  in  the  chapter. 
In  general  it  may  be  said  that  chromophore  groups  contain  the  ele- 

1 J.  Lifschitz,  "Studien  iiber  die  Chromophorfunktion,"  Zeitschr.  wiss.  Phot.,  16, 
101,  140,  149,  269  (1916);  19,  198  (1920);  Ber.,  50,  897,  906  (1917);  Zeitschr. 
physikal.  Chemie,  95,  1  (1920). 

2  Armstrong,  Proc.  Chem.  Soc.  1892,  101,  189,  195;  1893,  53,  55,  63,  206;  1897, 
228;  1902,  101;  Jour.  Chem.  Soc.  87,  1272,  (1905). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     405 

ments,  C,  N,  0,  and  S  either  in  union  with  each  other  by  double  or  triple 
bonds  or  present  in  so-called  free  organic  radicals  as  single  highly  unsat- 
urated  atoms.  Chromophore  groups  have  been  classified  as  either  inde- 
pendent or  auxiliary  in  character.  For  example,  certain  combinations 
such  as  NO  and  NCfe1  have  the  power  of  forcing  colored  compounds 
even  when  in  union  with  such  groups  as  CH3  and  others  which  possess 
only  very  slight  optical  activity.  The  properties  of  these  chromophores 
are  the  more  marked  2  when  they  are  compared  with  those  of  the  second 
class.  The  latter  are  individually  and  singly  incapable  of  producing 
color,  but  have  the  power  of  reinforcing  each  other  and  by  a  sort  of 
cumulative  action  they  are  able  to  contribute  to  the  generation  of  color. 
Thus,  for  example,  the  quinoid  grouping  which  is  so  effective  as  a  color 
bearer  represents  the  combined  action  of  four  pairs  of  unsaturated  atoms. 
In  general  it  may  be  said  that  the  great  majority  of  colored  substances 
owe  their  color  not  to  the  presence  of  one  but  of  many  chromophores. 

J.  Lifschitz 3  has  recently  classified  chromophores  according  to 
conceptions  embodied  in  Werner's  theory.  From  this  point  of  view  they 
may  be  regarded  as  coordinately  saturated  or  coordinately  unsaturated. 
In  the  latter  case  three  principal  classes  may  be  differentiated : 

1.  Monatomic   chromophores    (free  radicals,   dyes,  additive   com- 
pounds) . 

2.  Diatomic  chromophores.     This  class  includes  the  great  majority 
of  the  chromophores  which  have  been  mentioned  above. 

3.  Chromophore  systems  of  atoms.     This  class  includes  conjugated 
compounds  and  inner  complex  salts.     Such  systems  are  characterized 
in  particular  by  the  fact  that  they  appear  to  act  optically  as  separate 
units  within  the  molecule. 

In  every  case  it  must  be  remembered  that  the  property  of  unsatura- 
tion  represents  only  one  of  the  several  factors  which  operate  in  the 
formation  of  chromophores.  According  to  H.  Kauffmann  4  it  is  not  even 
the  controlling  factor  in  determining  color.  For  example  it  has  not  as 
yet  been  possible  to  demonstrate  conclusively  that  depth  of  color  bears  a 
definite  relation  to  the  degree  of  unsaturation.5  It  is  not  even  certain 

Compare  H.  Kauffmann,  "Uber  den  Zusammenhang  zwischen  Farbe  und 
Konstitution  bei  chemischen  Verbindungen " ;  Ahrens  Sammlung  chem.  und  chem.- 
techn.  Vortrage,  9;  also  "Die  Valenztheorie,"  p.  432,  Stuttgart,  1911  (Enke);  Ber., 
46,  781,  2333  (1912);  46,  3788,  3801,  3808  (1913);  49,  1324  (1916);  H.  Ley,  "Die 
Beziehungen  zwischen  Farbe  und  Konstitution  bei  organischen  Verbindungen," 
p.  19,  Leipzig,  1911  (Hirzel);  R.  Mohlau  and  R.  Adam,  Zeitschr.  f.  Farbenindustrie. 

2  Kauffmann,  Ber.,  40,  2341  (1907). 

3  Zeitschr.  wiss.,  Phot.,  16,  107  (1916). 
*  Ber.,  60,  635  (1917). 

^  Lifschitz,  Ber.,  60,  906  (1917);  Ley,  Ber.,  60,  243  (1917);  61,  1808  (1918). 


\  /^6ri5  V_y6A14\ 

\C=C<  and         |         >C=C      » 

X  / 


406  THEORIES  OF  ORGANIC  CHEMISTRY 

that  the  shifting  of  absorption  in  the  direction  of  the  red  is  exactly 
proportional  to  an  increase  in  the  unsaturated  character  of  a  compound. 
The  relative  position  of  chromophores  in  the  molecule  is  of  great 
importance  in  the  production  of  color.  From  the  point  of  view  of 
arrangement  chromophores  have  been  classified  as  cyclostatic  and  strep- 
tostatic,  the  former  representing  constituent  parts  of  a  cyclic  and  the 
latter  being  present  in  acyclic  combinations.  In  general  it  may  be  said 
that  density  of  atomic  groupings,  such  as  is  associated  with  ring  forma- 
tion, tends  to  produce  and  also  to  strengthen  color.  Innumerable 
instances  may  be  cited  in  illustration  of  this.  In  the  case  of  the  fol- 
lowing compounds: 

C6H. 

CeHs/  XC6H5 

I  II 

it  has  been  observed,  for  example,  that  the  first  is  colorless  1  while  the 
second  is  red;2  and  in  the  case  of  the  two  isomers,  benzene  and  fulvene, 

CH=CH 
and 

CH=CH 

one  is  colorless  while  the  other  is  yellow.  The  difference  in  color  in  both 
instances  is  generally  assumed  to  be  due  to  a  relatively  greater  density 
in  the  arrangement  of  the  atoms  in  the  colored  compound. 

The  cumulative  effect  of  chromophore  groups  in  strengthening  color  is 
strikingly  illustrated  by  a  comparison  of  the  derivatives  of  fulvene.3 
Thus,  while  fulvene  itself  is  yellow,  methyl  phenyl  fulvene  possesses  the 
color  of  a  solution  of  chromic  acid  and  diphenyl-f ulvene  is  deep  red : 4 

CH=CH  ^  CH=CH 

A_yG-tl5 


CH=CH 


C=C  and  C=C 


Carbonyl    groups    are    much    more    effective    chromophores    than 
ethylene  groups,  since  two  such  groups,  when  present  in  adjacent  posi- 

1  Klinger  and  Lonnes,  Ber.,  29,  2157  (1896). 
2Graebe,  Ber.,  26,  2354  (1893);  25,  3146  (1892). 

3  Ber.,  33,  668,  851,  3395  (1900);  36,  842  (1903);    Annalen  der  Chemie,  319,  226 
(1901). 

4  Ber.,  33,  666  (1900);   37,  2851    (1904);   Annalen  der  Chemie,   349,   333,   361 
(1906);   also  Ley,  Zeitschr.  angew.  Chemie,  1907,  1305. 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     407 


tions,  possess  the  power  of  imparting  color  to  the  molecule  containing 
them.  For  example,  CH3COCH3  is  colorless,  CH3CO  •  COCH3  is  yellow, 
CH3CO-CO-COCH3  l  is  reddish  orange,  while  CH3COCH2COCH3 
and  CH3CO-CH2CH2COCH3  are,  on  the  other  hand,  colorless.  The 
influence  of  density  in  atomic  groupings  is  illustrated  in  the  case  of  the 
following  two  pairs  of  substances : 


CO CO 

Orange 

Numerous   derivatives  of  butadiene   /3-X-dicarboxylic  acid  and  of 
its  anhydride, 

H2G=C— COOH  H2C=C-CO 

and 
H2C=C— COOH 

a      3 


H2C=C— CO 


have  been  studied  by  H.  Stobbe2  and  afford  striking  illustrations  of  the 
relation  existing  between  color  and  constitution.  Thus  a  comparison 
of  the  substances  given  in  the  following  table  shows,  that  the  aliphatic 
derivatives  of  these  two  substances  are  colorless,  while  both  phenyl  and 
furyl  (C4H30)  derivatives  are  colored.  This  color  deepens  from  yellow 
to  orange  to  deep  red  steadily,  as  the  number  of  such  groups  present 
in  the  molecule  is  increased  : 


(CH3)2C=C— COOH 
(CH3)2C=C— COOH 


(CH3)2-C=C— CO 


(CH3) 


! 

2  C=C—  CO 


Colorless 


Colorless 


•H 


i> 


Light  yellow 


C4H30-H 

Orange 


1  Ber.,  34,  3047  (1901);  35,  3309  (1902);  36,  3221  (1903). 
2Annalen  der  Chemie,  349,  333  (1906);  380,  1  (1911). 


408 


THEORIES  OF  ORGANIC  CHEMISTRY 
(CH3)2 


Yellow 

C6H5-H- 


C4H3O.H- 


Citron  yellow 

(C6H5)2C=C— COOH    (C6H5)2 


Reddish  brown 


C6H5-H-C=C— COOH 

Yellow 

(C6H5)2C=C— COOH 
(C6H5)2C=C— COOH 


CoHs  •  H 

Reddish  orange 


N02 


o-  and  m-orange 
p-deep  red 


Red 


(C6H5)2 

Red 


i> 


As  has  been  pointed  out,1  ring  formation  frequently  brings  about  a 
deepening  of  color,  as,  to  use  another  illustration,  in  the  case  of 

/\      CH=CH.C6H5 

;H  I         CH 

,/ 
CH 


5-Diphenylbutadiene 
Colorless 


CH 

Benzalindene 
Yellow 


but  this  is  not  always  the  case.  The  closing  of  a  straight  chain  of  carbon 
atoms  to  form  a  heterocyclic  ring  which  contains  oxygen  may,  in  fact, 
produce  the  opposite  effect: 

0 


C6H5-CH  CH-C6H5 

II       II 
HC      CH 


C6H5-C      C-C6H5 

II       II 
HC     CH 


Dibenzalacetone 
Yellow 


CO 

Diphenylpyrone 
Colorless 


Differences  in  color  have  been  observed  to  accompany  differences  in 
constitution  in  the  case  of  a  great  many  stereoisomers, — such  as,  for 
example,  the  two  modifications  of  dibenzoylethylene  discovered  by 
C.  Paal  and  Schulze.2 


C6H5CO 

Yellow 


HCCOC6H5  HC— COCeHs 

II  II 

/STT                                                             TJ/^       f~^f~\f~^  TT 
v>-Ll  Xlvv v_yvyv^6±l5 

Colorless 

1  Annalen  der  Chemie,  349,  349  (1906). 
2Ber.,  33,  3784  (1900);  35,  168  (1902). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      409 

The  stereoisomers  of  diethoxynaphthostilbene  l  and  benzaldesoxyben- 
zoin 2  represent  other  illustrations  of  phenomena  of  this  class.  To 
recapitulate,  chromophores  may  be  regarded  as  distinctly  unsaturated 
groups.  Two  or  more  such  groups  are  frequently  necessary  to  produce 
color  and  in  such  cases  density  in  arrangement  tends  to  deepen  or  even 
in  certain  instances  to  generate  color.  H.  Staudinger  has  recently 
pointed  out  as  a  result  of  his  investigation  of  carbonyl  groups,3  that  a 
direct  relation  exists  between  the  degree  of  unsaturation  of  a  substance 
and  its  color.  Since  conjugate  systems  of  double  bonds  have  been 
observed  to  be  more  unsaturated  than  two  single  pairs  of  unsaturated 
atoms,  it  should  follow  that  such  an  arrangement  is  marked  by  a  deepen- 
ing of  color.  Staudinger  maintains  that  this  is  in  fact  the  case  and  that, 
moreover,  the  still  less  saturated  systems  of  crossed  double  bonds  show 
corresponding  differences  in  color.4  The  relation  between  color  and 
chemical  reactivity  is  in  every  instance  definitely  marked,  as  may  be 
seen  by  reference  to  the  following  table : 

Quinones very  active,  colored 

Unsaturated  Ketone* I      Hpprpas:n0. 

Aldehydes decreasing 

Aromatic  Ketones [ 

Acid  derivatives only  slightly  reactive,     colorless 

The  relation  between  absorption  phenomena  and  a  condition  of 
unsaturation  in  organic  compounds  has  been  the  subject  of  serious 
interest  and  speculation  to  many  investigators.  According  to  Kauff- 
mann  5  the  common  cause  of  both  manifestations  is  to  be  found  in  the 
shattering  of  the  principal  valencies  of  the  atoms  and  their  resolution 
into  innumerable  and  scattered  component  forces  ("  Zersplitterung  der 
Valenz.")  To  understand  this  conception  it  is  necessary  to  develop 
Kauffmann's  views  in  some  detail. 

It  has  been  pointed  out  that  color  may  be  created  and  modified 
in  an  infinite  variety  of  ways  by  the  combination  of  chromophore 
groups.  The  mechanism  of  the  process  becomes  clearer  if  the  effect  of 
introducing  a  new  group  into  a  chromogen  of  definite  color  is  considered. 
In  such  a  case  the  resulting  change  of  color  may  be  due  either  to  the  fact 
that  the  absorption  bands  have  been  shifted  in  the  direction  of  the  violet 
or  the  red,  thus  deepening  or  lightening  the  color,  or  it  may  be  due  to 

1  Jour,  prakt.  Chemie,  47,  72  (1893). 

2  Ber.,  34,  3897  (1901);  46,  76  (1912). 
3Annalen  der  chemie,  384,  45  (1911). 

4  Compare  Friedlander,  Ber.,  47,  1919  (1914);  also  Ber.,  60,  243,  630,  906  (1917). 
E  "Die  Valenzlchre,"  p.  344  and  following,  Enke,  Stuttgart,  1911. 


410  THEORIES  OF  ORGANIC  CHEMISTRY 

the  fact  that  the  extinction  in  certain  regions  of  the  spectrum  has  been 
altered. 

Characteristic  changes  in  general  properties  and  in  color  are  caused 
by  the  action  of  auxochromes.  As  has  .already  been  stated  such  groups 
were  first  supposed  to  act  exclusively  as  salt-forming  groups,  but  recent 
investigations  show  that  they  possess  other  important  properties.  It 
has  been  demonstrated,  for  example,  that  OCHa  and  OC2H5  ex- 
ercise an  important  function  in  the  determination  of  color  even  though 
they  are  obviously  not  salt-forming  groups.  According  to  Kauffmann,1 
auxochromes  may  be  broadly  defined  as  groups  of  atoms  which,  while  not 
possessing  the  properties  of  chromophores,  are,  nevertheless,  able  to 
strengthen  the  color  of  a  chromogen.  In  cases  where  such  groups  possess 
in  addition  the  power  to  form  salts,  their  introduction  into  an  organic 
molecule  not  only  serves  to  modify  the  particular  color  of  a  given 
chromogen,  but  actually  changes  the  chromogen  into  a  dye. 

The  exact  mechanism  of  the  process  by  which  changes  in  color  are 
produced  has  been  found  to  be  much  more  complex  than  was  at  first 
supposed.  In  the  case  of  derivatives  of  benzene,  the  color  of  a  given 
chromogen  has  been  found  to  be  affected  by  the  action  of  auxochromes 
only  when  the  latter  substitute  in  the  ring.2  Even  under  these  circum- 
stances the  relative  positions  occupied  by  the  different  groups  seem  to 
constitute  an  important  factor  in  the  determination  of  color.  Thus 
o-nitraniline  is  orange  while  p-nitraniline  is  yellow,  or,  in  other  words,  the 
same  groups  may  strengthen  color  in  one  position  and  not  in  another. 
The  group  NEb  may  be  regarded  as  acting  in  the  capacity  of  an  auxo- 
chrome  only  when  by  direct  substitution  of  the  hydrogen  in  the  ring  it 
actually  strengthens  the  color  of  the  chromogen. 

Having  narrowed  the  process  down  to  the  substitution  of  ring  hydro- 
gen by  specific  groups  in  definite  positions,  the  question  as  to  the  ulti- 
mate cause  of  the  phenomena  still  remains  unanswered.  In  general  two 
opinions  prevail.  According  to  Kauffmann  substitution  brings  about 
changes  of  a  particular  kind  in  the  valency  relationships  of  the  carbon 
atoms  of  the  ring.  This  conception  does  not  find  full  expression  in 
any  of  the  structural  formulas  which  are  current  at  the  present  time, 
and  is  commonly  referred  to  as  the  "  auxochrome  theory."  In  the 
opinion  of  still  other  chemists  and  theorists  the  changes  which  are  brought 
about  by  the  action  of  auxochromes,  while  due  to  differences  in  the  dis- 
tribution of  affinity  in  the  molecule,  may  be  expressed  in  terms  of  struc- 
tural formulas  and  therefore  interpreted  in  terms  of  the  "  theory  of 
molecular  rearrangements." 

1  "Die  Valenzlehre,"  p.  482. 

2  "Die  Valenzlehre,"  p.  483. 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      411 

Kauffmann's  theory  may  now  be  considered  in  some  detail.  In 
general  it  may  be  said  to  be  based  upon  certain  observations  in  regard  to 
the  behavior  of  the  vapor  of  aromatic  compounds  under  the  action  of 
Tesla  rays.1  It  has  been  observed  that  many  substances  in  the  gaseous 
state  at  great  dilution  and  under  low  pressure  absorb  Tesla  radiations 
and  become  luminous,  emitting  blue,  violet,  green,  or  yellow  light.  In 
the  case  of  most  substances  this  luminescence  disappears  at  greater 
densities  and  higher  pressures  although  there  are  certain  notable  excep- 
tions to  this  general  rule.  For  example,  a  few  aliphatic  compounds  (espe- 
cially ke tones)  and  most  aromatic  compounds  retain  their  luminescence 
at  a  relatively  high  pressure.  These  substances,  according  to  Kauff- 
mann,  have  certain  characteristics  in  common.  Thus,  for  example, 
derivatives  of  benzene  which  show  a  violet  luminescence  in  Tesla  radia- 
tions also  dissolve  in  alcohol  with  a  violet  fluorescence.  A  comparative 
study  of  luminescing  substances  has  led  to  the  conclusion  that  the  phe- 
nomenon is  due  to  the  specific  arrangement  of  the  atoms  in  the  molecule, 
and,  since  a  great  many  derivatives  of  benzene  possess  this  property, 
the  benzene  nucleus  has  come  to  be  regarded  as  a  luminophore. 

It  has  been  observed  further  that  the  luminescence  of  different 
derivatives  of  benzene  varies  both  as  to  color  and  intensity,  and  that 
although  benzene  itself  is  not  luminous,  it  readily  yields  luminescent 
derivatives  as  a  result  of  the  substitution  of  such  groups  as  NH2, 
NHCH3,  N(CH3)2,  OH,  OCH3,  etc.,  in  place  of  the  hydrogen  of  the 
ring.  In  other  words,  those  groups  which  have  been  most  frequently 
observed  to  function  as  auxochrome  groups,  possess  the  power  of 
strengthening  luminescence  and  fluorescence  in  organic  compounds. 
In  order  to  explain  simultaneous  variations  in  these  various  properties 
among  derivatives  of  benzene  KaufTmann  supposes  that  the  ring  itself 
changes  as  a  result  of  substitutions,  and  that  it  is  actually  different  in 
different  substances.  It  is  assumed  that  the  energy  of  the  molecule  may 
be  distributed  in  a  variety  of  ways  and  that  such  variations  correspond  to 
differences  in  the  chemical  properties  as  well  as  in  the  physical  properties 
of  the  substances.  While  an  almost  infinite  number  of  arrangements 
is  possible,  certain  ideal  extreme  conditions  may  be  imagined  to  exist 
and  these  may  be  expressed  in  terms  of  constitutional  formulas. 

The  first  of  these  limiting  ideal  conditions  is  supposed  to  correspond 
roughly  to  Kekule*'s  formula  for  benzene  and  may  be  assumed  to  exist 
in  all  substances  which  possess  fluorescent  bands  in  the  regions  of  the 
extreme  ultra-violet,  as  for  example  benzene  and  its  homologues.  The 
second  corresponds  to  the  Dewar  formula  for  benzene  and  may  be 

iZeitschr.  physikal.  Chemie,  26,  719  (1898);  27,  519  (1898);  28,  688  (1899); 
50,  350  (1905),  etc.;  also  compare  Ber.,  33,  1725  (1900);  34,  682  (1901). 


412  THEORIES  OF  ORGANIC  CHEMISTRY 

assumed  to  be  present  in  all  substances  whose  vapors  emit  a  violet  light 
under  the  influence  of  Tesla  radiations.  This  condition  is  characterized 
by  certain  definite  physical  and  chemical  properties  as,  for  example, 
anomalous  magnetic  rotation  and  to  a  lesser  degree  anomalous  molecu- 
lar refraction,  accompanied  by  increased  chemical  reactivity,  i.e.,  a 
tendency  to  oxidize  readily,  to  form  substitution  products,  to  form 
quinoidal  derivatives,  etc.  According  to  Kauffmann  this  condition, 
which  is  sometimes  referred  to  as  the  "  D  "  condition,  finds  its  best 
expression  in  the  quinoid  formula: 


These  first  two  classes  include  all  substances  which  give  a  violet  fluores- 
cence when  dissolved  in  alcohol.  A  third  condition  of  the  benzene 
nucleus  is  characterized  by  the  complete  absence  of  luminescence  and 
fluorescence  and  by  decreased  chemical  reactivity.  It  results  from  the 
substitution  of  such  groups  as  NO2,  CHsCO,  Br,  etc.,1  and  was  orig- 
inally supposed  to  correspond  roughly  to  the  Glaus  formula  for  benzene. 
This  assumption  has  not,  however,  been  substantiated. 

If  three  typical  conditions  such  as  have  just  been  described  actually 
exist,  it  follows  that  substitution  in  any  given  case  must  favor  one  at  the 
expense  of  the  others,  and  the  particular  effect  of  auxochromes  must 
therefore  be  considered.  The  most  important  auxochromes  are  NH2 
and  OH,2  and  of  these  the  former  is  more  powerful  than  the  latter. 
Since  both  groups  continue  to  function  as  auxochromes  even  after  the 
hydrogen  has  been  replaced, — as  for  example  by  methyl, — it  follows  that 
the  characteristic  properties  of  the  auxochrome  depend  in  each  case  upon 
the  presence  of  the  non-metallic  element.  In  studying  the  effect  of  the 
substitution  of  hydrogen  in  the  amino  group  it  has  been  observed  that 
methyl,  as  in  N(CH3)2,  strengthens  the  auxochromic  properties  of 
the  radical,  while  negative  groups,  such  as  COCHs,  COCeHs, 
=CH-C6H5,  etc.,  when  substituted  for  hydrogen  as  in  NHCOCHs, 
effectively  weaken  these  properties.  Salt  formation  completely  neu- 
tralizes all  power  of  NH2  to  function  as  an  auxochrome  group.  The 
substitution  of  the  hydrogen  in  hydroxyl,  on  the  other  hand,  is  accom- 
panied in  the  case  of  methyl  by  a  slight  weakening  and  in  the  case  of 
acetyl  by  a  relatively  great  weakening  of  the  auxochromic  properties  of 

1  "Die  Valenzlehre,"  p.  500. 

2  Compare  E.  Noelting,  Chem.  Zeitung,  1910,  1016. 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     413 

the  radical.  Salt  formation  in  this  case  has  the  effect  of  greatly  increas- 
ing the  power  of  the  group.  Thus  ONa  is  almost  as  strong  an  auxo- 
chrome as  NH2. 

The  following  series  of  auxochromes  has  been  arranged  by  Kauff- 
mann  in  the  ratio  of  their  respective  opto-magnetic  anomalies: 

OCOCH3        OCH3        NHCOCHs        NH2          N(CH3)2    N(C2H5)2 
—0.26          +1.457          +1.949  +3.821         +8.587       +8.816 

The  figures  represent  the  relative  displacement  of  absorption  due  to  the 
substitution  of  the  various  groups  and  are,  therefore,  a  measure  of  the 
relative  activity  of  each  radical.  It  should  be  noted  that  an  acetylated 
hydroxyl  group  does  not  function  as  an  auxochrome,  while  methoxyl 
and  acetylamino  groups  are  very  weak  auxochromes. 

Two  methoxyl  groups  in  the  para-position  mutually  reinforce  each 
other,  and  have  a  combined  value  of  +2.999.  In  general  when  two 
or  more  auxochromes  are  present  in  a  molecule,  their  effect  upon  color 
seems  to  depend  to  a  very  great  degree  upon  their  relative  positions,  as 
is  strikingly  illustrated  in  the  case  of  the  dimethoxynitro-benzenes : 

OCH3  OCH3  OCH3  OCH3 

>N02 


Pale  yellow  Pale  yellow  Almost  colorless 

II  III  IV 

Condensed  ring  systems  such  as  naphthalene  and  anthracene  have 
the  same  effect  as  auxochrome  groups  and  produce  a  shifting  of  the 
absorption  bands  of  benzene  in  the  direction  of  the  visible  spectrum. 

It  must  be  clearly  recognized  that,  according  to  the  conceptions 
of  the  auxochrome  theory,1  the  change  in  color  which  accompanies  sub- 
stitutions is  due  on  last  analysis  not  solely  to  the  auxochrome  but  also  to 
the  changed  character  of  the  chromogen  itself.  The  substitution  of 
auxochrome  groups  is  undoubtedly  very  far  reaching  and  affects  not 
only  the  absorption,  but  also  the  refraction  and  the  magnetic  rotation 
of  the  substance.  Luminescence  is  also  affected,  as  has  already  been 
pointed  out.  Such  changes  and  others  have  a  common  basis,  accord- 
ing to  Kauffmann,  in  the  fact  that  substitution  always  involves  funda- 
mental readjustments  in  the  distribution  of  energy  in  the  case  of  both 

lBer.,  39,  1959(1906). 


414  THEORIES  OF  ORGANIC  CHEMISTRY 

the  auxochrome  and  of  the  chromogen.  In  particular  it  is  assumed 
that  in  this  process  the  unit  valencies  of  the  atoms  present  in  both  groups 
are  very  much  broken  up.  The  systematic  development  of  this  idea 
follows. 

The  fact  that  an  auxochrome  such  as  N(CHs)2  loses  its  power 
when  nitrogen  passes  from  the  trivalent  to  the  pentavalent  condition, 
can  be  explained  only  by  supposing  that  this  power  depends  in  some  way 
upon  the  presence  of  two  potentially  free  valencies.  It  is,  therefore, 
reasonable  to  assume  that,  when  an  auxochrome  such  as  N(CHs)2 
substitutes  for  hydrogen  in  the  benzene  nucleus,  this  process  involves 
an  exchange  of  affinity  not  only  between  nitrogen  and  the  carbon  atom 
with  which  it  is  in  direct  union,  but  also  between  it  and  the  other  five 
carbon  atoms  of  the  ring,  with  the  result  that  nitrogen  actually  passes 
into  what  approximates  the  pentavalent  condition.  Reasoning  by 
analogy  auxochromes  which  contain  oxygen  function  as  such  because  of 
the  tendency  of  oxygen  to  pass  into  the  quadrivalent  condition.  More- 
over, since  this  tendency  to  exercise  its  higher  valencies  is  less  marked 
in  the  case  of  oxygen  than  in  the  case  of  nitrogen  it  is  easy  to  under- 
stand why  the  former  element  is  a  less  effective  auxochrome  than  the 
latter.  The  distribution  of  the  two  higher  valencies  in  the  case  of  both 
elements  is  represented  as  taking  place  according  to  the  following 
scheme  : 


N(CH3)2 


It  should  be  noted  that  this  exercise  of  affinity  involves  the  carbon  atoms 
in  the  oriho-  and  para-  but  not  in  the  meta-positions  to  the  substituting 
group. 

This  exchange  of  valency  cannot  be  conceived  in  terms  of  a  single 
line  joining  the  central  atom  of  the  auxochrome  with  the  adjacent  carbon 
atom  of  the  ring,  but  must  rather  be  imagined  as  due  to  the  saturation  of 
innumerable  lines  of  force  which  radiate  in  all  directions  from  the  oxygen 
or  nitrogen  atom  respectively.  According  to  this  conception  the  oriho- 
and  para-positions  in  the  benzene  ring  may  be  regarded  as  centers 
toward  which  an  unusually  large  number  of  lines  of  force  converge  and 
they  should,  therefore,  be  centers  of  great  chemical  activity.  If  follows, 
also  that  the  quinoidal  arrangement  of  the  carbon  atoms  in  the  ring, 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     415 

which  has  already  been  referred  to  as  the  condition  D,  is  favored  by 
the  substitution  of  strong  auxochromes  in  the  para-position  to  each 
other.  This  becomes  apparent  by  reference  to  the  following  diagram : 


It  is  significant  in  this  connection  that  the  presence  of  even  weak 
auxochromes  in  the  para-positions  imparts  the  property  of  luminescence 
to  aromatic  compounds.  Indeed  the  strongest  luminophores  contain 
para-auxochromes.  So  important  are  these  particular  positions  in  the 
determination  of  the  specific  properties  of  a  given  substance  that  their 
mutual  reinforcement  is  referred  to  by  Kauffmann  as  the  "  law  of  dis- 
tribution of  auxochromes."1 

According  to  this  conception  the  saturation  of  the  affinity  of  a 
substituting  group  is  not  concentrated  at  a  given  point  in  the  benzene 
molecule  but  must  be  imagined  as  distributed  over  the  whole  radical. 
Under  such  circumstances  two  groups  in  either  the  ortJio-  or  para-posi- 
tions with  reference  to  each  other  interfere  with  each  other,  so  that  the 
full  exchange  of  affinity  between  either  auxochrome  and  the  benzene 
radical  becomes  impossible.  It  is,  therefore,  easy  to  understand  why 
substitution  in  these  positions  produces  what  may  be  termed  decentrali- 
zation of  valency.  Many  instances  of  the  decentralization  of  chemical 
functions  have  been  noted.2  It  has  been  observed,  for  example,  that 
while  a  substance  such  as  acetone  which  contains  only  one  carbonyl 
group  absorbs  in  the  region  of  the  ultra-violet  and  is,  therefore,  colorless, 
substances  which  possess  two  or  more  such  groups  in  adjacent  positions 
exhibit  the  phenomenon  of  color : 

CH3-CO-CO-CH3  CH3-CO-CO-CO-CH3 

Yellow.  Orange. 

This  is  explained  by  supposing  that  the  introduction  of  a  second  and  of  a 
third  carbonyl  group  in  positions  adjacent  to  the  first  produces  an 
increase  in  the  decentralization  of  the  chromophore  function  and  that 
this  is  accompanied  by  corresponding  changes  in  the  optical  properties 

1  Ber.,  39,  2724  (1906);  also  "Die  Valenzlehre,"  p.  503;  compare  Ber.,  46,  3792 
(1913);  and  47,  1919(1914). 

2  Ber.,  47,  1324  (1916);  also  "Die  Naturwissenschaften,"  6,  21  (1917). 


416  THEORIES  OF  ORGANIC  CHEMISTRY 

of  the  substance.  The  relationship  which  is  expressed  by  means  of 
conjugate  systems  of  double  bonds  seem  to  be  definitely  responsible  for 
the  phenomenon,  since  the  presence  of  two  isolated  carbonyl  groups  in  a 
molecule  is  not  productive  of  color.  This  is  strikingly  shown  by  a  com- 
parison with  the  following  substances : 

CH3-CO-CH2-CO-CH3  CH3.CO-CH2-CH2-CO-CH3 

Colorless.  Colorless. 

Kauffmann  1  recently  discovered  that  the  relative  acidity  or  basicity 
of  certain  compounds  is  a  direct  measure  of  the  degree  to  which  any  of 
the  principal  valencies  have  been  broken  up  into  smaller  units  of  affinity. 
This  observation  proved  to  be  so  general  in  its  nature  that  Kauffmann 
formulated  it  as  the  "  law  of  the  decentralization  of  chemical  func- 
tions/7 according  to  which  an  increase  in  the  acid  and  basic  properties 
of  a  compound  is  assumed  to  accompany  any  increase  in  the  decentrali- 
zation of  chemical  affinity.2 

Kauffmann  supposes  that  while  the  molecule  of  a  given  compound 
necessarily  possesses  a  fixed  and  definite  structure  it  may,  nevertheless, 
exist  in  an  almost  endless  variety  of  phases  which  depend  in  large  measure 
upon  external  conditions  such  as  heat,  etc.  For  example,  if  z  represents 
the  fraction  of  affinity  by  which  any  two  atoms  are  bound  together  in  a 
given  molecule,  it  follows  according  to  Kauffmann's  conception  that  this 
value  is  constant  only  under  perfectly  definite  conditions  and  that  it 
varies  continuously  with  slight  changes  in  conditions.  This  is  known 
as  the  "  principle  of  shifting  conditions  "  according  to  which  the  various 
forms  of  union  between  the  atoms  of  a  given  molecule  are  supposed  to 
vary  greatly  in  value,  depending  upon  conditions,  and  not  to  represent 
fixed  and  definite  units  of  affinity.3  The  most  reactive  positions  in  the 
molecule  are,  moreover,  those  which  are  characterized  by  the  greatest 
fluctuations  in  the  value  of  the  strength  of  union  between  any  two  of 
its  atoms  (z).  Further  applications  of  the  law  of  the  decentralization 
of  chemical  affinity  will  be  referred  to  again  later.4 

H.  Staudinger  and  N.  Kon  in  their  paper  on  "  The  Reactivity 
of  Carbonyl  ")5  differ  from  Kauffmann  in  their  interpretation  of  the 
relation  of  color  to  constitution.  They  explain  the  fact  that  auxo- 
chromes  increase  the  reactivity  of  carbonyl  and  other  unsaturated  groups 
by  supposing  that  the  presence  of  auxochromes  increases  the  partial 
valencies  on  these  groups.  Since,  however,  no  deepening  in  the  color  of 

'Ber.,  46,  3801  (1913);  52,  1425  (1919). 
2Ber.,  53,  263(1920). 
3Ber.,  49,  1324  (1916);  62,  1425  (1919). 
4Ber.,  47,  1324  (1916);  62,  1422  (1919). 
5  Annalen  der  Chemie,  394,  45  (1911). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      417 

a  substance  is  observed  unless  the  auxochrome  is  bound  to  the  carbonyl 
or  other  chromophore  by  means  of  the  benzene  ring,  it  is  necessary  to 
make  an  additional  assumption  and  to  suppose  that  under  the  influence 
of  the  auxochrome  the  normally  neutral  ethylene  bonds  of  the  ring 
become  actively  unsaturated.  In  other  words,  the  partial  valencies  on 
the  carbon  atoms  in  the  ring  are  simultaneously  increased: 

N  (CH3)2 


\ 


Speculation  along  these  lines  was  stimulated  in  1878  by  the  discovery 
that  fuchsine  and  related  dyes  may  be  regarded  as  derivatives  of  tri- 
phenylmethane.1  A  detailed  study  of  these  substances  by  E.  and  0. 
Fischer  demonstrated  that  dyes  are  not  formed  immediately  as  the 
result  of  introducing  hydroxyl  and  amino  groups  into  the  triphenyl- 
methane  molecule,  since  derivatives  such  as  CH(C6H4NH2)3,  for  exam- 
ple, are  colorless  even  in  the  form  of  their  salts.  They  readily  oxidize, 
however,  to  give  true  color  bases,  as  for  example  COH(C6H4NH2)3, 
and  the  latter  react  with  acids  to  give  salts  which  dissolve  in  water 
without  hydrolysis. 

In  studying  the  action  of  acids  upon  derivatives  of  triphenylcarbinol 
it  has  been  observed  that  salt  formation  is  always  accompanied  by 
the  loss  of  one  molecule  of  water  and  cannot,  therefore,  be  regarded  as 
a  process  involving  simply  the  addition  of  one  molecule  of  acid  to  an 
amido  group. 

Ci9Hi9N3O+HCl    -*    H20-fCi9Hi8N3Cl 

Pararosaniline  Parafuchsine 

Various  interpretations  of  this  reaction  have  been  suggested.     E.  and  O. 
Fischer  assume  that  it  takes  place  in  two  stages:2 

/C6H4NH2  /C6H4NH2  •  HC1 

!  /  +HC1=(H2NC6H4)2C< 

\HTT  > 


OH 


/ 


C6H4NH2 


\|OH 


H 


Cl 


1  Annalen  der  Chemie,  194,  286  (1878);  also  Ber.,  37,  3355  (1904). 
2Ber.,  12,2348  (1879). 


418  THEORIES  OF  ORGANIC  CHEMISTRY 

This  conception  supposes  a  complete  change  1  in  the  condition  of  one 
benzene  nucleus  since 


NH2C1 


is  evidently  directly  analogous  to  the  peroxide  formula  of  quinone. 

o-/ Vo 


Another  explanation  was  advanced  by  Rosenstiehl,2  who  assumed  that 
the  acid  reacted  directly  with  hydroxyl  and  that  the  amido  group  was 
in  no-wise  involved  in  the  reaction  : 


OH+HC1= 

Pararosaniline  Parafuchsine 

This  latter  interpretation  is  supported  by  the  fact  that  dyes  of  this  type 
are  capable  of  reacting  with  three  additional  molecules  of  hydrochloric 
acid  to  form  salts,  and  also  by  the  fact  that  parafuchsine  may  be  formed 
directly  from  carbon  tetrachloride  and  aniline.  It  was  not,  however, 
generally  accepted  at  that  time,  largely  because  it  failed  to  account  for 
the  obvious  analogy  existing  between  pararosaniline  and  hydrocyan 
pararosaniline  .3 

Although  Fischer's  formula  for  parafuchsine  was  undoubtedly 
quinoidal  in  character,  the  full  significance  of  this  was  not  realized 
until  1888,  when  R.  Nietzki  introduced  it  into  his  text-book  in  somewhat 
modified  form,  viz., 

=  H  Cl 

This  formula  was  preferred  to  Fischer's  original  formula  because  it  was 
closely  analogous  to  Fittig's  formula  for  quinone, 


which  was  then  in  current  use,  although  there  seemed  to  Nietzki  to  be  no 
very  fundamental  difference  between  the  two.4     Thus,  in  terms  of  the 

^er.,  26,  2223(1893). 

2  Bull.  soc.  Chimie  (2),  33,  342  (1880);  Compt.  rend.,  116,  194  (1893);  120,  192, 
264,331,740(1895). 

3  Compare  Fischer  and  Jennings,  Ber.,  26,  2222  (1893). 

4  "Chemie  der  Organischen  Farbstoffe,"  R.  Nietzki,  1901,  p.  120. 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     419 

Fischer-Nietzki  formula  fuchsine  and  other  substances  of  this  class  may 
be  regarded  as  derivatives  of  quinonimide  : 

>=NH 

This  conception  has  been  further  developed  and  somewhat  modi- 
fied recently  as  the  result  of  the  discovery  of  two  isomeric  orZ/io-quinones. 
R.  Willstatter  1  observed  that  when  pyrocatechol  is  carefully  oxidized 
by  means  of  silver  oxide  in  the  absence  of  moisture,  a  colorless  oxidation 
product  is  formed.  This  substance  is  very  unstable  and  isomerizes 
quickly  to  give  a  red  compound.  Both  of  these  products  possess  all  of 
the  characteristic  properties  of  quinone,  and  seem  therefore  to  corre- 
spond respectively  to  the  formulas  : 


and 


Colorless  Red 

Isomerism  of  this  sort  is,  of  course,  also  possible  in  the  case  of  the  para- 
quinones,  although  as  yet  no  colorless  compound  corresponding  to  the 
peroxide  formula  has  been  isolated. 

According  to  this  interpretation  it  is  necessary  to  assume  that  the 
color  bases  possess  a  very  different  structure  from  the  dyes  themselves, 
and  that  the  transformation  from  pararosaniline  into  parafuchsine, 
for  example,  takes  place  in  the  following  manner: 

(H2NC6H4)2C  -  C6H4NH2+HC1=(H2NC6H4)2C  •  C6H4NH3C1 

OH  OH 

H 


HO 


To  explain  the  formation  of  salts  containing  three  additional  molecules  of 
acid  it  is  supposed  that  two  molecules  of  acid  combine  with  the  two 
remaining  nitrogen  atoms,  and  that  the  third  combines  with  the  quinone 
nucleus  as  a  whole.  This  latter  reaction  is  conceivable,  since  it  is  known 
that  quinone  itself  possesses  the  characteristic  property  of  adding  one 
molecule  of  hydrochloric  acid.  Similar  interpretations  apply  to  a 
large  number  of  phenomena.  Indeed  the  Fischer-Nietzki  formula  was 
found  to  offer  a  satisfactory  explanation  in  so  many  cases  that  it  came  to 
to  be  very  generally  accepted,  and  further  discussion  of  the  problem 
gradually  ceased. 

1  Ber.,  41,  2580  (1908);  44,  2171  (1911). 


420 


THEORIES  OF  ORGANIC  CHEMISTRY 


In  1900  the  structure  of  dyes  of  the  triphenylmethane  type  again 
became  the  subject  of  controversy  as  the  result  of  Gomberg's  discovery 
of  the  so-called  triphenylmethyl.  This  discovery  led  to  the  revision  of 
certain  fundamental  conceptions  regarding  both  the  physical  and 
chemical  properties  of  the  element  carbon  and  most  therefore  be  con- 
sidered in  some  detail. 

The  discovery  of  the  triphenylmethyl  radical  has  been  referred 
to  earlier  in  this  text  and  the  fact  has  been  noted  that  although  con- 
centrated solutions  of  hexaphenylethane  are  colorless,  these  solutions 
become  yellow  on  dilution.  The  application  of  the  chromophore 
theory  in  the  interpretation  of  this  phenomenon  is  obviously  difficult,  if, 
as  has  been  assumed,  the  color  is  due  to  the  formation  of  the  free  tri- 
phenylmethyl radical  (CeHs)  sC The  first  theory  to  account  for 

color  was  advanced  by  Heintschel 1  and  was  based  upon  the  assumption 
that  the  substance  is  bimolecular : 


C6H5 


But  while  a  quinoidal  configuration  served  to  account  for  the  instability 
and  also  for  the  color  of  the  substance,  it  failed  to  explain  certain  other 
properties  as,  for  example,  the  tendency  of  the  substance  to  form  perox- 
ides and  other  addition  products.  This  explanation  was  therefore  soon 
abandoned  in  favor  of  another  which  was  advanced  by  P.  Jacobson  2 
and  which  assumed  the  following  configuration  of  hexaphenylethane  : 


(C6H5)2C 


(C6H5)3 

According  to  this  formula  the  substance  may  be  regarded  as  a  deriva- 
tive of  quinol  : 

)H 


and  might,  therefore,  be  expected  to  undergo  rearrangement  in  the  sense 

^JJ  TT 

(CeH^sC.+tCeHs^C^ 
'  (Cells)  3 

,H 

->    (C6H5)2C- 
H 

^er.,  36,  320  (1903). 
2Ber.,  38,  196  (1905). 


(CeH5)2C= 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      421 

since  in  quinol  both  the  hydroxyl  and  methyl  groups  are  mobile  and 
readily  migrate  into  the  ring.  This  formula  affords  a  satisfactory 
explanation  for  the  color  and  instability  of  the  substance  and  also  serves 
to  account  for  such  rearrangements  as  are,  for  example,  represented  in  the 
formation  of  benzhydrol  tetraphenylmethane  : 


(C6H5) 

(C6H5)3 

It  presupposes  a  bimolecular  formula  and  is  thus  in  agreement  with  the 
earlier  molecular  weight  determinations.  Further,  by  assuming  the 
possibility  of  rearrangements  which  involve  the  formation  of  the  radical 
(CeHs^C,  it  explains  why  the  substance  reacts  as  if  it  were  itself  tri- 
phenylmethyl.  This  very  ingenious  formulation  of  the  constitution  of 
hexaphenyle  thane  was,  however,  abandoned  in  turn  and  a  third  theory 
was  advanced  to  take  its  place.  The  latter,  which  is  current  at  the  pres- 
ent time,  resulted  from  a  very  exact  investigation  of  the  behavior  of 
solutions  of  hexaphenylethane,  but  in  order  to  understand  the  facts 
upon  which  it  is  based  it  is  now  necessary  to  review  certain  of  the  prop- 
erties of  this  very  interesting  substance. 

It  has  been  stated  that  triphenylmethyl  is  a  colorless  crystalline 
solid  which  dissolves  in  various  solvents  to  give  yellow  solutions. 
Gomberg  l  attempted  to  explain  this  phenomenon  by  supposing  that 
the  substance  is  capable  of  existing  in  the  form  of  two  isomeric  modifica- 
tions : 


C—  CoHs  /H 


c4 

C(CeH6)3  XC(C6H5)3 

Benzoid  Quinoid 

I  II 

Of  these  the  first  is  colorless  and  on  solution  passes  into  the  second  which 
is  yellow.  The  yellow  modification  on  evaporation  of  the  solvent  rear- 
ranges to  give  a  colorless  compound.  In  order  to  explain  the  great 
reactivity  of  solutions  containing  triphenylmethyl,  Gomberg  made  the 
further  assumption  that  the  substance  in  solution  dissociates  into  a 
benzoid  and  a  quinoid  ion: 

(C6H5)2C  :  CeH^  *±  [(CeH^C  :  C6H4<f    ]' 

XC(C6H5)3       L 
1  Compare  Schmidlin,  Ber.,  41,  2471  (1908). 


422  THEORIES  OF  ORGANIC  CHEMISTRY 

By  assuming  that  benzoid  ions  are  stable  in  certain  solutions,  and  that 
under  such  conditions  the  quinoid  modification  undergoes  immediate 
isomerization  into  the  more  stable  form,  it  is  possible  to  account  for 
the  great  reactivity  of  the  given  solutions,  since  the  benzoid  ion  is 
itself  nothing  more  or  less  than  triphenylmethyl. 

The  collective  reactions  of  triphenylmethyl  may  be  accounted  for 
on  the  basis  of  the  following  closely  related  hypotheses: 

1.  Tautomerism  in  the  sense  of  I  ^  II  as  shown  above. 

2.  The  partial  dissociation  of  II  in  all  solvents  to  form  quinoid  and 
benzoid  ions. 

3.  Tautomerism  in  the  sense  of  benzoid  ions  *=±  quinoid  ions.1 

It  has,  however,  already  been  pointed  out  in  the  chapter  on  free 
radicals,  where  the  investigations  of  W.  Schlenk  and  others  were  fully 
reviewed,  that  the  relations  which  have  just  been  described  are  much 
more  simple  in  character  than  the  present  discussion  might  lead  one  to 
suppose.  It  may  be  remembered  that  Schlenk  succeeded  in  preparing 
the  following  triaryl  derivatives  of  triphenylmethyl,  and  that  while  I 


C6H5\  C6H5\ 

C6H5^C...       CeHgCeH^C...      and 

C6H5-C6H4/ 


I  II  III 

and  II  closely  resemble  triphenylmethyl  in  being  colorless  and  bimolec- 
ular  in  the  solid  state,  III  is  deep  violet  in  color,  monomolecular  in  the 
solid  state,  and  is  in  general  an  exceedingly  unstable  substance.  These 
derivatives  of  triphenylmethyl  dissolve  in  various  solvents  to  give  solu- 
tions which  are  respectively  orange  and  deep  red  in  color  the  intensity 
of  color  deepening  as  we  approach  III.  As  in  the  case  of  triphenyl- 
methyl, molecular  weight  determinations  show  that  such  solutions 
contain  equilibrium  mixtures  of  mono-  and  bi-molecular  modifications: 

Ar3C-CAr3     <=>    2Ar3C... 

and  that  the  intensity  of  color  due  to  dilution  corresponds  to  the  degree 
of  dissociation  of  the  bimolecular  into  the  monomolecular  form.  The 
reverse  reaction  corresponds,  as  J.  Piccard  has  demonstrated,  to  diminu- 
tion of  color  and  a  simultaneous  increase  in  the  values  representing  the 
molecular  weights  until  finally  in  concentrated  solutions  the  color 
almost  disappears  and  the  molecular  weights  approximate  those  of  the 
respective  hexaphenylethanes. 

These  observations  lead  definitely  to  the  important  conclusion  that 
the  appearance  of  color  is  directly  associated  with  the  existence  of  a  free 

1  Ber.,  40,  1883  (1907) 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     423 

triaryl  radical  and  that  it  therefore  depends  upon  the  imsaturated 
condition  of  the  methane  carbon  atom  or,  in  other  words,  to  the  presence 
of  free  valency.  This  interpretation  has  had  an  important  influence 
upon  the  development  of  the  theory  of  the  relation  between  color  and 
constitution,  but,  before  this  matter  can  be  considered  in  detail,  it 
will  be  necessary  to  review  some  of  the  earlier  discussions  in  regard 
to  the  possible  configuration  of  colored  triarylmethyl  radicals.  Two 
possible  configurations  have  been  suggested,  namely: 


and 


or 

4\n_/~     \ / 

/\ 

The  relative  merits  of  these  formulas  depend  upon  certain  considera- 
tions which  may  now  be  briefly  reviewed. 

Following  closely  upon  the  discovery  of  triphenylmethyl  Norris  and 
Sanders  1  and  also  Kehrmann  and  Wentzel 2  observed  that  the  color- 
less triphenylmethyl  chloride  and  triphenyl  carbinol  dissolve  in  con- 
centrated sulphuric  acid  to  give  yellow  solutions.  Parafuchsine  behaves 
in  the  same  way  and  in  both  cases  the  solution  loses  its  yellow  color 
on  dilution,  and  a  colorless  carbinol  is  precipitated.  These  reactions 
are  difficult  to  explain.  In  general  they  resemble  the  decomposition 
of  salts  of  hydrochloric  acid  by  the  action  of  H2SC>4,  and  if  this  analogy 
holds  the  triphenylmethyl  radical  must  be  assumed  to  possess  basic 
properties  : 

(C6H5)3CC1+HOS02OH=HC1+ (C6H5)3C  •  OS02OH 

This  does  not,  however,  account  for  the  color  of  the  resulting  solution 
since  it  does  not  presuppose  the  formation  of  a  chromophore,  and  if 
(C6H5)3C  •  Cl  is  colorless,  a  sulphate  similarly  constituted  must  also  be 
assumed  to  be  colorless. 

Kehrmann  and  Wentzel  then  discovered  that  triphenylmethyl 
chloride  exists  in  two  modifications  one,  of  which  is  white  and  the  other 
orange  yellow,  the  latter  being  formed  when  triphenyl  carbinol  is  dis- 

1  Am.  Chem.  Jour,  25,  54  (1901). 
2Ber.,  34,3815  (1901). 


424  THEORIES  OF  ORGANIC  CHEMISTRY 

solved  in  acetic  acid  and  then  treated  with  concentrated  hydrochloric 
acid.  The  resulting  yellow  solution  was  found  to  be  stable  for  hours 
if  allowed  to  stand,  but  lost  its  color  on  the  addition  of  water.  The 
colorless  chloride  was  also  found  to  dissolve  in  liquid  sulphur  dioxide 
with  a  bright  yellow  color.  Triphenylmethyl  also  forms  a  colored 
perchlorate,  (CcH^aC-ClC^,  which  has  been  isolated,  as  well  as  a 
series  of  colored  double  salts,  as  for  example  (Cells)  3C-C1- Aids; 


Kehrmann  and  Wentzel  interpret  these  phenomena  by  supposing 
that  triphenylmethyl  chloride  and  similar  substances  exist  in  two 
tautomeric  modifications,  one  of  which  is  colorless,  ether-like,  and  ben- 
zoid  in  character,  while  the  other  is  yellow,  salt-like,  and  semi-quinoid  or 
quinolic  in  character: 

CeH5V 

Cells/ 

The  essentially  salt-like  properties  of  the  yellow  modification  have 
received  confirmation  as  the  result  of  a  series  of  investigations  under- 
taken by  Gomberg  and  Cone.1  They  observed  for  example  that  when 
tribromtriphenylmethyl  chloride,  (BrCeH^sC-Cl,  was  dissolved  in 
liquid  sulphur  dioxide,  the  solution  on  evaporation  gave  monochlor- 
dibromtriphenylmethyl  bromide.  This  product  is  essentially  different 
from  the  original  substance  in  that  it  reacts  readily  with  silver  chloride : 

/T>     /"^    TT    N  /"Dv/^    TT     \ 

\&  1^6114)  2  f->\-r>^\     A  ,^i         *     -p      il-Dri^G^M^  or1! 

C1C6H4  '   C1C6H4 

Thus  while  tribromtriphenylmethyl  chloride  contains  three  bromine 
atoms  all  of  which  are  in  union  with  ring  carbon  and  therefore  relatively 
inactive,  its  isomer  contains  one  bromine  atom  which  is  in  union  with 
methane  carbon  and  is,  therefore,  readily  displaceable. 

Monobromtriphenylmethyl  chloride  behaves  similarly  when  dis- 
solved in  sulphur  dioxide.  In  this  case  Gomberg  and  Cone  interpret 
the  reaction  as  taking  place  in  the  following  way: 

C6H5x       xC6H4Br  C6H5\          / \   /Cl 

>c\  -*        ^=(     A    - 

C6H5X    XC1  C6H5/  -/ \Br+AgCl 

CeHsv  , \    /Cl 

\P /  xX  \r*/ 

/^ — \  /\  ~^  /C< 

C6H5/  XC1  CeHs/    \C1 

1Annalen  der  Chemie,  376,  183  (1910). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     425 

Doubt  has  been  expressed  as  to  the  value  of  this  interpretation.  It 
offers,  for  example,  no  satisfactory  explanation  for  the  phenomenon 
of  color  since,  substances  of  the  general  formula 


must  be  regarded  as  derivatives  of  quinol  and  are  not  in  any  sense  true 
quinone  derivatives.  Baeyer  and  Villiger  1  point  out,  moreover,  that  if 
true  quinones  are  actually  present  in  solutions  of  triphenylmethyl 
chloride  in  concentrated  sulphuric  acid,  and  in  solutions  of  its  colored 
double  salts,  the  existence  of  these  substances  should  manifest  itself  in 
the  increased  sensitiveness  of  such  solutions  to  the  action  of  oxidizing 
and  reducing  agents,  etc.  Such  is  not,  however,  the  case. 

Baeyer  and  Villiger  2  therefore  advanced  the  theory  that  in  all 
such  cases  color  could  be  explained  as  due  to  salt  formation: 


(C6H5)3COH+HOS03H(HC1,  etcO^^O+CCeHsJaC-OSOaHCCl,  etc.) 

Colorless  Yellow 

They  had  discovered,  as  a  result  of  earlier  investigations,  that  tertiary 
alcohols  possess  basic  properties3  and  they,  therefore,  concluded  that 
salt  formation  was  directly  analogous  to  the  action  which  takes  place 
between  inorganic  acids  and  bases,  as  for  example  KOH  and  HC1.  At 
first  sight  this  explanation  seems  to  be  the  same  as  that  which  Rosenstiehl 
advanced  in  1888  to  explain  the  formation  of  fuchsine,  but  it  differs  in 
one  important  .  respect.  Baeyer  and  Villiger  conceived  that  the  carbon 
atom  which  is  present  in  the  triarylmethyl  radical  functions  in  exactly 
the  same  way  as  the  nitrogen  atom  in  the  ammonium  radical,  and  that 
the  complex  (CeHsJsC  therefore  plays  the  part,  of  a  metal  in  the  resulting 
salt.  They  assume,  moreover,  that  this  change  in  function  is  accom- 
panied by  a  change  in  the  character  of  one  of  the  valencies  of  the  carbon 
atom,  and  in  order  to  distinguish  between  the  ordinary  non-ionizable 
valencies  of  carbon  and  such  a  metallic  ionizable  valence,  they  represent 
the  latter  by  means  of  a  waving  line  -  and  refer  to  it  as  a  car- 
bonium  valence. 

Both     triphenyl    carbinol    and    triphenylmethyl    chloride   may   be 
regarded  as  possessing  the  usual  formulas 

(C6H5)3COH    and     (C5H5)3CC1 

iBer.,  36,  1195  (1902). 

2  Baeyer,  Villiger,  and  others  on  the  Carbonium  Theory:  Ber.,  35,  1754,  3013 
(1902);  36,  2774  (1903);  37,  597,  1183,  2848,  3191  (1904);  38,  569,  1156  (1905); 
40,  3083  (1907);  42,  2624  (1909). 

3  Ber.,  36,  3015  (1902). 


426  THEORIES  OF  ORGANIC  CHEMISTRY 

when  present  in  the  solid  state  since  their  basic  properties  are  not 
apparent  in  this  form.  The  basic  functions  of  these  substances  become 
evident,  however,  when  they  are  dissolved  in  the  presence  of  certain 
metallic  chlorides,  concentrated  sulphuric  acid,  etc.,  and  under  such 
conditions  they  may  be  formulated  respectively  as : 

(C6H5)3CCl+SnCl4  =  (C6H5)3C~  Cl  -  SnCU 
(C6H5)3C .  OH+H2S04  =  H20+ (C6H5)3C-OS03H 

Salts  of  methoxy  and  halogen  derivatives  of  triphenyl  carbinol  may  be 
formulated  in  an  analogous  way. 

The  phenomenon  itself  is  referred  to  as  halochromism,1  and  is  denned 
by  Baeyer  as  the  transformation  of  a  colorless  or  weakly  colored  sub- 
stance into  an  intensely  colored  salt  without  the  simultaneous  formation 
of  a  chromophore  group,  such  as,  for  example,  the  quinoid  group. 

In  order  to  verify  this  theory  it  was  necessary  to  demonstrate  that 
alcohols  such  as  triphenyl  carbinol  actually  possess  basic  properties 
and  that  they  combine  with  acids  to  give  true  salts.  With  this  end  in 
view  Baeyer  and  Villiger  endeavored  to  increase  the  basicity  of  sub- 
stances of  this  type  by  substitutions  of  hydrogen  in  the  benzene  ring. 
They,  therefore,  prepared  the  ortho-,  meta-,  and  para-trianisyl  carbinols 
and  found  that  all  of  these  compounds  possess  greater  basicity  than  the 
corresponding  triphenyl  carbinols,  as  is  evident  from  the  fact  that  they 
react  even  with  dilute  acids  to  form  well-defined  salts.  Substitution 
in  the  para  position  was  observed  to  produce  the  greatest  relative 
increase  in  basic  properties  while  that  in  the  meta  position  produced  the 
least. 

The  chloride  (CHsOCeH^sC  •  Cl  is  colorless,  but  combines  with 
an  excess  of  hydrochloric  acid  to  form  a  red  salt.  A  nitrate  having 
the  composition  (CH3O-C6H4)3C-ONO2+liHNO3  is  also  intensely 
colored.2 

The  essentially  salt-like  character  of  substances  of  this  type  has  been 
demonstrated  by  P.  Walden,  who  discovered  that  triphenylchlormethane 
and  triphenylbrommethane  dissolve  in  liquid  sulphur  dioxide  to  give 
solutions  which  are  yellow  in  color  and  which  resemble  electrolytically 
dissociated  salts  in  the  way  in  which  they  conduct  the  electric  current. 
Schlenk  and  Marcus  3  were  subsequently  able  to  show  that  the  sodium 
salt  of  triphenymethyl  actually  suffers  electrolytic  dissociation  during 
the  passage  of  the  electric  current 

(C6H5)3C-Na    ->     [(C6H5)3C]-+Na+ 
^er.,  61,  1828  (1918). 
2Ber.,  35,  1200  (1902). 
3Ber.,  47,  1678  (1914). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION      427 

and  that  free  triphenylmethyl  is  deposited  in  considerable  quantities 
at  the  anode  following  the  neutralization  of  the  electric  charge  of  the 
dissolved  ion. 

But  even  if  triphenyl  carbinol  and  its  derivatives  may  be  assumed  to 
be  true  bases  which  form  salts  in  the  normal  way  with  acids,  the  final 
constitution  of  these  substances  still  remains  to  be  determined.  It  will 
be  recalled  that  the  Kehrmann-Gomberg  formula  assumes  a  quinoidal 
configuration  for  the  molecule.  If,  however,  the  sulphate  of  trianisyl 
carbinol  is  formulated  in  this  way 

/        \    /'OCRs 
(H3COC6H4)2C=< 

OjSO3H 

it  should  follow  that  methyl  sulphuric  could  be  readily  split  off  in  the 
sense  indicated  by  the  dotted  lines;  but  as  a  matter  of  fact  this  is  not 
the  case.  Baeyer  and  Villiger  have  pointed  out  other  facts  which  are 
in  direct  contradiction  to  the  Kehrmann-Gomberg  formula.  For 
example  during  the  process  of  introducing  successively  one,  two,  and 
three  methoxy  groups  into  the  molecule  of  triphenyl  carbinol,  they  dis- 
covered that  the  increase  in  basicity  is  not  an  additive  function  of  the 
molecule  but  that  it  takes  place  according  to  the  law  of  geometrical 
progression.  Thus  if  1  equals  the  basicity  of  triphenyl  carbinol,  \-\-n 
will  equal  the  basicity  of  monomethoxytriphenyl  carbinol,  (l+ri)2  that 
of  dimethoxytriphenyl  carbinol  and  (1+r*)3  that  of  trimethoxytriphenyl 
carbinol.  From  this  it  follows  that  the  three  aryl  groups  function  in 
exactly  the  same  manner.  Such  a  relation  would  be  obviously  impos- 
sible if  one  of  these  groups  were  quinoid  while  the  other  two  were  benzoid 
in  structure. 

The  strongest  evidence  in  support  of  the  Kehrmann-Gomberg  hy- 
pothesis is  offered  by  the  fact  that  tribromtriphenylmethyl  chloride  on 
solution  in  liquid  SO2  and  evaporation  of  the  solvent  gives  an  isomeric 
substance  containing  a  reactive  bromine  atom.  This  transformation 
has  already  been  referred  to  and  has  been  interpreted  in  the  following 
manner : 

(BrC6H4)2C  (BrC6H4)2C  •  Br 

(BrC6H4)2C.Cl  )|  I 


Br     Cl 

This  explanation  of  the  phenomenon  is,  however,  opposed  by  Schlenk 
and  Marcus  who  hold  that  the  assumption  of  such  a  fundamental  trans- 


428  THEORIES  OF  ORGANIC  CHEMISTRY 

formation  in  the  structure  of  the  molecule  is  not  justified  by  the  facts  of 
the  case.  An  increase  in  the  chemical  reactivity  of  one  bromine  atom  as 
a  result  of  the  change,  by  no  means  necessitates  the  assumption  of  quinoid 
structure,  since  similar  phenomena  have  been  observed  in  cases  where 
no  color  changes  have  been  involved.  For  example,  tribrombenzene 
diazonium  chloride  readily  rearranges  into  dibrommonochlorbenzene 
diazonium  bromide. 

_    Br  Br 

Br/      ~V-N2C1     ->    Cl<f        V-N2Br 


when  dissolved  in  alcohol  in  the  cold  or  even  in  the  solid  state  upon 
standing,  and  this  transformation  takes  place  without  any  apparent 
change  in  color  during  the  process.1 

Certain  instances  of  changes  in  color  due  to  salt  formation,  and  similar 
in  character  to  those  which  have  been  described  in  the  case  of  the  triary- 
carbinols,  have  been  observed  in  cases  where  any  explanation  of  the 
phenomenon  on  the  basis  of  a  quinoid  structure  is  difficult.  For  exam- 
ple, A.  Tschitschibabin  and  Gawrilow,2  on  the  one  hand,  and  W.  Schlenk 
and  R.  Ochs3  on  the  other,  have  succeeded  in  preparing  trithienyl 
carbinol. 

OH     HC CH 


and  have  found  that  this  substance  dissolves  in  acids  to  give  orange- 
brown  solutions.  According  to  Schlenk  and  Ochs  the  perchlorate 
resembles  triphenylmethyl  perchlorate  in  a  most  amazing  manner. 

Although  it  at  first  seemed  impossible  to  explain  the  facts  by  sup- 
posing a  quinoid  rearrangement,  the  following  formula  for  trithienyl- 
carbinol  : 


/v        TT 

=C 


(C4H3S)2C=C  C 

104 


s 

'Ber.,  30,  2334  (1897). 

2 Jour.  Russ.  Physikal.  Chem.  Ges.,  46,  1614  (1914);  Chem.  Centralbl.,  1915,  II,  78. 

3Ber.,  48,  676  (1915). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     429 


was,  nevertheless,  finally  constructed.  This  interpretation  of  the  phe- 
nomenon was  due  in  part  to  the  discovery  of  the  quinoid  character  of 
maleic  anhydride  and  its  derivatives  by  P.  Pfeiffer  and  T.  Bottler.1 

Hantzsch  has,  moreover,  recently  pointed  out  on  the  basis  of  optical 
investigations  that  it  is  by  no  means  necessary  to  abandon  entirely 
quinoid  formulas  for  the  triphenylmethane  dyes,  since  many  of  the 
properties  of  these  substances  may  be  satisfactorily  accounted  for  by 
assuming  that  they  represent  conjugated  quinoid  combinations.2  If 
the  absorption  curves  of  triphenyl  carbinol,  and  hexamethyltriamino- 
triphenyl  carbinol  are  compared  (Fig.  1),  it  is  apparent  that  the  latter 
(curve  2)  possesses  a  stronger  absorption  than  the  first  (curve  1),  but 
that  both  represent  relatively  simple  types  of  color-bases.  The  trans- 


Number  of  Oscillations 
1500      2000      2500      3000      3500      4000 


d.b 
q  f\ 

£ 

\ 

9  ^ 

/ 

I 

\ 

\ 

\ 

I 

L 

9  0 

/ 

/ 

***, 
N 

\ 

1  5 

i3 

I 
/ 

\\ 

fS 

A. 

\ 

1  0 

1 

/ 

I       ' 

c 

\  \ 

t 

/' 

s 

0  n 

t 

i   / 

: 

» 

V_ 

dumber  of  Oscillations 
1500    2000      2500      3000      3500      4000 


II 
If 


/  \ 


FIG.  1. 


FIG.  2. 


formation  from  hexamethyltriaminotriphenyl  carbinol  into  its  salt 
(crystal  violet)  is,  on  the  other  hand,  accompanied  by  marked  changes  in 
optical  properties,  since  the  latter  substance  is  characterized  by  banded 
absorption  extending  into  the  region  of  the  visible  spectrum  (curve  3). 
Many  other  instances  might  be  cited  to  show  that  as  a  rule  free  color 
bases  possess  relatively  simple  absorptions  and  that  banded  absorption 
arises  as  the  direct  result  of  salt  formation.  It  is  rather  remarkable  to 
find  in  this  connection  that  two  dyes  which  appear  to  the  eye  to  be  so 
distinctly  different  as  crystal  violet  and  fuchsine,  should  possess  absorp- 
tion curves  which  vary  so  little  as  those  which  are  represented  in  Fig.  2. 
Indifferent  solvents  seem  to  have  very  little  effect  upon  the  absorp- 
tion of  dyes  as  is  apparent  from  a  study  of  the  absorption  curves  of 
crystal  violet  in  solution  in  chloroform,  alcohol,  and  water  respectively 

(Fig.  3). 

i  Ber.,  61,  1828  (1918).  2  Ber.,  52,  509  (1919). 


430 


THEORIES  OF  ORGANIC  CHEMISTRY 


The  flattening  of  the  curve  in  the  case  of  the  aqueous  solution  may  be 
accounted  for  on  the  assumption  that  an  unstable  addition  product  is 
formed  by  the  action  of  the  solvent  upon  the  quinoid-cation  of  the  dye. 
In  comparing  the  absorption  of  quinoid  dyes  derived  from  mono-,  di-, 
and  tri-phenylmethane  Hantzsch  made  an  important  discovery.  Baeyer 
had  previously  demonstrated  that  while  quinoid  monoamino-salts  are 
strongly  colored  they  cannot  be  regarded  as  true  dyes  and  that  the 
distinct  characteristics  of  a  dye  appear  only  after  the  introduction  of  a 
second  amino  group.  This  was  somewhat  difficult  to  explain  in  view 
of  the  fact  that  the  introduction  of  a  third  amino  group  did  not  have  the 
effect  of  further  intensifying  the  color  of  the  dye  as  was  to  be  expected 
from  an  auxochrome  group  but,  on  the  contrary,  actually  played  the 


Number  of  Oscillations 
1500      2000      2500      3000       3500     4000 


Number  of  Oscillations 
2000     2500      3000      3500      4000     4500 


4.0 
3.5 
3.0 
2.5 
2.0 
1.5 

i  n 

3\'.2 

\  ; 

\ 

\  \ 

\\ 

\\ 

/•" 

/ 

//• 

\\ 

\ 

\\ 

I 

V 

\ 

S 

\\ 

\v 

f 

FIG.  3. 


FIG.  4. 


part  of  a  weak  hypsochrome.  Similar  observations  were  made  by 
Hantzsch  and  F.  Hein  in  connection  with  a  study  of  the  alkali  salts  of 
nitrome thane  and  nitrotriphenylmethane.  Thus  mononitrome thane 
forms  colorless  salts  which  show  no  absorption  in  the  region  of  the  pene- 
trable ultra-violet  while  dinitromethane,  on  the  other  hand,  forms  yellow 
salts  which  are  characterized  by  strongly  banded  absorption  (Fig.  4). 
The  introduction  of  a  third  nitro  group  into  the  molecule,  on  the  other 
hand,  produces  nothing  which  even  approximates  the  radical  change 
induced  by  the  introduction  of  the  second  nitro  group.  In  other  words 
salt  formation  in  the  case  of  dinitro  and  diamino  derivatives  of  triphenyl- 
methane  is  attended  by  marked  changes  in  optical  properties  which 
cannot  be  accounted  for  on  the  basis  of  the  older  conceptions  of  the 
so-called  theory  of  auxochromes. 

It  would  seem  to  follow  from  a  consideration  of  these  facts  that  only 
two  nitro  or  two  amino  groups  play  an  important  part  in  the  production 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     431 


of  color  during  salt  formation  and  that  in  cases  where  a  third  such  group 
is  present  in  the  molecule  its  role  is  distinctly  subordinate.  In  other 
words  the  chromophore  group  may  in  every  instance  be  assumed  to 
result  from  the  interaction  of  two  and  only  two  amino  or  nitro  groups. 
This  conception  finds  expression  in  the  so-called  conjunction  formulas 
which  have  been  introduced  by  Hantzsch  to  represent  these  dyes: 


M 


and 


N0< 


•C/ 
\ 


M 


M      and      NO2-C6H4C 


M 


x, 
C6H5C<f 


•  NR2\  ^,V>6J-L4  •  -1A  AV2\ 

>X      and       NR2C6H4Cf  >X 

•NR2'X  N^6H4.NR2/ 

These  formulas  assume  that  the  difference  in  the  form  of  union  of 
the  two  conjugated  nitro  or  amino  groups, — which  is  shown  by  the 
fact  that  one  forms  part  of  a  quinoid  grouping  while  the  other  does  not, — 
involves  no  fundamental  change  in  the  constitution  of  these  groups 
which  are  in  all  other  respects  identical.  No  special  form  of  expression 
is,  therefore,  necessary  to  indicate  this  difference  or  to  differentiate  these 
groups  from  the  third  nitro  or  amino  group  which  differs  from  both  of  the 
other  two  in  that  it- does  not  form  an  integral  part  of  the  conjugated 
complex  and  is  not  in  direct  union  with  the  acid  or  metallic  ion. 

It  has  been  observed  that  the  absorption  curves  of  the  simplest 
aminoazobenzenes  and  the  simplest  fuchsines  very  closely  resemble  each 
other. 

Number  of  Oscillations 

2000      2500      3000      3500     4000 


1.  Dimethylaminoozo  benzene; 

2.  Fuchsine.  Both  substances  in  very 
dilute  HC1. 


432  THEORIES  OF  ORGANIC  CHEMISTRY 

This  is  not  very  surprising,  however,  in  view  of  the  fact  that  both  classes 
of  dyes  contain  the  chromophore  grouping 

=NR2C1 

and  may,  therefore,  be  supposed  to  form  conjugated  quinoid  complex 
salts.  In  the  case  of  the  azo  dyes  the  acid  ion  is  assumed  to  be  simul- 
taneously in  union  with  the  quinoid  amino  group  and  a  second  amino 
group : 

C6H4:NR2 

N<  >X 

NNC6H4-H  (orR) 

In  summary  it  may  be  said  that  this  interpretation  of  dye  formation 
is  significant  because  according  to  it  the  optical  properties  of  three 
important  groups  of  dyes  may  be  regarded  as  due  to  the  same  general 
causes,  viz., 

1.  The  presence  in  the  molecule  of  a  quinoid  complex  containing 
nitrogen  which  is  strongly  colored  and  which  plays  the  role  of  a  chromo- 
phore. 

2.  The  presence  of  a  second  group  containing  nitrogen  which  is  not 
itself  colored  but  which  when  coupled  with  a  quinoid  group  plays  the 
role  of  a  strong  auxochrome. 

The  well-known  fact  that  all  of  the  above  dyes  "are  materially  changed 
by  the  action  of  acids  may  be  accounted  for  on  the  basis  of  this  concep- 
tion by  assuming  that  the  acid  enters  into  chemical  combination  with 
the  nitrogen  residue  and  thus  destroys  its  power  of  forming  an  inner 
complex  salt. 

The  fact  that  the  introduction  of  a  third  amino  or  nitro  group  into 
triphenylmethane  and  its  derivatives  produces  a  lightening  of  the 
color  and  not  a  deepening  as  might  be  expected,  may  be  explained  by  a 
slight  modification  of  H.  Kauffmann's  theory.1  It  will  be  recalled  that 
halochromism  has  been  investigated  from  two  entirely  independent 
points  of  view.  On  the  one  hand  attention  has  been  concentrated  upon 
the  phenomenon  of  salt  formation  and  an  effort  has  been  made  to  ascer- 
tain the  causes  which  under  varying  conditions,  operate  to  produce 
salts  while,  on  the  other  hand,  attention  has  been  concentrated  upon  the 
phenomenon  of  color  and  effort  has  been  directed  to  ascertain  its  cause. 
Baeyer  and  Villiger  2  were  able  as  early  as  1902  to  define  the  character  of 
a  salt  in  terms  of  the  ease  with  which  it  tends  to  hydrolyze  and  they  even 

ifier.,  52,  1422  (1919). 
2  Ber.,  35,  3019  (1902). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     433 

formulated  certain  general  rules  in  regard  to  salt  formation.  The 
phenomenon  of  color  does  not,  however,  conform  to  the  same  rules 
since  it  has  been  observed  that  while  a  deepening  in  color  is  sometimes 
proportional  to  the  basicity  of  a  substance,  the  opposite  is  also  sometimes 
true.  Indeed  the  investigation  of  halochromism  in  the  case  of  the 
triphenyl  carbinols  convinced  Kauffmann  that  salt  formation  and  color 
represent  entirely  independent  phenomena  and  that  each  is  governed 
by  its  own  particular  laws. 

In  applying  the  conception  of  the  decentralization  of  chemical 
function  to  the  colored  salts  of  the  triphenyl  carbinols,  Kauffmann 
assumes  that  the  cation  does  not  necessarily  have  its  seat  on  a  single 
atom  but  that  the  total  positive  charge  is  frequently  made  up  of  smaller 
units  which  originate  respectively  on  different  atoms  located  in  different 
parts  of  the  molecule.  Since  all  such  fractions  of  a  given  cation  may  be 
supposed  to  represent  small  positive  charges,  they  will  mutually  repel 
each  other.  This  conception  is  embodied  in  Kauffmann's  first  principle 
governing  the  partition  of  cation  valencies,  "  cationic  valence  parts  do 
not  saturate  each  other." 

According  to  Kauffmann's  principle  of  shifting  relationships  the 
different  components  of  any  molecule  are  in  a  condition  of  constant 
flux  and  it  may,  therefore,  be  supposed  that  the  parts  which  together 
constitute  the  total  charge  of  the  positive  complex  do  not  represent  fixed 
and  definite  values  but  that  they  vary  incessantly, — an  increase  in  the 
positive  value  of  the  charge  at  certain  points  in  the  molecule  corre- 
sponding to  a  decrease  at  other  points.  Kauffmann  assumes,  moreover, 
that  cationic  partial  valencies  may  be  present  in  the  molecule  even  when 
the  molecule  in  question  does  not  possess  the  power  of  ionic  dissociation. 
This  follows  from  the  fact  that  inner  complex  salts  exist  in  which  cationic 
partial  valencies  are  neutralized  inside  the  molecule.  The  presence  of 
valencies  of  this  type  cannot  be  detected  by  means  of  electrolytic 
measurement  and  is  only  apparent  from  a  study  of  the  shifting  relation- 
ships within  the  molecule.  Kauffmann's  second  principle  in  regard  to 
cationic  partial  valencies  is  based  upon  the  above  considerations  and 
supposes — "those  partial  valencies  which  act  in  competition  with 
cationic  valencies  are  themselves  cationic  in  character." 

Auxochromes,  according  to  Kauffmann,  represent  groups  of  atoms 
which  possess  cationic  partial  valencies.  This  follows  from  the  fact 
that  the  ammonium  valency  of  the  nitrogen  in  the  amino  group,  for 
example,  is  not  required  for  the  saturation  of  the  phenyl  group  and  n 
certain  fraction  of  the  total  affinity  of  the  nitrogen  atom  is,  theivfore, 
available  in  the  form  of  free  positive  partial  valencies  which  may  be 
exercised  in  saturating  other  positions  within  the  molecule.  This  con- 


434  THEORIES  OF  ORGANIC  CHEMISTRY 

ception  finds  expression  in  the  following  formulas  for  the  colored  salt 
of  tetramethyldiaminobenzhydrol  (I)  and  crystal  violet  (II). 


4  -  Aux..., 

4  -  Aux.  ^.:--An  ^C6Ht  •  Aux.  *-> 

'' 


I  II 

where  the  ionic  partial  valencies  are  represented  by  means  of  dotted 
lines  while  other  fractions  of  the  total  affinity  of  any  atom  which  are  less 
than  a  so-called  unit  valence  are  represented  by  curved  lines  terminating 
in  dots.  According  to  the  latter  formula  the  fourth  valency  of  the 
methane  carbon  atom  is  represented  as  broken  up  into  a  number  of  parts 
which  are  distributed  among  the  three  phenyl  groups  and  the  anion. 
Since  depth  of  color  is  supposed  to  correspond  to  the  degree  of  decen- 
tralization of  valency  crystal  violet  (II)  might  be  expected  to  possess  a 
deeper  color  than  tetramethyldiaminobenzhydrol  but  as  a  matter  of  fact 
this  is  not  the  case.  Kauffmann  accounts  for  this  apparent  discrepancy 
by  his  third  principle  in  regard  to  cationic  partial  valencies  according  to 
which  —  "  the  decentralization  of  ionic  charge  does  not  function  in  the 
production  of  color."  This  is  further  illustrated  by  the  fact  that  mala- 
chite green  (III). 


in 

exhibits  greater  decentralization  of  its  cationic  valence  than  does  the 
salt  of  tetramethyldiaminobenzhydrol,  but  less  than  the  salt  of  crystal 
violet.  Here  again  these  properties  seem  to  bear  no  relation  to  the 
optical  properties  of  these  respective  substances. 

In  summary  it  may  be  said  that  while  a  decentralization  of  the 
cationic  charge  has  been  observed  to  correspond  to  an  increase  in  the 
basic  properties  of  the  methane  carbon  atom,  this  is  not  proportional  to 
a  deepening  in  the  color  in  the  case  of  certain  compounds.  From  this  it 
would  seem  to  follow  that  the  methane  carbon  atom  is  not  the  seat  of 
color  in  these  substances.  Similar  relationships  exist  in  the  case  of  a 
number  of  different  compounds  which  have  been  investigated  by 
Hantzsch  and  even  in  certain  instances  where  OCH3  replaces  N(CH3)2 
in  the  molecule.  To  explain  the  phenomenon  of  color  Kauffmann 
supposes  that  the  seat  of  color  is  located  on  a  ring  carbon  atom.  This 
assumption  holds  in  the  case  of  nitro  as  well  as  in  the  case  of  amino 
derivatives  of  triphenylmethane.  Salts  of  triphenyl  carbinol,  for 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     435 

example,  contain  three  distinct  centers  of  color,  namely,  the  three  ring 
carbon  atoms  which  are  in  union  with  the  methane  carbon  atom. 

H.  Kauffmann  1  carries  the  analysis  further  and  is  of  the  opinion 
that  the  basic  properties  which  have  been  observed  in  the  case  of  these 
substances  are  not  actually  properties  of  the  methane  carbon  atom  but 
that  they  denote  a  general  decentralization  of  the  chemical  functions 
not  only  of  the  central  carbon  atom  but  also  of  other  atoms  in  union 
with  it.  Kauffmann  assumes  that  the  exchange  of  affinity  within  the 
molecule  regulates  the  degree  of  basicity  and  simultaneously  the  color 
of  any  given  compound.  In  the  case  of  fcriphenyl  carbinol,  for  example, 
the  basicity  is  increased  by  the  introduction  of  auxochrome  groups  in 
favored  positions  and  is  most  marked  when  two  such  groups  occupy 
para  positions  with  reference  to  each  other.  This  relationship  may  be 
expressed  by  means  of  the  following  diagram 


in  which  the  dotted  lines  that  are  supposed  to  represent  the  fourth 
valency  of  the  carbon  atom  decrease  in  size  in  proportion  to  the  strength 
of  the  particular  auxochrome.  The  ability  of  the  methane  carbon  atom 
to  hold  a  fourth  atom  or  group  is  represented  as  weakened  by  the  intro- 
duction of  substituents  in  the  place  of  hydrogen  in  the  benzene  ring,  and 
it,  therefore,  follows  that  an  increase  in  the  number  of  auxochrome  groups 
will  be  accompanied  by  a  decrease  in  the  stability  of  the  union  between 
the  methane  carbon  atom  and  a  fourth  atom  or  group.  This  will  become 
apparent  in  a  tendency  to  ionization,2  and  trianisylmethyl  chloride,  for 
example,  will  be  found  to  possess  perfectly  definite  salt-like  properties. 
In  the  case  of  parafuchsine  the  decentralization  of  valency  on  both  the 
carbon  and  the  chlorine  atom  may  be  assumed  to  be  very  great,  as  shown 
by  the  following  diagram: 


The  interdependence  of  these  two  atoms  is  obvious  and  can  be  followed 
experimentally  since  the  breaking  up  of  the  total  affinity  of  the  carbon 
atom  into  a  number  of  relatively  small  partial  valencies  has  long  been 

1  Ber.,  46,  3794  (1913). 

2  Ber.,  46,  3794  (1913). 


436 


THEORIES  OF  ORGANIC  CHEMISTRY 


associated  with  the  phenomena  of  color,  while  the  same  condition  in  the 
case  of  the  chlorine  atom  has  been  associated  with  the  phenomena  of 
ionization. 

Halochromism  has  been  observed  in  widely  separated  fields  of 
organic  chemistry  but  even  the  most  divergent  groups  of  colored  com- 
pounds have  been  interpreted  and  related  by  means  of  Pfeiffer's  theory, 
which  may  now  be  considered  in  some  detail. 

Many  colorless  or  weakly  colored  compounds  dissolve  in  mineral 
acids  to  give  solutions  which  are  intensely  colored.  Dibenzal  acetone, 
for  example,  and  other  allied  substances  behave  in  this  way.  The  color 
of  the  product  depends  in  any  given  case  upon  the  character  of  the  sub- 
stituted phenyl  group  and  of  the  acid  radical.  Thus  dibenzal  acetone 
reacts  with  concentrated  sulphuric  acid  to  give  a  product  which  is 
reddish  orange,  while  with  hydrochloric  acid  the  product  is  dark  red, 
and  with  hydrogen  iodide,  black.  In  each  case  the  color  is  destroyed 
upon  the  addition  of  water  and  dibenzal  acetone  is  precipitated.  The 
compounds  which  are  formed  in  this  way  have  been  shown  to  be  addition 
products  of  dibenzal  acetone  with  one  or  more  molecules  of  acid. 
H.  Stobbe  has  undertaken  a  series  of  very  exact  investigations  which 
tend  to  show  that  in  the  case  of  hydrochloric  acid  such  addition  products 
have  the  general  formula  ketone+zHCl,  and  that  the  value  of  x  varies 
inversely  as  the  temperature. 

Variations  of  color  due  to  substitutions  in  the  phenyl  group  are 
shown  in  the  following  table : 

Ar  •  CH=CH— CO— CH=CH  •  Ar'  • 


Moles  HC1  at 

Color  of  Salt  at 

Ar 

Ar' 

15° 

-75° 

15° 

C6H5 

C6H5 

2 

4 

Red 

CH3OC6H4  (1—4) 

CH3OC6H4  (1—4) 

2 

5 

Violet  black 

C2H5OC6H4  (1—2) 

C2H5OC6H4  (1—2) 

2 

4 

Violet 

CeHs 

CH3OC6H4 

1.5 

4 

Reddish  violet 

C6H6—  CH=CH— 

C6H5—  CH=CH— 

2    ' 

4 

Violet  black 

That  the  relation  which  exists  between  colored  compounds  of  this  type 
is  complicated  may  be  seen  from  the  fact  that  it  is  possible  to  prepare 
both  a  colored  and  a  colorless  addition  product  by  the  addition  of  one 
molecule  of  dibenzal  acetone  and  one  molecule  of  hydrochloric  acid  and 
that  two  products  are  also  formed  by  the  addition  of  one  molecule  of 
dibenzalacetone  and  two  molecules  of  hydrochloric  acid.  In  general 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    437 

two  series  of  salts  are  possible, — one  colorless,  the  other  colored.  It  has 
been  observed,  moreover,  that  these  two  series  are  quite  separate,  and 
that  the  colored  salts  cannot  be  obtained  from  their  colorless  isomers. 

A  number  of  theories  have  been  advanced  as  to  the  structure  of 
these  substances.  It  does  not  seem  probable  that  they  represent 
additions  of  hydrochloric  acid  to  the  unsaturated  carbon  linkages  since 
this  would  mean  a  decrease  in  the  number  of  conjugate  systems  present 
in  the  molecule  and  would  serve  to  weaken  the  color  by  lessening  the 
number  of  chromophores.  Baeyer  and  Villiger  l  have  suggested  that 
colored  salts  of  this  type  might  possess  quinoidal  structure: 

OH 
=CH— CH==C— CH=CH— 


but  an  objection  to  this  is  to  be  found  in  the  fact  that  the  above  con- 
figuration is  quinolic  and  not  quinoid  and  that  the  formula  does  not, 
therefore,  serve  to  explain  the  color  of  the  compound.2 

Another  explanation  of  the  phenomenon  presupposes  addition  of 
hydrochloric  acid  to  the  carbonyl  group  in  one  of  three  ways  : 

CkxOH 
C6H5-CH=CH—  C—  CH=CH-CGH5   .....        (I) 


o 

C6H5-CH=CH—  C—  CH=CH-CGH5  .....      (II) 
HC1 


6 

[— C— C 


C6H5-CH=CH— C— CH=CH-C6H5 (Ill) 

Straus  and  Caspari3  have  recently  succeeded  in  preparing  substances 
corresponding  to  the  first  formula,  and  have  shown  that  such  products 
are  entirely  colorless  and  are  not  decomposed  by  the  action  of  alkali. 
It  might  therefore  be  assumed  that  the  colored  compounds  under  con- 
sideration correspond  to  the  second  formula  and  that  they,  therefore, 
belong  to  the  general  class  of  oxonium  salts. 

iBer.,  36,  1191  (1902). 

2  See  Vorliinder  and  Mumme,  Ber.,  36,  1482  (1903). 

"Ber.,40,  2689  (1907). 


438  THEORIES  CF  ORGANIC  CHEMISTRY 

P.  Pfeiffer  l  has  recently  made  a  series  of  special  investigations  in 
regard  to  halochromism  among  ketones.  Since  phenomena  of  this 
type  have  been  observed  most  frequently  in  the  case  of  substances  which 
contain  carbonyl  groups,  Pfeiffer  decided  to  study  the  subject  system- 
atically and  to  this  end  prepared  a  large  number  of  addition  products 
of  the  general  formula : 

SnX4-2RCOR' 

where  R  equals  phenyl,  methoxyphenyl,  cinnamyl,  or  furyl,  and  R' 
equals  H,  CH3,  OH,  OC2H5  or  NH2.  These  substances  crystallize  well 
and  may  be  readily  prepared  by  treating  SnCU  and  SnBr4  with  alde- 
hydes, ketones,  acids,  esters,  and  amides. 

In  terms  of  Werner's  theory  compounds  of  this  type  may  be  regarded 
as  belonging  to  the  same  general  class  as  other  double  salts  of  tin. 
Since  the  coordination  number  of  tin  is  six  such  substances  may  be 
assumed  to  possess  the  formula: 

/OC<R, 

X4Sn 


X)C< 


These  substances  resemble  the  acid  addition  products  in  all  essentials 
and  the  latter  may  therefore  be  assumed  to  possess  a  similar  configura- 
tion. The  structure  of  compounds  formed  by  the  addition  of  acids  to 
carbonyl  narrows  itself  down  to  two  possibilities,  viz., 

H      X  HX 

I 
or  O 

R— C-R/  R— C— R' 

I  II 

but  since  the  former  (I)  fails  to  account  for  the  striking  similarity  in 
properties  which  has  been  observed  in  the  case  of  the  two  types  of  molecu- 
lar compounds  which  are  formed  by  the  action  of  acids  and  metallic 
salts  respectively,  it  has  been  discarded  in  favor  of  the  second  for- 
mula (II). 

The  mechanism  of  this  addition  has  been  explained  by  Pfeiffer  and 
has  already  been  referred  to  in  a  previous  chapter.  In  brief  it  assumes 
that  the  molecule  of  acid  or  salt  adds  to  the  free  affinity  on  the  oxygen 

1Annalen  der  Chemie,  370,  99  (1909);  376,  285  (1910);  383,  92  (1911);  404, 
1  (1914);  412,  253  (1916);  also  compare  F.  Straus  and  H.  Blankenhorn,  Annalen 
der  Chemie,  415,  232  (1918). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    439 

atom  and  that  this  union,  which  is  at  first  very  unstable,  changes  to 
one  of  greater  stability,  thus  engaging  a  greater  fraction  of  the  total 
affinity  of  the  oxygen.  The  change  is  accompanied  by  the  evolution  of 
heat  and  by  a  corresponding  increase  in  the  free  affinity  of  the  adjacent 
carbon  atom,  the  latter  gradually  approaching  a  trivalent  condition  as 
more  and  more  of  its  bound  chemical  energy  is  released  by  the  carbonyl 
oxygen  atom.  Since  this  condition  is  essentially  the  same  as  that  which 
has  been  observed  in  the  case  of  triphenylmethyl  its  existence  is  to  be 
associated  with  the  appearance  of  color.  Pfeiffer  1  originally  expressed 
these  conceptions  by  means  of  the  following  symbols : 

Rx  +MXn      Rx  Rx 

>C=0—  >C=O MXn    -*         >C— O— MXn 

R;/  !      I  R'/    i  R'/  I 


but  he  has  recently  modified  the  formula  for  the  product  to 

C=0  ......  MXn 


R\ 


The  arrow  serves  to  indicate  the  presence  of  free  affinity  on  the  carbon 
atom  and,  therefore,  accounts  for  its  increased  chemical  reactivity. 

According  to  Pfeiffer  other  chromophore  groups  such  as  C=N,  C=C, 
N=0,  etc.,  behave  in  exactly  the  same  way  as  carbonyl  and  may, 
therefore,-  be  responsible  for  the  phenomenon  of  halochromism.  In  all 
cases  the  appearance  of  color  in  a  substance  can  be  referred  to  the 
presence  of  an  unsaturated  atom  in  its  molecule  and  in  this  way  the 
relationship  which  exists  between  color  and  chemical  constitution  is 
brought  into  harmony  with  modern  physical  conceptions  in  regard  to  the 
cause  of  the  phenomenon. 

It  has  been  noted  that  the  addition  products  which  are  formed  by 
the  action  of  SnCU  and  HC1  upon  ketones  are  in  some  cases  colorless 
while  in  other  cases  the  color  varies  from  yellow  to  orange-red,  to 
bordeau-red,  to  black,  as  for  example, 


( 


;>C=O )  SnCU 

2 

Colorless 


( 


Nc—O )  SnCU 

•  /  /  r* 


Yellow 
lAnnalcn  der  Chemie,  383,  93  (1911). 


440  THEORIES  OF  ORGANIC  CHEMISTRY 


\ 
=O  ......  )  SnCU 

/2 

Reddish  orange 


/C6H5  •  CH=CH  •       =X  \ 

(  >C=0  ......  )  SnCU 

\  C6H5/  /2 

Reddish  orange 

/CeHs  •  CH=CH  •  CH=CHX  \ 

(  >C=0  ......    SnCU 

\         •    H=CH  •  CR=CR/  1 


C6H5  •  CH=CH 

Black, 

Such  differences  in  color  may  be  readily  explained  in  terms  of  Pfeiffer's 
theory  since  the  above  formulas  indicate  that  depth  of  color  bears  a 
direct  relation  to  the  number  of  .ethylene  groups  which  are  present  in 
the  molecule  of  any  given  ketone.  It  should  be  noted  that  the  ethylene 
groups  in  addition  to  being  unsaturated  themselves  may  act  to  increase 
the  unsaturation  of  the  carbonyl  carbon  atom.  Indeed  the  presence  of 
a  number  of  ethylene  groups  in  a  given  molecule  may  release  so  much  of 
the  bound  chemical  energy  of  this  atom  as  to  cause  it  to  approximate 
the  trivalent  condition. 

A  marked  difference  has  been  observed  in  the  halochromic  properties 
of  aldehydes  and  ketones  as  compared  with  acids,  esters,  and  acid 
amides,  the  former  giving  much  more  deeply  colored  addition  products 
with  SnCU  than  the  latter.  This,  too,  may  be  readily  explained  in 
terms  of  Pfeiffer's  theory  if  the  general  formulas  for  these  five  classes 
of  substances  are  compared  : 


R— C=0 

|  and 

H  R 

I 


R— C=0  R— C=0 

I  |  and          | 

OH  OR'  NH2 

II 

If  X  is  used  to  represent  R',  H,  OH,  OR',  and  NH2  it  is  obvious  that 
the  unsaturated  character  of  the  carbonyl  group  will  depend  upon  two 
factors,  namely  R  and  X,  and  that  if  R  remains  constant,  it  will  vary 
solely  according  to  the  nature  of  X.  If  now  X  is  represented  by  such 
unsaturated  groups  as  OH,  OR',  NH2,  NR2,  etc.,  it  follows  that  the 


RELATION   BETWEEN   COLOR  AND  CHEMICAL  CONSTITUTION    441 

effect  will  be  to  decrease  the  unsaturation  of  the  carbonyl  group.     It  is 
easy  to  understand  by  means  of  the  following  formulas  : 


OH  OR'  NH2 

R—  C=O  ......  MXn    R—  C=O  ......  MXn        and        R—  C=O  ......  MX 


d  d 

R—  C 

why  the  addition  products  which  are  formed  by  the  action  of  SnCU, 
HC1,  etc.,  upon  acids,  esters,  and  acid  amides  should  be  colorless  or  only 
slightly  colored.  These  formulas  are  in  marked  contrast  to  those  which 
are  used  to  represent  the  atomic  relationships  in  the  corresponding 
addition  products  formed  by  aldehydes  and  ketones  : 

H  R' 


R—  C=O  ......  MXn        R—  C=O  ......  MXn 

I  I 

and  which  serve  to  explain  the  relatively  deep  color  of  such  compounds. 
These  speculations  lead  to  other  interesting  conclusions  which 
may  be  noted  at  this  point.  If  the  colored  addition  products  which  are 
formed  by  the  action  of  metallic  halides  upon  unsaturated  ketones 
actually  possess  the  formula 

R\ 

>C  =  0  ......  MXn 

Kr/  | 

they  should  by  virtue  of  their  free  affinity  be  capable  of  entering  into 
chemical  combination  with  other  molecules  such  as  water,  alcohol,  etc., 
to  give  tertiary  compounds  of  the  general  formula 

R\ 

>C=O  ......  MXn 

R'/  ! 
OH2 

These  deductions  have  been  verified  experimentally  by  Pfeiffer  1  who 
has  succeeded  in  preparing  substances  of  this  type  and  who  has  found 
that,  as  was  to  be  expected,  they  are  actually  lighter  in  color  and  cor- 
respondingly more  saturated  than  the  parent  substances. 

Pfeiffer  has  carried  his  analysis  still  further  and  attempts  to  explain 
the  catalytic  action  of  hydrogen  ions  in  the  saponification  of  esters  on 

1Annalen  dcr  Chemie,  383,  119  (1911). 


442  THEORIES  OF  ORGANIC  CHEMISTRY 

the  assumption  that  tertiary  molecules  of  the  above  type  form  as 
intermediate  products  during  the  progress  of  the  reaction.  He  supposes 
that  the  primary  action  consists  in  addition  of  the  hydrogen  ion  in  a 
coordinate  position  to  the  oxygen  atom  of  the  carbonyl  group.  This  is 
accompanied  by  an  increase  in  the  free  affinity  of  the  adjacent  carbon 
atom  and  is  followed  by  the  addition  of  a  molecule  of  water  and  the 
formation  of  a  tertiary  compound.  The  latter  then  decomposes  with 
the  separation  of  one  molecule  of  alcohol  and  the  formation  of  the  free 
acid: 

OC2H5  OC2H5 

R— C=O    ->    R— C=O H-   -> 

i      I  I 


->        R— C=O H-    ->    R— C=O 

OH2  OH  OH 

It  is  scarcely  necessary  to  add  that  Pfeiffer's  theory  offers  a  satis- 
factory explanation  for  the  phenomenon  of  halochromism  which  has 
been  observed  in  connection  with  the  action  of  sulphuric,  nitric  and 
perchloric  acid  upon  triphenyl  carbinol  and  with  the  action  of  metallic 
halides  upon  triphenylchlormethane.  As  in  the  preceding  cases  color 
is  supposed  to  be  due  primarily  to  the  presence  of  free  affinity  upon  the 
methane  carbon  atom  and  not  to  the  quinoid  configuration  of  a  benzene 
ring. 

The  application  of  this  theory  to  the  interpretation  of  the  phenom- 
enon of  color  in  the  case  of  the  quinhydrones  is  of  especial  interest  and 
may  be  considered  briefly  at  this  point.  This  important  group  of 
organic  compounds  represents  products  which  are  obtained  by  the 
direct  addition  of  quinones  to  phenols  and  amines,  and  the  question  of 
their  constitution  has  been  a  perplexing  one  for  many  years.  The  best 
known  example  of  a  substance  of  this  type  is  quinhy drone.  This  sub- 
stance is  formed  by  the  interaction  of  quinone  and  hydroquinone  and 
has  the  formula  C6H4O2  •  C6H4(OH)2.  It  is  dark  green  in  color  although 
quinone  is  yellow  and  hydroquinone  is  colorless.  Substances  of  this 
class  may  in  general  be  described  by  saying  that  they  are  much  more 
deeply  colored  than  their  components  and  that  they  dissociate  into  these 
components  with  great  ease.  These  characteristics  had  not  been 
accounted  for  in  a  satisfactory  way  by  any  of  the  earlier  theories  which 
were  advanced  to  explain  their  constitution.  For  example,  it  was  at 
first  supposed  that  addition  took  place  as  a  result  of  the  mutual  satura- 
tion of  carbonyl  and  ethylene  double  bonds  located  on  the  quinone  and 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    443 


hydroquinone  molecules  respectively  and  that  the  substance  therefore 
possessed  either  one  of  two  possible  configurations,  i.e., 

O 


or 


Both  formulas  suppose  that  the  addition  of  the  two  molecules  is 
effected  by  a  saturation  of  two  principal  valencies  and  therefore  neither 
affords  a  satisfactory  explanation  for  the  color  of  the  substance  nor  for 
the  ease  with  which  it  dissociates  into  its  components.3  To  overcome 
this  objection  it  was  then  assumed  that  the  addition  of  the  two  com- 
ponents takes  place  as  the  result  of  the  mutual  saturation  of  partial 
valencies  on  the  two  molecules  and  that  quinhydrones  may  thus  be 
regarded  as  typical  examples  of  molecular  compounds.  This  conclu- 
sion seemed  at  the  time  to  afford  a  satisfactory  explanation  of  the 
phenomena  and  was  in  general  harmony  with  the  experimental  data 
obtained  as  a  result  of  the  investigations  of  Urban,4  Willstatter  and 
Piccard,5  Kurt  H.  Meyer,6  Schlenk7  and  others.  The  question  has 
recently  received  further  elucidation,  but  before  these  latest  develop- 
ments can  be  considered  it  will  be  necessary  to  review  the  researches  of 
Willstatter  and  his  students  in  some  detail.8 

In  1879  C.  Wurster  9  discovered  two  dyes,  one  of  which  was  red 
and  the  other  was  blue,  corresponding  respectively  to  the  formulas 
C8Hi2N2Br  and  CioHi5N2Br.  Later  A.  Bernthsen  10  discovered  that  the 
red  dye  resembled  quinone  in  some  of  its  properties  and  he,  therefore, 
gave  it  the  quinoid  formula 

N(CH3)2C1 


1  L.  Jackson,  Ber.,  28,  1614  (1895). 

2Th.  Posner,  Annalen  der  Chemie,  336,  85  (1904). 

3  Compare  Ber.,  43,  3603  (1910);  44,  1503  (1911). 

4  Monatsh.  Chemie,  28,  299  (1907). 

5  Ber.,  41,  1458  (1908). 

6  Ber.,  43,  157  (1910). 

7  Annalen  der  Chemie,  368,  271  (1909). 

8  Ber.,  37,  1494,  3761,  4605  (1904);  38,  1232,  2244,  2348  (1905);  39,  3474, 
3482,  3765  (1906);  40,  1406,  1432,  2665  (1907);  41,  1458  (1908). 

9  Ber.,  12,  1803,  1807,  2071  (1879);  also  19,  3195,  3217  (1886). 

10  Annalen  der  Chemie,  230,  162  (1885);  251,  11,  49,  82  (1889). 


444  THEORIES  OF  ORGANIC  CHEMISTRY 

which  was  later  slightly  modified  by  Nietzki l  to 
HN=C6H4=N(CH3)2C1 

In  either  case  the  red  compound  may  be  regarded  as  belonging  to  the 
general  class  of  quinonium  salts. 

Following  this  Willstatter  and  his  students  undertook  a  systematic 
investigation  of  quinoids.  They  succeeded  in  preparing  the  simplest 
quinone-imides  and  discovered  that  these  are  not  intensely  colored  as 
was  to  be  expected  from  theoretical  considerations,  but  are  on  the  con- 
trary either  colorless  or  yellow.  For  example  O=C6H4=NH  and 
NH=C6H4=NH  are  colorless,  O=C6H4=NCH3  is  slightly  yellow  in 
solid  form  and  bright  yellow  in  solution,  and  CH3N=C6H4=NCH3  is 
colorless  in  solid  form  and  bright  yellow  in  solution.  When  these  sub- 
stances were  compared  with  the  red  compound  which  had  been  dis- 
covered by  Wurster  and  which  was  supposedly  so  closely  related  to  them 
in  structure,  it  was  discovered  that  the  difference  was  so  great,  both  as 
to  shade  and  intensity  of  color,  as  to  preclude  the  possibility  of  a  similar 
constitution.  A  doubt  as  to  whether  the  red  salt  was  a  true  quinone- 
imide  had  been  voiced  by  Willstatter  and  Pf annenstiel 2  as  early  as 
1905,  but  it  was  not  until  three  years  later  that  Willstatter  and  Piccard  3 
actually  discovered  that  Wurster's  salt  was  not  a  true  quinoneimide 
(holoquinoid)  but  a  semi-quinoneimide  (meriquinoid) .  They  then 
succeeded  in  preparing  the  holoquinoid  of  p-aminodimethylaniline  and 
found  that  it  was  entirely  colorless  but  that  on  partial  reduction  it 
readily  passed  into  the  red  meriquinoid.  The  latter  could  also  be  pre- 
pared by  the  combination  of  one  molecule  of  leuco-base  (p-aminodi- 
methyl  aniline)  with  one  molecule  of  holoquinoid. 

These  facts  lead  to  the  conclusion  that  Wurster's  salt  represents 
a  transition  or  intermediate  product  between  quinones  and  hydro- 
quinones,  or,  in  other  words,  belongs  to  the  class  of  quinhydrones. 
This  discovery  helps  to  substantiate  an  opinion  previously  expressed 
by  Kehrmann  in  regard  to  colorless  and  colored  di-imides,  namely, 
"  many  of  the  deeply  colored  substances  of  quinoid  character  which 
are  formed  by  the  oxidation  of  simple  amines  and  which  on  reduction 
revert  to  them  again,  are  in  fact  quinhydrones."  In  order  to  account 
for  the  two  most  important  characteristics  of  these  substances,  viz., 
their  color  and  the  ease  with  which  they  dissociate,  Willstatter  and 
Piccard  assume  that  quinhydrones  are  formed  as  the  result  of  the 

1  "Organische  Farbestoffe,"  5th  Ed.,  190fi  D.  199. 

2  Ber.,  38,  2244  (1905). 
3Ber.,  41,  1462  (1908). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION   445 

mutual  saturation  of  the  partial  valencies  of  their  components  as  rep- 
resented by  the  following  scheme: 


O--       HO 


BrH2N 


N— H2 


O          HO 

Quinhydrone. 
I 


Br(CH3)2N  N(CH3)2 

Wurster's  red  salt. 
II 


Since  then  Piccard  has  succeeded  in  preparing  the  simplest  dye  of  this 
type,  namely: 

NH2Br  NH2 


NH2Br 


NH2 


An  objection  to  this  interpretation  is  to  be  found  in  the  fact  that 
Wurster's  salt  differs  markedly  from  the  quinhydrones  in  that  it  is  a 
much  more  stable  compound.  Although  it  dissociates  in  acid  solu- 
tion, it  dissolves  in  water  without  any  appreciable  decomposition  into 
its  components  while  quinhydrones,  on  the  other  hand,  break  down 
very  readily  on  solution.  This  important  difference  may  be  explained 
by  supposing  a  corresponding  difference  in  the  constitution  of  the 
molecules  of  two  types  of  meriquinoids  (I  and  II).  The  red  salt,  being 
the  more  stable  in  its  properties,  may  be  assumed  to  possess  the  more 
saturated  molecule,  and  this  difference  in  degree  may  be  visualized  as 
corresponding  roughly  to  the  difference  in  the  degree  of  saturation 
represented  respectively  by  the  following  two  formulas  for  benzene:2 


CH 


HCl< 


CH 
CH 


and 


Oil 


CH 

HCiACH 

HOOCH 
CH 


Compare  Annalon  dor  Chemie,  381,  351  (1911). 

2  Ber.,  41,  1458  (1908);  see  also  Kehrmann,  Ber.,  41,  2340  (1908). 


446  THEORIES  OF  ORGANIC  CHEMISTRY 

Just  how  the  saturation  of  free  affinity  takes  place  in  the  case  of  the 
substances  under  consideration  remains  a  question,  although  there  is  at 
the  present  time  much  speculation  in  regard  to  the  constitution  of  the 
meriquinoids.  It  has  been  suggested  that  the  two  components  exist  in  a 
condition  of  dynamic  equilibrium  with  reference  to  each  other  and  that 
the  pronounced  color  of  this  type  of  compound  as  compared  with  the 
very  slight  coloration  of  its  components  (quinones  and  imines  respect- 
ively) may  thus  be  accounted  for  on  the  basis  of  isorropesis,  or,  in  other 
words,  a  make-and-break  in  the  linkage  of  residual  valencies.  While 
this  term  has  usually  been  applied  to  an  infra-molecular  condition  it  may 
be  extended  to  include  an  wfer-molecular  condition  such  as  is  repre- 
sented in  the  recurrent  making  and  breaking  of  linkages  between  differ- 
ent molecules.  In  either  case  vibrational  disturbances  are  produced. 
These  are  supposed  to  occasion  molecular  oscillations  which  in  terms  of 
Hartley's  theory  cause  the  phenomenon  of  color.  Since  the  stability 
of  certain  solutions  of  meriquinonium  salts  as  compared  with  solutions 
of  quinhy drones  is  explained  as  due  to  a  relatively  greater  degree  of 
saturation,  it  follows  that  the  color  of  these  substances  must  be  assumed 
to  be  due  to  isorropesis  of  the  m£ra-molecular  rather  than  of  the  inter- 
molecular  type,  for  the  latter  conception  is  obviously  possible  only  on 
the  assumption  of  a  relatively  loose  union  between  the  component 
molecules. 

Interpretations  of  this  kind  may  be  applied  not  only  to  Wurster's 
red  salt  but  also  to  fuchsine,  which  closely  resembles  it  in  the  color 
of  its  aqueous  and  acid  solutions  and  in  the  color  of  its  compounds.  In 
general,  it  may  be  assumed  that  substances  of  this  type  are  formed  by 
the  union  of  one  molecule  of  a  weakly  colored  ammonium  salt  with  one 
or  two  molecules  of  amines  and  that  they  are  usually  meriquinoid  in 
structure.1  H.  Kauffmann  2  explains  the  phenomenon  in  terms  of  his 
particular  conceptions  by  assuming  that  the  dotted  lines,  which  are  used 
in  the  above  formulas  to  express  partial  valencies,  indicate  a  breaking  up 
("  Zersplitterung  ")  of  affinity.  As  has  been  noted  such  a  condition  of 
split  affinity  is  supposed  by  Kauffmann  to  account  for  the  phenomenon 
of  color.3 

The  investigations  of  Kurt  H.  Meyer4  in  the  field  of  the  so-called 
phenoquinones  aided  materially  in  the  further  elucidation  of  this  prob- 
lem. Phenoquinones  are  formed  by  the  addition  of  phenols  to  quinone 
and  resemble  the  quinhy  drones  in  their  general  properties.  In  the 

iBer.,  41,  1458  (1908);  also  Kehrmann,  Ber.,  41,  2340  (1908). 

2  "Die  Valenzlehre,"  p.  510. 

3  Compare  Werner,  Ber.,  42,  4324  (1909). 

4  Ber.,  41,  2568  (1908);  42,  1149  (1909);  43,  157  (1910). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION   447 

course  of  their  investigation  Meyer  made  the  important  discovery 
that  quinones  possess  the  power  to  combine  with  other  substances,  such 
as  acids  and  metallic  halides,  with  which  they  form  salt-like  addition 
products.  The  latter  are  relatively  deeper  in  color  than  the  corre- 
sponding quinones  and  readily  dissociate  into  their  components.  In 
short  quinones  in  common  with  other  ketones  exhibit  the  phenomenon 
of  halochromism.  In  the  case  of  benzoquinone,  for  example,  metallic 
addition  products  of  this  type  may  be  represented  as  possessing,  in 
terms  of  Pfeiffer's  theory  the  formulas  : 

CH=CH 
0=C<  >C=0.  .....  MeXB 


and 

CH=CH 
XnMe  ......  O=C<  >  C=0  ......  MeXn 

j  \CH=CHj 

The  application  of  this  discovery  to  the  problem  of  the  constitution 
of  quinhydrones  was  developed  by  Pfeiffer  following  the  discussion  of 
certain  objections  which  had  been  raised  to  the  Willstatter-Piccard 
formulas  : 

0       HO  -Cells  0          HO 


and 


HOC6H5  O          HO 

Phenoquinone  Quinhydrone. 

The  question  arose  as  to  the  particular  atoms  in  the  molecule  which 
served  to  unite  the  two  components.  While  there  was  little  doubt  that 
oxygen  represented  the  seat  of  the  partial  valencies  in  the  quinone 
molecule,  the  seat  of  the  corresponding  valency  in  phenol  was  uncertain. 
In  order  to  decide  the  question  as  between  hydrogen  and  oxygen  of  the 
hydroxyl  group,  Pfeiffer  attempted  to  prepare  addition  products  by  the 
action  of  phenyl  ethers  upon  quinone.  His  experiments  proved  suc- 
cessful and  in  the  case  of  C6H4(OCH3)2,  for  example,  he  obtained  a 
deeply  colored  readily  dissociable  product  which  resembled  qumhydrone 
in  all  essentials.  This  seemed  to  settle  the  question  in  favor  of  the 
oxygen  atom  but  it  was  opened  again  almost  immediately  by  the  dis- 
covery by  Haakh  l  and  Pfeiffer  2  that  similar  addition  products  could  be 

1  Ber.,  42,  4595  (1909). 

2  Annalen  der  Chemie,  404,  5  (1914). 


448  THEORIES  OF  ORGANIC  CHEMISTRY 

prepared  by  the  action  of  aromatic  hydrocarbons  upon  quinones. 
Stilbene,  fluorene,  naphthalene  and  anthracene  when  melted  with 
quinone  and  chloranil  give  deep  colorations,  although  as  yet  it  has  been 
impossible  to  isolate  these  products.  .A  red  crystalline  substance  was, 
however,  separated  as  a  product  of  the  addition  of  durene  to  chloranil 
and  a  similar  substance  was  obtained  as  a  result  of  the  reaction  between 
durene  and  bromanil.  These  substances  have  been  purified  and  ana- 
lyzed and  correspond  respectively  to  the  formulas  : 

C6Cl402-2C6H2(CH3)4     and    CeB^Cfe^CelfcCCHa^ 

They  are  therefore  in  all  respects  directly  analogous  to  the  pheno- 
quinones. 

It  follows  from  this  discovery  that  the  seat  of  the  free  affinity  on  the 
phenol  molecule  is  neither  the  hydrogen  nor  the  oxygen  of  the  hydroxyl 
group  and  the  discussion,  therefore,  shifts  to  a  decision  between  the 
hydrogen  and  the  carbon  of  the  benzene  ring.  Since  hexamethyl- 
benzene  combines  with  chloranil  to  give  a  reddish  brown  addition 
product  having  the  formula 


and  since  chloranil  dissolves  in  dimethylbutadiene, 
H2C=C  --  C=CH2 
CH3    CH3 

to  give  an  orange  colored  solution,  it  may  be  assumed  that  unsaturated 
carbon  atoms  represent  the  seat  of  the  free  affinity. 

In  general  it  may  be  said  that,  according  to  the  conceptions  of 
Werner's  theory,  the  formation  of  quinhydrones  takes  place  as  a  result 
of  the  saturation  of  partial  valencies  present  on  the  oxygen  of  the 
carbonyl  group  in  the  quinone  molecule,  by  other  valencies  which  are 
present  on  the  unsaturated  carbon  atoms  of  the  benzoid  molecule. 
The  two  classes  of  addition  products  which  may  be  obtained  in  any  case 
are  represented  by  means  of  the  following  formulas  : 

0  ......  C6H6  O  ......  C6H4(OH)2  O  ......  C6H6 


.C6H6 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    449 
O C6H5OH  0 C6H5  •  NH2 


) C6H5OH  0 C6H5NH2 

The  fact  that  the  quinone  oxygen  atom  appears  to  be  coordinately 
univalent  is  not  surprising  but  the  fact  that  the  benzene  ring  behaves  in 
the  same  way  is  rather  remarkable.1 

That  the  phenomena  may  be  included  under  the  general  head  of 
halochromism  is  obvious  from  a  comparison  of  the  type  formulas 

Rv  Rv 

>C=O..  HX          >C=0 MXn 

R'/  R'/ 


Ketonic  combinations  with  acids 

and  metallic  salts 

r>                                             -p 
ftv                                          Kv 

>C=0  C6H6          >C=0. 

RV 
C6H5-OH           ) 

€=0  C6H5-NH2 

V'                          R'/ 

R'/ 

Quinhydrones 

The  applications  of  the  theory  of  halochromism  have  been  by  no 
means  exhausted  as  a  result  of  the  present  discussion  of  addition  reac- 
tions which  involve  the  carbonyl  group.  Corresponding  colored  addi- 
tion products  may  be  obtained  if  nitro  groups  are  substituted  for 
carbonyl  as  for  example, 

R2C=O A     and      RN02 A 

I  I 

where  A  is  used  to  represent  any  adding  molecule.  It  even  seems  as  if 
halochromism  were  a  property  which  is  possessed  in  common  by  all 
unsaturated  groups,  since  unsaturated  hydrocarbons,  ketimines, 

R 

=NH 


> 
R'/ 


triaryl  halides,  azo-compounds,  tetraaryl-hydrazines,  etc.,  etc.,  all 
react  with  acids  and  metallic  halides  to  give  highly  colored  and  readily 
dissociable  addition  products.  In  every  case  the  deepening  in  color 

1  See  P.  Pfeiffer  and  T.  Bottler,  Ber.,  61,  1828  (1918)  for  the  more  recent  develop- 
ments along  these  lines. 


450  THEORIES  OF  ORGANIC  CHEMISTRY 

may,  in  terms  of  Pfeiffer's  theory,  be  assumed  to  be  due  to  a  consider- 
able increase  in  the  free  affinity  of  some  one  atom  which  is  present  in 
the  molecule.  The  mechanism  of  the  process  is  always  described 
in  the  same  simple  way  by  supposing  that  the  saturation  of  a  partial 
valency  in  one  part  of  the  molecule  leads  to  a  lack  of  balance  in  the 
energy  relationships  in  some  other  part  of  the  molecule  and  that  this 
frequently  takes  the  form  of  a  high  degree  of  unsaturation  at  some  one 
given  point.  This  theory,  together  with  the  corollary  that  increase  in 
the  free  affinity  of  an  atom  is  accompanied  by  an  increase  in  its  power 
to  act  as  a  chromophore,  offers  a  satisfactory  general  explanation  for 
the  phenomenon  of  halochromism.  It  should  again  be  noted,  however, 
that  the  amount  of  free  affinity  which  the  arrow  represents  in  these 
formulas 
R  -  N02 HX  R  -  N02 MX«  R  •  N02 C6H6  RN02 C6H5  •  OH 

II  II 

R-NO2 C6H5-NH2 

I 

varies  not  only  with  the  character  of  A  but  also  with  the  nature  of  the 
substituents  (OH,  OCHs,  NH2,  CH=CH,  etc.),  which  are  present  in  R 
and  that  such  variations  are  accompanied  by  corresponding  changes 
in  color.1 

The  application  of  Pfeiffer's  theory  to  the  case  of  derivatives  of 
triphenyl  carbinol  must  now  be  considered.  In  general  it  may  be  said 
that  while  the  relationships  are  much  more  complicated  in  the  case  of 
these  substances  than  in  the  case  of  any  which  have  yet  been  considered, 
it  seems  probable  that  in  some  cases  at  least  color  is  due  to  a  quinoid 
configuration  of  the  molecule.  Baeyer  found,  for  example,  as  the  result 
of  an  exhaustive  investigation  of  a  large  number  of  derivatives  of  tri- 
phenyl carbinol,  that  all  oxy-  and  amino-derivatives  resemble  the  parent 
substance  in  being  entirely  colorless.  Since  the  appearance  of  color 
in  these  compounds  is  associated  in  every  case  with  the  elimination  of 
one  molecule  of  water,  the  simplest  explanation  of  the  phenomenon  is 
one  which  supposes  that  this  process  involves  a  change  from  the  benzoid 
to  the  quinoid  state : 

(C6H4NH2)2C-OH    (C6H4NH2)2C  +     H20 


7 

NH2-HC1 
Annalen  der  Chemie,  412,  253  (1916). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    451 


The  various  types  of  dyes  which  belong  to  this  group  have  there- 
fore been  formulated  in  the  following  way: 


Fuchsone 


HO-H4C6 

Benzaurine 


HO-H4C6 
HO-H4C 


Aurine 


Fuchsoneimide 


H2N-H5Cf 


H2N.H5C6' 

Aminofuchsoneimide 


H2N.H5C6- 

Diaminofuchsoneimide 


=NH 


=NH 


Of  these  substances  the  fuchsoneimides  stand  somewhat  apart  from  the 
others  in  that  they  possess  a  slightly  different  brown  to  orange  color 
and  when  treated  with  acids  they  form  addition  products  which  possess 
every  variety  of  color.  Since  in  this  case  a  weakly  colored  ketone- 
imide  passes  into  a  highly  colored  molecular  compound  on  the  addition 
of  acids,  the  phenomenon  represents  a  clear  case  of  halochromism  and 
may  be  formulated  as  follows : 


H2NC6H4> 
C6H5- 


>=NH 


H2N-C6H4, 

/ 


=NH......HX 


Thus  even  in  the  triphenylmethane  series  certain  types  of  colored  com- 
pounds must  obviously  be  accounted  for  on  the  basis  of  Pfeiffer's  theory. 

The  fuchsones  and  oxyfuchsones  also  form  intensely  colored  addi- 
tion products  with  acids  and  metallic  salts  but  these  differ  from  those 
which  have  just  been  described  in  the  case  of  the  fuchsoneimides  in 
that  they  are  about  the  same  shade  as  the  parent  substance  or  even 
lighter  as  is  illustrated,  for  example,  in  the  salts  of  benzaurine  and  aurine. 
From  this  it  would  seem  to  follow  that  the  fuchsones  and  oxy-fuchsones 
must  differ  in  constitution  from  the  ordinary  types  of  unsaturated 
oxyketones. 

The  relation  which  exists  between  the  absorption  spectrum  of  a 
substance  and  the  chemical  constitution  of  its  molecule  has  been  dis- 
cussed at  some  length  in  a  preceding  chapter  where  it  was  pointed  out 
that  all  changes  in  the  atomic  relationships  within  the  molecule  are 
accompanied  by  corresponding  changes  in  absorption.  Such  changes,  in 


452  THEORIES  OF  ORGANIC  CHEMISTRY 

the  greatest  possible  variety  from  the  finest  to  relatively  the  crudest, 
have  been  followed  in  connection  with  a  study  of  the  phenomenon  of 
salt  formation  in  the  case  of  organic  acids,  and  as  a  result  many  of  the 
laws  which  govern  these  particular  relationships  have  been  discovered.1 
The  graphic  representation  of  changes  in  the  absorption  of  a  substance 
by  means  of  absorption  curves  has  led  in  the  first  place  to  the  classifica- 
tion of  all  such  changes  under  two  heads: 

I.  Those  in  which  the  number  and  relative  position  of  the  absorption 
bands  remain  unchanged  and  where  the  general  character  of  the  absorp- 
tion spectrum  of  the  substance  is  unaltered.      Under  these  circum- 
stances changes  in  constitution  are  accompanied  either  by  a  shifting 
of  the  absorption  band  of  the  substance  in  the  direction  of  longer  or 
shorter  wave  lengths,  or  by  an  increase  in  the  intensity  of  absorption. 

II.  Those  in  which  the  whole  character  of  the  absorption  spectrum 
of  the  substance  is  fundamentally  changed.     For  example,  salt  forma- 
tion in  the  case  of  benzoic  acid,  its  homologues  and  its  substitution 
products  is  accompanied  by  a  more  or  less  pronounced  shifting  of  the 
absorption  curves  of  these  substances  in  the  direction  of  the  shorter 
wave  lengths  and  is  accompanied  by  a  simultaneous  decrease  in  the 
intensity.     In  the  case  of  phenol  and  its  homologues,  polyphenols, 
hydroxystilbenes,  hydrooxyquinolines,  etc.,  the  effect  of  salt  formation 

C6H5-OH    ->    C6H5-ONa 

is  much  more  pronounced  than  in  the  preceding  case  and  is  accom- 
panied by  a  shifting  of  the  absorption  curve  in  the  opposite  direction, 
i.e.,  in  the  direction  of  the  longer  wave  lengths.2 

Both  types  of  neutralization  are  alike  in  that  they  are  accompanied 
by  only  minor  changes  in  the  absorption  spectrum  of  the  substance  and 
therefore,  quite  obviously  fall  within  the  category  defined  by  Class  I. 
In  interpreting  the  change  in  constitution  which  is  suffered  by  the 
molecule  in  the  transformation  of  CeHsOH  — *  CeHsONa,  for  example, 
it  must  be  assumed  that  the  oxygen  atom  maintains  relatively  the 
same  general  relation  to  the  adjacent  carbon  atom  before  and  after  the 
change.  In  other  words  the  amount  of  affinity  with  which  this  partic- 
ular atom  is  bound  to  the  carbon  of  the  ring  may  vary  considerably 
without  fundamentally  affecting  the  absorption  of  the  substance — that 
is  to  say  so  long  as  this  variation  does  not  transcend  the  limits  of  what 
is  commonly  referred  to  as  the  benzoid  condition  of  the  molecule.  As 
soon  as  this  limit  is  passed,  however,  and  the  molecule  as  a  whole 

1 J.  Lifschitz,  "Die  Anderungen  der  Lichtabsorption  bei  der  Salzbildung  organ- 
ischer  Sauren,"  Stuttgart,  1914. 

2  Compare  H.  Ley,  Zeitschr.  physikal.  Chemie,  94,  405  (1920). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION   453 

undergoes  a  rearrangement  from  the  benzoid  to  the  quinoid  condition, 
the  absorption  spectrum  suffers  an  abrupt  and  fundamental  change. 
Such  changes  fall  in  the  category  of  Class  II  and  have  frequently  been 
observed  during  the  process  of  salt  formation. 

In  all  of  the  instances  which  have  been  cited  changes  in  constitution, 
together  with  the  accompanying  changes  in  absorption,  may  be  assumed 
to  be  due  to  differences  in  either  the  character  or  the  distribution  of 
the  principal  valencies  and  they  must,  therefore,  be  clearly  distinguished 
from  another  type  of  change  which  results  from  the  interaction  of 
partial  valencies  and  which  must  now  be  considered  in  detail. 

In  the  case  of  nitro-compounds  and  other  atomic  complexes  which 
are  capable  of  undergoing  intramolecular  rearrangements,  the  change 
in  color  involved  in  salt  formation  becomes  much  more  complicated. 
Thus,  for  example,  according  to  Ley  and  Kissel l  nitroform  CH(NO2)3 
gives  a  mercury  salt  HgC(NO2)3  which  is  colorless  in  solid  form  and 
which  dissolves  in  ether  and  other  indifferent  solvents  to  give  colorless 
solutions.  On  the  other  hand,  this  substance  dissolves  in  alcohol  and 
in  water  to  give  yellow  solutions.  Under  these  circumstances  it  is 
necessary  to  assume  that  it  exists  in  two  modifications,  namely, 

O 

/N02  /N02 

hg— Cf-N02         and          hgO— N=C< 

\NO2  XNO2 

Pseudosalt,  colorless,  undissociated  True  salt,  yellow  dissociated 

In  the  case  of  the  nitroparaffines  the  relations  are  even  more  com- 
plicated. The  mononitroparaffines  are  colorless  and  give  colorless 
salts : 

H    O 

R.C=N— ONa 
I 

The  dinitroparaffines  are  also  colorless  but  give  yellow  salts: 

NO2   O 

I          II 
R-C=NONa 

II 

A  comparison  of  the  simplest  formulas  for  these  two  classes  of  sub- 
stances, as  represented  by  I  and  II,  would  seem    to   show  that  the 
appearance  of  color  in  the  second  case  is  due  merely  to  the  substitution 
'Ber.,  32,  1357  (1899);  38,  973  (1905). 


454  THEORIES  OF  ORGANIC  CHEMISTRY 

of  a  second  nitro  group.  If  this  is  true  it  might  be  supposed  that  the 
substitution  of  other  negative  groups,  such  as  cyanogen,  etc.,  would  have 
the  same  effect,  but  the  sodium  salt  of  phenylcyan-nitromethane 

CN     O 
C6H5-C=NONa 

is  actually  colorless.  From  this  it  would  seem  to  follow  that  the  second 
nitro  group  plays  a  specific  part  in  salt  formation,  and  the  theory  has 
been  advanced  that  its  residual  affinities  are  engaged  in  saturating  resid- 
ual affinities  already  present  on  the  — NO-ONa  group: 

N02 

R-C         \ 
\NO-ONa 

In  order  to  arrive  at  a  definite  decision  in  regard  to  the  constitution 
of  these  substances  Hantzsch  undertook  a  systematic  investigation  of 
the  changes  brought  about  by  salt  formation  in  the  absorption  spectra 
of  a  large  number  of  organic  compounds  as,  for  example,  nitro-,  nitroso-, 
azo-,  pyridine,  quinoline,  acridine  and  other  bodies.  The  investiga- 
tion proved  to  be  one  of  considerable  importance  and  led  to  a  number 
of  very  interesting  discoveries  which  may  now  be  reviewed  in  some 
detail. 

It  may  be  assumed  that  in  the  formation  of  colored  salts  the  color  is 
conditioned  either  by  the  acid  or  by  the  metal  or  by  both  and  Hantzsch, 
therefore,  made  a  preliminary  study  of  the  action  of  colorless  metals 
such  as  Li,  Na,  K,  Rb,  Cs,  Be,  Mg,  Ca,  Sr,  Ba,  Zn,  Cd,  Pb,  Ag,  and  Tl 
upon  colorless  acids.  His  results  demonstrated  that  under  normal 
conditions  and  in  a  great  variety  of  instances,  colorless  acids  combine 
with  these  metals  to  give  colorless  salts  and  that  in  cases  where  the 
acid  has  a  definite  color  it  reacts  with  these  metals  to  give  salts  which 
possess  the  same  color.1  For  example  antibenziloxime  is  colorless  and 
so  are  its  salts  with  the  above  metals.  Isatinoxime, 

C=NOH 
C6H4/  ^CO 
NH 

is  yellow  as  are  likewise  its  salts  and  esters.  In  such  cases  the  organic 
acid  is  referred  to  as  a  monochromic  acid. 

1  Ber.,  42,  967  (1909). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    455 

There  are,  however,  a  great  many  exceptions  to  this  general  rule. 
For  example  violuric  acid, 

NH— CO 

\C=NOH 
^NH— CO 

which  is  itself  colorless  forms  not  only  colorless  salts  with  these  metals 
but  also  yellow,  red,  blue,  green  and  violet  salts  in  great  abundance 
and  variety.  Such  an  acid  is  referred  to  as  a  polychromic  acid  because 
it  is  capable  of  forming  salts  which  are  characterized  by  many  different 
colors.  Since  these  salts  are  believed  by  Hantzsch  to  be  isomeric,  the 
phenomenon  is  known  as  chromoisomerism.  Other  organic  compounds 
have  been  observed  to  possess  this  property  as,  for  example,  mono- 
and  di-methyl  violuric  acids,  diphenyl  violuric  acid,  p-bromphenyloxi- 
mino-oxazolone,1 

C6H4Br 


A 


C=NOH 

I        I 
O — CO 

as  well  as  certain  derivatives  of  nitrophenol,  azophenol,  pyridine, 
quinoline  and  acridine.2  The  salts  of  the  latter  will  be  referred  to 
again  later.  At  present  it  need  only  be  added  that  even  substances 
which  are  similar  to  ethyl  acetoacetate  in  structure  have  been  observed 
to  exhibit  the  phenomenon  of  chromoisomerism.3 

A  few  illustrations  will  suffice  to  bring  this  matter  clearly  before  the 
attention  of  the  reader.  The  freshly  precipitated  silver  salt  of  violuric 
acid,  for  example,  is  white  but  if  left  in  contact  with  its  solution,  it 
first  changes  to  an  amorphous  green  and  then  to  a  crystalline  black- 
brown  compound.  If  the  freshly  precipitated  colorless  salt  is  filtered 
and  dried,  it  changes  to  a  dirty  rose  color,  and  this  change  is  not  due  to 
the  action  of  light.  The  potassium,  rubidium  and  caesium  salts  occur 
in  blue  and  red  modifications.  If  the  blue  salt  is  crystallized  from  hot 
water  a  mixture  of  red  and  blue  salts  separates,  while  if  it  is  Subjected 
to  the  action  of  steam  it  is  completely  transformed  into  the  red.  The 
latter  are  stable  in  perfectly  dry  condition,  but  in  the  presence  of  moist 
air  revert  to  the  blue.  The  stability  of  the  different  colored  salts  varies 
greatly.  It  is  probable  that  each  salt  of  a  definite  color  has  its  own 

^er.,  42,  969  (1909). 
'Ber.,  44,  1783,3290(1911). 
»Ber.,  48,  785  (1915). 


456  THEORIES  OF  ORGANIC  CHEMISTRY 

definite  sphere  of  existence  outside  of  which  it  is  metastable  or  labile. 
In  dry  condition  and  solid  form  the  tendency  to  isomerization  is  very 
slight,  but  even  traces  of  solvent  seem  to  act  catalytically  in  bringing 
about  rearrangements.  It  frequently  happens  that  under  a  given  set 
of  conditions  one  and  only  one  modification  of  a  given  salt  will  be 
formed.  This  may  isomerize  spontaneously  into  another  modifica- 
tion having  a  different  color,  but  more  frequently  subsequent  changes 
can  be  traced  to  the  presence  of  a  minute  quantity  of  some  catalyst 
or  to  changes  in  temperature  or  the  nature  of  the  solvent. 

Chromotropism  is  a  term  used  by  Hantzsch  in  referring  to  varia- 
tions of  color  in  one  and  the  same  salt,  and  should  be  distinguished  from 
variochromism  which  is  used  to  express  the  existence  of  a  given  salt 
in  different  colored  modifications.  Silver  and  potassium  salts  frequently 
exhibit  the  phenomenon  of  variochromism,  illustrations  of  which 
have  already  been  noted  in  the  case  of  violuric  acid.  For  example, 
Hantzsch  has  discovered  an  oximido-ketone  which  forms  colorless, 
yellow,  red,  and  blue  primary  salts,  each  of  which  represents  a  distinct 
and  separate  modification.  These  substances  by  secondary  reaction, 
were  also  observed  to  give  different  colored  mixed  salts. 

Many  theories  have  been  advanced  to  explain  the  phenomena  of 
chromoisomerism.  It  has  been  supposed  for  example  that  differences 
in  color  could  be  accounted  for  on  the  basis  of  benzoid-quinoid  isomerism. 
The  colorless  salts  and  ethers  of  nitrophenol  were  thus  assumed  to 
have  the  formula, 


-  OCH3 


while  the  corresponding  colored  derivatives  were  given  one  or  the  other 
of  the  following  quinoid  formulas: 


O 
— OM     O=<  >=N— OCH3    C6H4 


NO2M 


This  explanation  was  soon  abandoned  since  it  does  not  account  for  the 
multiplicity  of  different  colored  salts  which  can  be  obtained  from  a 
given  acid.  These  salts  seem,  moreover,  to  bear  a  closer  chemical  rela- 
tion to  each  other  than  that  indicated  by  the  above  formulas  since 
they  react  in  all  cases  to  give  identical  products.  That  they  exhibit 
slight  differences  in  chemical  properties  is,  however,  obvious  from  the 
fact  that  the  deeper  colored  salts  react  in  general  much  more  readily 
than  their  lighter  colored  isomers. 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    457 

It  was  then  supposed  that  the  different  colored  salts  of  a  given 
acid  might  be  polymorphic  modifications  1  of  one  and  the  same  chemical 
individual.  The  main  objection  to  such  an  explanation  is  to  be  found 
in  the  fact  that  individual  salts  having  characteristic  colors  frequently 
dissolve  in  organic  solvents  to  form  stable  solutions  and  that  in  so  doing 
they  impart  their  particular  colors  to  their  solutions.  Molecular  weight 
determinations  have,  moreover,  served  to  demonstrate  that  in  all 
cases  these  salts  are  present  in  such  solutions  in  mono-molecular  con- 
dition and  that  therefore  any  explanation  of  color  which  is  based  upon 
polymorphism  is  untenable. 

Stereoisomerism  has  been  employed  by  Lifschitz2  to  explain  the 
chromoisomerism  of  the  red  and  green  salts  and  esters  of  p-nitro- 
benzyl  cyanide  to  which  he  gives  the  formulas : 


but  this  explanation  is  capable  of  only  a  limited  application  and  there- 
fore contributes  very  little  toward  the  solution  of  the  general  problem 
of  chromoisomerism. 

The  suggestion  has  also  been  made  that  the  color  of  different  salts 
may  be  due  to  the  action  of  the  solvent  since  according  to  Ley  it  may 
be  assumed  that  changes  in  the  degree  of  solution  occur  during  the 
process  of  salt  formation.  For  example,  the  complex  which  is  formed 
by  the  union  of  the  solute  with  one  or  more  molecules  of  the  solvent  may 
alter  its  composition  during  the  process  of  neutralization  and  this  change 
in  composition  may  be  accompanied  by  corresponding  changes  in 
color.  Such  an  explanation  presupposes  a  change  in  the  equilibrium 
relations  in  the  solution  as  represented  by 

A+nLm     +±     (A-nLm) 

A  •  nLm     *=±     (A-nLm)H-pLm 

where  A  represents  the  dissolved  substances  and  Lm,  the  solvent. 
Experimental  evidence  does  not,  however,  support  this  explanation 
since  it  has  been  demonstrated  that  the  solvent  frequently  plays 
no  part  in  the  process.  In  fact  it  is  generally  conceded  that  even  under 
the  most  favored  conditions  association  between  the  solvent  and  the 

'Ber.,  43,  84  (1910). 
2Ber.,  48,  1730  (1915). 


458  THEORIES  OF  ORGANIC  CHEMISTRY 

solute  plays  a  very  minor  part  in  color  changes.     The  effect  of  electro- 
lytic dissociation  also  appears  to  be  negligible. 

In  order  to  reach  a  decision  in  regard  to  the  constitution  of  sub- 
stances ,of  this  type  A.  Hantzsch l  undertook  a  systematic  investi- 
gation of  the  optical  properties  of  the  polychromic  alkali  salts  of  oxi- 
mido  ketones  dissolved  in  indifferent  solvents.  The  results  of  this 
investigation  may  be  summarized  as  follows : 

I.  All  polychromic  salt  solutions  contain  monomolecular  salts  (i.e., 
isomers  and  not  polymers). 

II.  The  color  of  the  solution  changes  from  yellow  — >  orange  — > 
red  — >  violet  — >  blue  depending  upon  the  positive  character  of  the 
metal — as  indicated  by  the  series  Li,  Na,  K,  Rb,  Cs,  NIU — and  also 
upon  the  character  of  the  solvent — as  indicated  by  the  series  phenol, 
chloroform,  acetic  ester,  acetone,  and  finally  pyridine. 

III.  The  absorption   curves   of  the  weakly  colored   (yellow)   salt 
solutions  indicate  that  they  are  very  closely  related  to  solutions  of  the 
free  oximido-ketones  (or  their  acyl  and    alkyl    derivatives).      These 
solutions  all  show  general  absorption  in  the  region  of  the  ultraviolet 
and  are  in  marked  contrast  to  the  blue  solutions  of  the  more  deeply 
colored  salts.     The  latter  show  strong  selective  absorption  and  resemble 
the  blue  solutions  of  the  aliphatic  nitroso  compounds  very  closely. 
They  may,  therefore,  be  assumed  to  contain  nitroso-enolic  salts. 

IV.  All  other  colored  solutions  may  be  regarded  as  intermediate 
between  these  two  extremes  since  the  change  in  color  from  blue  — -> 
violet  — »  red  — >  orange  — >  yellow  corresponds  to  a  similar  change  in 
the   absorption    curves    of   these    solutions.     Thus    the    nitroso-band 
becomes  gradually  smaller  and  is  shifted  more  and  more  in  the  direc- 
tion of  the  ultraviolet  until  finally  in  certain  yellow  solutions  it  com- 
pletely disappears.     It  should  be  noted  that  these  solutions  differ  from 
solutions  of  the  free  oximido-ketones  and  their  alkyl  and  acyl  deriva- 
tives in  that  they  show  a  very  pronounced  absorption  band  in  the  ultra- 
violet, while  the  latter  show  either  no  absorption  or  very  weak  absorp- 
tion in  this  region  of  the  spectrum. 

V.  The  intermediate  colors  orange,  red,  and  violet  which  correspond 
to  intermediate  absorption  curves  may  thus  actually  be  regarded  as 
mixed  colors,  but  it  must  be  remembered  that  in  such  cases  a  mixture 
of  blue  and  yellow  produces  red  and  not  green.       There  are  relatively 
few  colorless  alkali  salts  of  the  true  oximido-ketones  and  where  these 
do  exist  in  solid  form  they  fail  to  give  colorless  solutions,  being  iso- 
merized  more  or  less  into  salts  of  the  nitroso-enolic  type. 

The  theoretical  deductions  which  may  be  drawn  from  this  investi- 
'Ber,,  43,  82  (1910). 


RELATION  BETWEEN  COLOR    AND  CHEMICAL  CONSTITUTION    459 

gation  seem  to  Hantzsch  to  point  to  the  conclusion  that  changes  in 
color  are  to  be  attributed  in  all  cases  to  changes  in  constitution.  These 
may  be  due  either  to  rearrangements  of  the  atoms  within  the  molecule 
or  to  rearrangements  in  the  distribution  of  affinity,1  in  which  case  the 
general  relations  between  the  atoms  remain  the  same.  Phenomena 
which  involve  changes  in  the  distribution  of  affinity  without  accom- 
panying changes  in  the  relative  positions  of  the  atoms  in  the  molecule 
are  referred  to  as  allodesmism,2  and  is  illustrated  by  means  of  the  follow- 
ing formulas  : 


—  CH3X         I     IN—  CH3X 


The  strongest  argument  in  support  of  such  an  explanation  of  the 
phenomena  under  consideration  is  to  be  found  in  the  fact  that  poly- 
chromism  has  never  been  observed  in  the  case  of  the  simplest  oximino 
salts  of  the  type  C=N  •  OM  and  thus  seems  to  be  conditioned  by  the 
presence  of  both  an  oxime  and  a  ketone  group. 

-C=0 
-C=N— OM 

Such  compounds  resemble  the  mono-  and  di-nitrophenols  in  the  fact 
that  they  readily  tend  to  undergo  intramolecular  rearrangements.  In 
the  case  of  the  oximido-ketones  it  may  be  assumed  that  in  general  all 
varieties  of  colored  salts  may  be  represented. by  the  composite  formula: 


Such  a  composite  formula  may  be  resolved  into  individual  formulas 
by  simply  indicating  the  particular  distribution  of  affinity  in  any  given 
case,  as  for  example, 

/OM 
__C_  OM  — C^- — O  — C— O 

— C_NO  —  C=N  -C-N— OM 

I  II  III 

The  theory  that  polychromism  is  due  to  differences  in  the  dis- 
tribution of  affinity  within  the  molecule  is  supported  at  least  in  certain 

^er.,  44,  1803  (1911). 
2Ber.,  44,  1803(1911). 


460  THEORIES  OF  ORGANIC  CHEMISTRY 

cases  by  the  fact  that  the  salts  in  question  are  chemically  identical 
and  it  is  therefore  impossible  to  explain  their  relationship  on  the  basis 
of  the  usual  types  of  structural  and  stereo-isomerism.  Differences  of 
this  character  may  be  regarded  in  their  broader  aspects  as  manifesta- 
tions of  vakncy-isomerism,  to  which  brief  consideration  must  now 
be  given. 

The  theory  of  valency-isomerism  represents  an  outgrowth  of 
Werner's  theory  of  partial  valencies  and  was  originally  advanced  to 
explain  the  mechanism  of  certain  addition  reactions.  It  has  been 
noted  that  a  number  of  intensely  colored  substances  are  formed  as  the 
result  of  the  addition  of  colorless  to  weakly  colored  molecules  as,  for 
example,  by  the  addition  of  amines  to  quinones  and  of  aromatic  bases 
or  hydrocarbons  to  polynitro  bodies.  Since  in  all  of  these  cases  the 
reactions  must  be  considered  as  simple  additions,  Werner  1  suspected 
that  they  were  brought  about  by  the  union  of  residual  or  partial  val- 
encies. Following  Hantzsch's  investigation  of  the  nitroquinone 
ethers2  and  polynitrobenzenes 3  and  his  application  of  the  conception 
of  residual  affinities  to  explain  the  constitution  of  these  bodies,  Werner 
devoted  himself  to  a  comprehensive  study  of  the  problem.4  His  con- 
clusions may  be  summed  up  by  saying  that  he  came  to  the  belief 
that  in  general  color  phenomena  of  this  particular  type  may  be  attrib- 
uted to  the  saturation  of  residual  valencies.  One  such  valency  may 
with  some  certainty  be  allocated  to  the  nitro  groups.5  The  position 
of  the  other  is  not  certain,  but  since  aliphatic  hydrocarbons  of  the 
methane  series  do  not  form  colored  addition  products  with  nitro- 
compounds,  it  is  possible  that  the  presence  of  free  affinity  on  the  unsatu- 
rated  carbon  of  the  adding  molecule  forms  a  second  important 
factor  in  the  reaction. 

In  the  special  case  of  colored  salts  it  is  necessary  to  ascertain 
the  particular  role  which  the  metal  plays  in  the  production  of  color. 
This  naturally  presupposes  a  study  of  the  conceptions  which  Werner 
has  developed  in  regard  to  the  partial  valencies  of  the  metals,6  and  also 
of  the  applications  of  these  views  made  by  Ley7  and  Tschugaeff. 
On  the  basis  of  such  considerations,  Hantzsch  was  at  first  inclined  to 
believe  that  residual  valencies  of  the  metal  were  involved  in  the  pro- 

1  Ber.,  42,  4324  (1909). 
2Ber.,  40,  1570(1907). 

3  Ber.,  41,  1212  (1908). 

4  Ber.,  42,  4324  (1909);  also  Werner's  "Neuere  Anschauungen  auf  dem  Gebiete 
der  anorganischen  Chemie,"  p.  241. 

5  Ber.,  42,  4327  (1909). 
e  Ber.,  42,  4327  (1909). 

7  "Farbe  und  Konstitution  bei  organischen  Verbindungen." 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION   461 

duction  of  color  and  that  therefore  the  isomeric  chromo-salts  possessed 
the  formulas 


A 

/Y 


I  II 

Yellow  Blue 

but  later  he  completely  abandoned  this  view. 

Such  a  conception  is  open  to  the  objection  that  it  presupposes  a 
much  greater  change  in  color  on  ionization  than  actually  is  observed. 
Moreover  it  also  presupposes  the  presence  of  residual  valencies  upon 
the  alkyl  groups  in  the  case  of  the  colored  ethers  of  the  oximidoketones, 
and  this  according  to  Hantzsch  is  likewise  not  in  accord  with  the  facts. 
To  avoid  such  difficulties  Hantzsch  substituted  the  following  formulas 
for  I  and  II  respectively 

N  N 

•C     OM  -C     O 


•C=O  -C=O-M 

Yellow  Blue 

The  latter,  while  still  open  to  objection,  seem  to  explain  the  relation- 
ship better  than  any  formulas  at  present  available.  Salts  of  other 
colors  than  yellow  or  blue  may  be  regarded  as  mixtures,  equal  parts 
of  yellow  and  blue  producing  red,  etc.,  etc.1 

The  chromoisomerism  of  nitro-compounds  may  now  be  considered 
somewhat  more  closely.2  The  phenomenon  occurs  in  connection  with 
the  metallic  derivatives  of  a  great  variety  of  different  compounds. 
For  example,  phenyldinitro-methane,  which  is  itself  pale  yellow  in  color, 
in  some  cases  forms  salts  of  the  same  color  while  in  other  cases  it  forms 
salts  which  are  either  deep  yellow  or  red.  A  detailed  optical  investi- 
gation of  the  phenomena  seems  to  indicate  that  combinations  which 
contain  the  group  NO2  are  present  in  solutions  of  free  nitro-compounds. 

1  Lifschitz,  Ber.,  46,  3233  (1913). 
2Ber.,  40,  1523  (1907). 


462 


THEORIES  OF  ORGANIC  CHEMISTRY 


Weakly  colored  salts  on  the  other  hand  may  be  assumed  to  contain  the 
combination 

MONO 

This  allows  the  possibility  of  stereoisomerism  in  certain  instances  as, 
for  example,  in  the  case  of  the  metallic  derivatives  of  phenyldinitro- 
methane 

C6H5— C— N02 

MONO 

The  yellow  and  red  salts  may,  because  of  their  deeper  color,  be  assumed 
to  possess  the  partial  valency  formulas: 

C6H5— C N02  C6H5— C— N02 


and 


C=N— OM 


0=N-OM 


Ethyl  nitrolic  acid  has  been  very  carefully  investigated  by  Hantzsch 
and  0.  Graul 1  and  hds  been  observed  to  give  two  isomeric  potassium 
salts,  one  red  and  one  colorless.  Of  these  the  former  is  unstable  and 
readily  rearranges  to  give  the  latter  at  a  temperature  of  from  45°  to 
50°.  This  reaction  is  not  reversible.  A  spectroscopic  investigation 
of  these  substances,  as  represented  below,2 


3.5 


3.0 


2500 


Oscillation  frequency 
3000  3500  4000 


4500 


fc 

!„ 


2.0 


1.5 


Curve  of  ethylnitrolic  acid.  1    T          ,,    , 

Curve  of  the  potassium  salt  of  ethylnitrolic  acid       [    in  metnyi 
Curve  of  the  potassium  salt  of  iso-ethylnitrolic  acid  J 

ifier.,  31,  2863  (1898);  42,  889  (1909). 

2  Dessertation  of  G.  Kanasirski,  Leipzig,  1908. 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION   463 

reveals  the  very  interesting  fact  that  of  these  three  substances  the 
red  salt  alone  possesses  selective  absorption.  While  the  colorless  salt 
and  the  free  acid  may  be  readily  distinguished  one  from  the  other, 
both  are  characterized  by  continuous  absorption  spectra.  In  this  case 
chromoisomerism  may  be  explained  on  the  basis  of  the  following 
formulas : 

N 

CH3-C=NV  H3C— C^\0 

|         >0        and  D 

KON— (X  O=N\S  /K 

6 

Colorless  Red 

These  formulas  are  in  general  agreement  with  the  facts  of  experiment 
in  so  far  as  it  has  been  demonstrated  by  Lifschitz  that  the  conductivity 
of  the  colorless  salt  is  15  per  cent  greater  than  that  of  the  red  salt,  and 
such  a  difference  can  be  accounted  for  most  readily  by  supposing  that 
the  postassium  atom  is  bound  by  means  of  a  partial  valency  in  one 
case  and  not  in  the  other. 

Nitrophenols  have  also  been  observed  to  exhibit  the  phenomenon 
of  chromoisomerism,1  and  the  following  types  of  salts,  all  of  which  may 
be  formed  by  one  and  the  same  metal,  have  been  isolated: 

1.  Colorless  (leuco)  salts. 

2.  Colored  (chromo)  salts. 

(a)  Yellow. 
(6)  Red. 

Additional  facts  which  seem  to  contribute  to  the  solution  of  these 
and  similar  questions  as  to  constitution  have  recently  been  discovered 
by  Hantzsch  who  has  observed  that  relatively  simple  molecules  which 
are  similar  to  ethyl  acetoacetate  in  structure  as,  for  example, 

R-CO-CH2-COOR'    and    R-CO-CH2-CO-R' 

possess  the  power  of  combining  with  colorless  metals  to  give  different 
colored  salts.  Since  the  phenomenon  is  never  met  with  in  connection 
with  compounds  such  as 

RCO-CH2-COR 

'Ber.,  39,  1073,  1084  (1906);  42,  2119  (1909);  43,  3049,  3366  (1910);  44,  1771 
(1911);  46,  85  (1912);  also  Lifschitz,  "Anderungen  der  Lichtabsorption  bei  der 
Salzbestimmung,"  pp.  55,  73. 


464  THEORIES  OF  ORGANIC  CHEMISTRY 

in  which  the  two  residues  are  identical,  it  would  seem  to  be  conditioned 
by  the  unsymmetrical  structure  of  the  molecule  and  may  therefore  be 
expressed  in  terms  of  the  formulas 

R .  C=CH— C— Ri  R— C— CH=C  -  Ri 

I  II  and  ||  | 

OM          O  O  OM 

Here  again  partial  valencies  may  be  supposed  to  function  in  a  manner 
which  involves  ring  formation,1  viz., 


.  C=CH— C— R  Ri  •  C— CH=C  •  R 

I  II  and  ||  | 

OM          O  O          MO 


Isomerism  of  this  type  has  not  been  observed  in  connection  with  ethyl 
acetoacetate  itself  but  very  interesting  discoveries  have  recently  been 
made  in  the  case  of  an  ester  of  succinyl  succinic  acid.  The  structure 
of  this  substance  has  been  the  subject  of  much  debate  and  arguments 
have  been  advanced  for  and  against  the  following  formulas 

ROCO  -  CH— CH2— CO  ROCO  •  C— CH2— COH 

I  I  and  ||  || 

CO— CH2— CHCOOR  HOC— CH2— C  •  COOR 

Keto-form  Enol-form 

Hantzsch  is  now  of  the  opinion  that  the  chemical  and  optical  relation- 
ships of  the  substance  are  correctly  expressed  by  means  of  the  enol 
formula,  since  the  substance  readily  adds  bromine  to  give  a  tetrabromide 
and  since  its  absorption  spectrum  is  radically  different  from  that  of 
c-dimethylsuccinylsuccinic  ester 

CH3 
ROCO-C— CO— CH2 

H2C— CO— C-COOR 
H3 

1  Ber.,  43,  3052  (1910);  45,  85  (1912);  48,  785,  800,  1407  (1915);  also  Gibbs  and 
Brill,  Chem.  Centralbl.,  1915,  II,  392;  Philippine  Jour.  Sci.,  10A,  51  (1915); 
Liebermann,  Annalen  der  Chemie,  404,  272  (1914);  H.  Kauffmann,  Ber.,  48,  1269 
(1915);  H.  Pauly,  Ber.,  48,  934,  2010  (1915);  Hantzsch,  Ber.,  48,  1332  (1915). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    465 


The  stability  of  the  enol  modification  is  evident  from  the  fact  that  the 
absorption  curve  of  the  substance  is  almost  the  same  in  all  solvents. 
This  would  seem  to  indicate  that  but  one  modification  is  present  in 
solution  instead  of  equilibrium  mixtures  of  the  two  forms  such  as  are 
observed,  for  example,  in  the  case  of  ethyl  acetoacetate.  According 
to  Hantzsch  this  remarkable  stability  of  the  enoWorm  may  be  accounted 
for  by  supposing  that  the  free  affinity  of  the  carbonyl  oxygen  atom  is 
saturated  in  such  a  manner  as  to  produce  the  effect  of  ring  formation, 
viz., 

O     CH2C-OR 

/VY\ 

I      II     II     ! 

O     C     C     H 
RO-C     CH20 

The  chromoisomerism  of  the  yellow  and  red  salts  which  are  derived 
from  this  acid  is  interpreted  by  means  of  the  following  formulas: 

0     CH20  CH2        /OR 

/N/\/\  °v  /S  >°C 

M    C     C     M  ^C     C^    XOM 

II       II  II 

0/-S         f^\  /-\  TVT/~\  O          O 

^        AJ      <J\        A)  MIX         Aj      O 

>cx  \/  NOT  >c^  x/V) 

RCK  CH2      XOR  RCK  CH2  l 

Succinylsuccinic    ester    oxidizes    to    give    dioxyterephthalic    ester 
C6H2(OH)2(COOR)2 

which  exists  in  two  isomeric  modifications.  Of  these  one  is  colorless 
and  labile  and  has  an  absorption  spectrum  which  is  practically  identical 
with  that  of  the  corresponding  dimethyl  ether 

C6H2(OCH3)2(COOR)2 

so  that  it  may  reasonably  be  assumed  to  possess  a  benzoid  configur- 
ation, i.e., 


466  THEORIES  OF  ORGANIC  CHEMISTRY 

The  second  modification,  on  the  other  hand,  is  yellow  in  color,  stable, 
and  shows  a  quite  different  absorption.  It  may  be  assumed  to  possess 
one  or  the  other  of  the  following  quinoid  structures: 


R(X 

C  0 

/\/\/\ 


o 

H 


H 
6 


and 


N)R 

Mono-quinoid  Di-quinoid 

Of  these  formulas  the  first  seems  best  fitted  to  the  facts  since  the  diagonal 
linkage  which  is  represented  as  present  in  the  second,  has  not  as  yet 
been  shown  to  occur  under  similar  conditions.  Both  the  colorless 
and  the  yellow  modifications  react  to  give  salts  which  are  respectively 
yellow  and  red  and  for  which  corresponding  configurations  may  be 
assumed. 

Chromoisomerism  in  the  pyridine,  quinoline,  and  acridine  series 
has  already  been  referred  to  and  the  statement  was  then  made  that 
the  phenomena  are  similar  to  that  which  has  been  described  in  the  case 
of  the  oximido-ketones.  Here  again  molecular  structure  and  stereo- 
isomerism  alone  seem  to  afford  no  adequate  explanation  for  the  rela- 
tionships which  have  been  observed  to  exist  among  the  different  colored 
isomers  and  these  must  therefore  be  interpreted  as  due  to  differences 
in  the  distribution  of  valencies  within  the  unsaturated  chromophore.1 
Since  the  relationships  are  somewhat  complicated  the  subject  must  be 
considered  in  some  detail. 

It  has  been  known  for  a  long  time  that  certain  pyridine,  quino- 
line and  acridine  salts,  C5H5N-HX  and  CioHyN-HX,  etc., — exist 
in  different  colored  modifications.2  This  is  conspicuously  true  of  the 
alkyl  iodides,  as  for  example  C5H5N-CH3I,  Ci0H7N-CH3I,  etc.  In 
the  pyridine  series  these  substances  are  either  colorless  or  yellow,  in 
the  quinoline  series  almost  colorless  or  dark  yellow  to  deep  orange, 
while  in  the  acridine  series  the  variations  in  color  are  still  more  marked. 

iBer.,  44,  1783,  1801-1803  (1911). 

2  Glaus  and  Decker,  Jour,  prakt.  Chemie,  39,  305  (1889);  Decker,  Ber.,  37, 
2939  (1904);  Jour,  prakt.  Chemie,  79,  342  (1909). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    467 

Hantzsch l   has,   for  example,   recently  discovered  polychromic   salts 
of  N-phenyl-  and  N-methyl-phenylacridinium  halides, 


C6H5-C-        - 


-C- 
\C6 


with  colorless  anions  which  can  be  obtained  in  almost  every  variety 
of  color. 

In  the  case  of  these  substances  polychromism  is  exhibited  both  by 
the  solid  salts  and  their  solutions  and  has  been  made  the  subject  of 
very  thorough  optical  investigation  by  Hantzsch  and  Schuler.2  Their 
results  seem  to  indicate  "  that  all  acridinium  salts,  whether  in  solid 
form  or  in  solution,  consist  of  equilibrium  mixtures  of  three  simple 
types,  namely  yellow,  green  and  red, — although  at  times  certain  con- 
ditions may  favor  the  independent  existence  of  one  to  the  exclusion 
of  the  other  two." 

Hantzsch  is  of  the  opinion  that  the  isomerism  of  these  different 
ammonium  salts  is  conditioned  by  the  fact  that  one  of  the  four  atoms 
or  groups  in  union  with  nitrogen  is  less  securely  bound  than  the  other 
three.  If  this  conception  is  expressed  in  terms  of  Werner's  theory, 
according  to  which  salts  of  this  type  may  be  formulated  as 

H3N_....H— Cl 

it  follows  that  the  labile  atom  or  group  is  the  one  in  union  with  the 
acid  radical. 

It  will  be  recalled  that  according  to  Werner's  theory  two  isomeric 
modifications  are  theoretically  possible  in  the  case  of  all  quaternary 
ammonium  salts  in  which  one  of  the  four  residues  is  different  from  the 
other  three,  viz. 

RRRN Ri— X      and      RRRiN R— X 

In  cases  where  either  the  radical  R  or  Ri  is  capable  of  tautomeric  rear- 
rangement, as,  for  example,  benzoid  <=^  quinoid,  the  number  of  isomeric 
modifications  will  obviously  be  increased. 

Following  a  line  of  argument  which  is  based  more  or  less  upon 
probabilities,  Hantzsch  has  formulated  these  relationships  in  the  follow- 
ing manner:3 

iBer.,  42,  68(1909). 
2Ber.,  44,  1799  (1911). 
"Ber.,  44,  1805  (1911). 


468 


THEORIES  OF  ORGANIC  CHEMISTRY 


Benzoid,  Colorless, 
Optically  Normal 

Quinoid,  Yellow, 
Optically  Abnormal 





Pyridonium  salt, 
C6H6N-HX 

0  0N......H-X 

x—  ^       -")N—  H 

\-  sS 

Only  stable  solution  of  the 
iodide 

Alkylpyridonium  salt, 
C6H6N.CH3X 

\}  \/N  CH3~X 

X—  ^          \N—  CH3 

Quinoline  and 
Isoquinoline  salts 

Stable  form 
C9H7=N  HX 

Only  stable  as  iodide  in 
chloroform  solution 

X—  C9H7ZHN—  H 

All  other  colored  modifications  must  be  regarded  as  mixed  salts. 
From  the  ease  with  which  the  two  groups  of  salts,  isomerize  Hantzsch 
deduced  certain  rules  according  to  which  a  particular  salt  may  be  recog- 
nized as  belonging  to  one  or  the  other  class:1  "  the  so-called  benzoid 
salts  are  characterized  by  the  presence  of  a  stable  pyridine  ring  which 
like  the  true  benzene  ring  possesses  very  little  free  affinity.  The  absorp- 
tion spectra  resemble  that  of  pyridine.  The  so-called  quinoid  salts, 
on  the  other  hand,  are  characterized  by  the  presence  of  an  unstable 
acridine  ring  and  possess  a  relatively  great  amount  of  free  affinity. 
They  do  not,  however,  resemble  acridine  in  their  absorption,  being  much 
more  deeply  colored  and  showing  a  much  stronger  selective  absorp- 
tion." 

It  must  be  emphasized  that  all  such  attempts  to  explain  these  phe- 
nomena are  more  or  less  premature,  since  the  whole  question  is  in  a 
state  of  flux  at  the  present  time  and  ideas  as  to  the  best  mode  of  repre- 
senting these  relationships  have  not  yet  become  settled.2  Kauffmann's 
particular  interpretation  is  given  in  the  "  Valenzlehre,"  p.  512. 

Still  another  type  of  isomerism  has  been  described  by  Hantzsch 
under  the  title  of  homochromisomerism.3  While  chromisomers  have 
the  same  chemical  properties  but  differ  optically  as  to  their  color  and 
absorption,  homochromisomers  are  almost  identical  in  all  of  these 
respects  and  also  possess  the  same  molecular  extinctions  and  refractions. 
Such  modifications  can  be  distinguished  only  by  differences  in  their 
melting  points,  solubilities,  etc.  Since  in  solution  they  show  the  same 

^er.,  44,  1783,  1809  (1911);  49,  1865,  2169  (1916). 

2  Compare  Kehrmann,  Ber.,  49, 1338  (1916) ;  60,  24  (1917) ;  also  60, 1204  (1917) . 

3Ber.,  43,  1651  (1910). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    469 

molecular  weight  and  are  recovered  unchanged,  they  must  be  regarded 
as  isomeric  and  not  polymeric  forms. 

Examples  of  homochromisomerism  are  to  be  found  in  the  stereoiso- 
meric  quinonoximes,  nitranilines  and  possibly  also  among  the  colored 
salts  of  the  dinitro-compounds.  Methylphenyl  picramide, 

N02 

. 

N02 

affords  perhaps  one  of  the  best  illustrations  of  this  phenomenon.  This 
substance  is  formed  by  the  action  of  methylaniline  upon  picryl  chloride 
and  appears  as  either  a  low  melting  (a)  or  a  high  melting  ((3)  modifi- 
cation, depending  upon  whether  benzene  or  alcohol  respectively  is  used 
as  the  diluent  in  the  reaction.  The  a  form  melts  at  108°  and  crystal- 
lizes from  methyl  alcohol,  acetic  acid,  ether,  chloroform,  carbon  bisul- 
phide and  pyridine  in  the  form  of  dark  red  prisms.  The  /3  modifica- 
tion possesses  the  same  color,  melts  at  128  to  129°,  and  crystallizes 
unchanged  from  benzene,  pyridine  and  carbon  bisulphide.  Both  forms 
are  monomolecular.  The  a  modification  is  readily  transformed  into 
the  /3  without  suffering  any  change  in  weight,  upon  heating  at  100° 
and  also  upon  recrystallization  from  benzene.  The  high-melting  isomer, 
on  the  other  hand,  rearranges  to  give  the  a  form  without  change  in 
weight  at  ordinary  temperatures,  and  also  upon  recrystallization  from 
acetone  l  alcohol,  ether,  chloroform  and  carbon  tetrachloride. 

Both  isomers  give  the  same  ultraviolet  absorption  spectra.  Their 
molecular  extinctions  are  identical  within  the  limits  of  experimental 
error  and  their  molecular  refractions  show  only  slight  differences. 
Biilmann,2  who  has  verified  the  majority  of  Hantzsch's  experimental 
results,  discovered  the  additional  fact  that  when  melted  each  of  these 
substances  can  be  made  to  isomerize  into  the  other  upon  seeding. 
This  observation  led  him  to  the  conclusion  that  they  represent  poly- 
meric modifications  and  not  a  new  kind  of  isomerism.  Hantzsch,3 
however,  continues  of  the  opinion  that  such  is  not  the  case.  No  satis- 
factory explanation  of  homochromisomerism  has  as  yet  been  found. 

The  number  of  isomers  which  have  been  discovered  in  the  case  of 

^er.,  44,  835  (1911). 

2Ber.,  44,  834;  Ber.,  43,  1651;  Ber.,  44,  3153  (1911);  also  Hantzsch,  Ber.,  44, 
2001  (1911). 

3  Ber.,  44,  2007  (1911). 


470  THEORIES  OF  ORGANIC  CHEMISTRY 

this  and  other  classes  of  substances,  cannot  be  accounted  for  on  the 
basis  of  the  old  structural  or  stereo-formulas.  This  becomes  even 
more  apparent  if  the  following  facts  are  considered.  H.  Stobbe  l  has 
discovered  that  m-nitrobenzaldesoxylbenzoin 

N02  •  C6H4  •  CH=C(C6H5)  •  COC6H5 

exists  in  three  isomeric  forms  all  of  which  are  yellow  in  color  and  mono- 
molecular  in  composition.  If  these  isomers  are  represented  respect- 
ively by  A,  B  and  C  it  may  be  said  that  A  has  the  lowest  melting-point 
and  that  its  properties  are  essentially  different  from  B  and  C.  Thus 
although  it  gives  the  same  chemical  derivatives  as  B  and  C,  it  reacts 
much  more  readily  and  the  absorption  spectra  of  its  solutions  are  dif- 
ferent from  those  of  B  and  C.  In  short  all  of  the  chemical  and  physical 
properties  of  the  substance  indicate  that  it  is  a  chemical  isomer  of  B 
and  that  the  relationship  is  in  all  probability  correctly  represented  by 
means  of  the  following  formulas : 

N02  •  C6H4 .  CH  N02C6H4  •  CH 

II  and  || 

C6H5  •  C  •  COC6H5  C6H5CO  •  CC6H5 

A  B  and  C 

The  difference  between  B  and  C  is  much  less  pronounced  and  may 
be  explained  by  supposing  either  that  the  two  substances  are  closely 
related  chemical  isomers  or  that  they  are  dimorphic.  In  the  foimer 
case  it  may  be  assumed  that  the  differences  are  due  to  differences  in 
the  degree  of  saturation  of  the  affinity  of  their  respective  atoms  and  that 
the  relationship  is  such  that  a  given  molecule  can  pass  from  one  condi- 
tion to  the  other  upon  solution,  melting,  seeding,  etc.  In  the  second 
case  the  atomic  relationship  within  the  two  molecules  A  and  B  may 
be  supposed  to  be  identical  and  the  observed  differences  in  the  prop- 
erties of  the  two  substances  must  be  accounted  for  on  the  basis  of 
differences  in  the  arrangements  of  the  molecules,  accompanied  by  dif- 
ferences in  crystalline  form. 

In  order  to  decide  this  question  in  one  way  or  the  other  it  is  obviously 
necessary  to  know  whether  the  two  different  crystalline  individuals 
lose  their  identity  when  dissolved,  or  melted,  or  when  in  the  gaseous 
state,  for  if  their  solutions  possess  the  same  absorptions,  refractions, 
viscosity,  etc.,  the  substances  in  question  must  of  necessity  be  physically 
and  chemically  identical.  Experiment  shows  that  while  both  B  and  C 
dissolve  to  give  solutions  which  ultimately  become  identical  in  all  respects, 
such  solutions  nevertheless  exhibit  perfectly  definite  although  slight 
i  Annalen  der  Chemie,  374,  260  (1910);  Ber.,  44,  1481  (1911). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    471 

differences  if  examined  immediately  after  solution  of  the  substance 
has  taken  place.  Stobbe,  therefore,  concludes  that  B  and  C  actually 
represent  different  chemical  individuals  but  that  upon  solution  or  melt- 
ing they  pass  instantly  into  equilibrium  mixtures.  Many  similar  phe- 
nomena have  been  observed.  For  example,  Pfeiffer  describes  yellow 
and  orange-colored  modifications  of  this  class  of  compounds  which 
are  stable  at  low  and  high  temperatures  respectively.  The  yellow 
form  dissolves  in  acetic  acid  to  give  a  yellow  solution,  which  if  heated 
above  100°  gradually  changes  to  an  orange  color  and  on  cooling  pre- 
cipitates the  isomeric  orange  colored  modification.  Both  forms  dis- 
solve in  any  given  solvent  to  give  solutions  which  are  identical  but  the 
color  varies  in  the  case  of  different  solvents.  For  example,  benzene 
solutions  are  almost  colorless,  while  alcohol  and  acetic  acid  solutions 
have  a  somewhat  deeper  color  and  trichloracetic  acid  solutions,  a  very 
deep  color.  In  general,  the  variation  is  from  straw  color  to  yellow  to 
orange. 

The  property  of  forming  yellow  or  orange  solutions  depending  upon 
the  temperature  must  be  regarded  as  a  characteristic  which  is  common 
to  all  unsaturated  aromatic  compounds  but  which  may  be  intensified 
by  the  bathochromic  effect  of  substituents  such  as  the  methoxy  groups. 
The  nitro-methoxystilbenes,  for  example,  resemble  the  unsaturated 
ketones  in  their  ability  to  form  addition  products  with  acids  and  metal- 
lic salts.  Thus  Pfeiffer  l  and  his  students  have  obtained  an  orange 
colored  compound  having  the  formula 

[NC  •  C6H3(N02)  •  CH=CH  •  C6H4OCH3]2SnCl4 

by  the  action  of  stannic  chloride  upon  the  yellow  modification  of  4-cyan- 
2  nitro-4  methoxystilbene.  This  product  on  decomposing  regenerates 
the  original  yellow  modification  of  the  parent  substance.  On  the  other 
hand  4-cyan-  2-nitro-4-methoxystilbene  dissolves  in  benzene  to  give  a 
yellow  solution  from  which  a  yellow  addition  product 

[NC  •  C6H3(N02)  •  CH=CH  •  C6H4OCH3]2C6H6 

separates.  The  latter  decomposes  to  give  a  free  stilbene  which  is  orange 
in  color  and  which  appears  to  represent  a  second  distinct  modification. 
In  an  analogous  manner  benzoylaminonitromethoxystilbene  combines 
with  acetic  acid  to  give  an  addition  product  which  is  yellow,  and  with 
trichloracetic  acid  to  give  an  orange  colored  compound.  Both  sub- 
stances decompose  to  give  respectively  orange  and  yellow  modifications 

'Ber.,  48,  1777  (1915). 


472  THEORIES  OF  ORGANIC  CHEMISTRY 

of  the  free  stilbene.  It  is  rather  remarkable  that  the  yellow  addition 
product  gives  a  yellow  modification  of  the  parent  substance,  but  this 
may  be  explained  by  supposing  that  both  modifications  of  the  free 
stilbene  coexist  in  a  condition  of  dynamic  equilibrium  in  solution  and 
that  the  preponderance  of  one  or  the  other  form  depends  upon  specific 
conditions.  If  this  assumption  is  correct  yellow  solutions  might  be 
expected  to  contain  the  yellow  modification  while  as  a  matter  of  fact 
it  has  been  observed  that  yellow  benzene  solutions  give  the  orange  and 
not  the  yellow  form  of  4-cyan-2  nitro-4-methoxy  stilbene. 

The  latter  observation  led  Pfeiffer  to  assume  that  the  phenomena 
could  not  be  explained  on  the  basis  of  normal  isomerism  and  that  the 
two  modifications  must,  therefore,  be  regarded  as  polymorphic  rather 
than  as  isomeric  forms.  Differences  in  color  may  be  explained  on  the 
basis  of  halochromism  if  different  molecular  compounds  are  assumed 
to  possess  varying  amounts  of  free  affinity.  In  the  case  of  different 
colored  addition  products  this  conception  may  be  expressed  by  means 
of  the  general  formulas 

RN02......SnCl4,    R  •  NO2......HOCO  -  CC13 

I  I 

where  the  arrow  serves  to  locate  the  position  of  the  free  affinity  present 
in  the  molecule  but  does  not  in  any  way  define  the  variations  in  quantity 
upon  which  a  lightening  or  a  deepening  of  color  depends. 

Differences  both  in  the  color  and  in  the  crystalline  structure  of 
different  .polymorphic  modifications  of  one  and  the  same  chemical 
individual  may  be  explained  simply  by  extending  this  conception  to 
the  higher  orders  of  molecular  compounds.1  If,  as  von  Laue,  W.  L. 
and  W.  H.  Bragg  suppose,  crystals  may  be  regarded  as  complex  molec- 
ular compounds,2  difference  in  crystalline  structure  in  the  case  of  one 
and  the  same  chemical  individual  must  be  due  to  differences  in  the  way 
in  which  the  molecules  are  combined.  Assuming  that  combination 
takes  place  according  to  the  rules  embodied  in  Werner's  doctrine  of 
coordinate  relationships  among  atoms,  it  is  easy  to  see  how  one  such 
complex  might  differ  from  another  in  the  relative  concentration  of  free 
affinity  in  definite  positions.  The  association  of  molecules  might, 
for  example,  result  from  the  union  of  either  principle  or  partial  valencies. 
Thus  two  molecules  of  nitromethoxystilbene  might  combine  by  the 
mutual  saturation  of  two  ethylene  groups.  In  this  case  the  associated 
molecules  would  probably  be  lighter  than  the  unassociated  molecules 

1  Pfeiffer,  Ber.,  48,  177*7  (1915). 

2  Zeitschr.  anorg.  Chemie,  92,  376  (1915). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    473 

since  reactions  of  this  type  are  usually  accompanied  by  increased  satura- 
tion or  in  other  words  by  a  decrease  in  the  free  affinity  of  the  substance. 
Another  and  very  different  condition  might  result  from  the  saturation 
of  partial  valencies  present  on  different  groups.  For  example,  nitro- 
methyoxystilbene,  02NC6H4-CH==CH-C6H4OCH3,  may  be  regarded 
both  as  a  nitro-compound  and  as  a  phenol  ether  and  since  it  is  known 
that  these  two  classes  of  atomic  groupings  interact  by  the  saturation 
of  their  partial  valencies,  it  may  be  assumed  that  nitromethyoxystil- 
bene  molecules  are  capable  of  reacting  in  the  same  way.  Associated 
molecules  of  this  type  would  be  coordinately  bound  by  the  saturation 
of  partial  valencies  on  the  nitro-groups  and  unsaturated  carbon  atoms 
respectively  and,  because  of  an  increase  in  the  free  affinity  of  the  nitro- 
gen, should  possess  a  deeper  color  than  the  unassociated  molecules. 
The  relative  depth  of  color  would  vary  with  the  relative  amount  of  free 
affinity  in  any  given  case.  Whatever  the  merits  or  demerits  of  this  con- 
ception it  at  least  serves  to  elucidate  the  relationships  which  exist 
between  true  chemical  isomers  on  the  one  hand  and  polymorphic  modi- 
fications on  the  other. 

The  more  recent  theories  in  regard  to  the  relation  between  color 
and  chemical  constitution  may  be  applied  in  the  elucidation  of  cer- 
tain phenomena  which  have  been  observed  in  connection  with  the 
chemistry  of  dyes  of  the  indigo  group  and  also  of  the  so-called  mordant 
dyes.  According  to  the  formula  which  was  originally  brought  forward 
by  Baeyer  to  explain  the  chemical  constitution  of  indigo 


NH 


the  color  of  the  substance  may  be  accounted  for  by  supposing  that  the 
two  conjugated  systems  CO — C=C — CO  act  as  chromophores,  each 
being  reinforced  respectively  by  an  NH  group  which  functions  as  an 
auxochrome.  This  formula  does  not,  however,  serve  to  explain  the 
color  of  indigo  in  terms  of  the  more  recent  conception  in  regard  to  the 
relation  between  color  and  constitution  so  that  it  has  now  been  super- 
seded by  another  expression  which  was  first  suggested  by  M.  Claass  1 
and  which  has  been  made  the  basis  for  new  theoretical  developments. 
Claass  made  the  important  observation  that  the  characteristic  deep 
blue  color  of  indigo  is  not  affected  by  the  substitution  of  the  group  SO 

ifier.,  49,  2079  (1916). 


474  THEORIES  OF  ORGANIC  CHEMISTRY 

for  the  group  CO.  This  discovery  was  rather  remarkable  in  view  of 
the  fact  that  sulphoxyl  has  not  been  found  to  act  as  a  chromophore 
in  other  of  its  combinations  and  the  presence  of  the  ethylene  group, 
C=C,  is  not  in  itself  sufficient  to  account  for  the  deep  color  of  the  com- 
pound. In  order  to  explain  the  phenomenon  Claass  was  led  to  assume 
that  in  the  case  of  indigo  and  its  sulphoxyl  derivative  the  color  is  con- 
ditioned by  two  factors  (a)  the  formation  of  an  inner  salt  by  the  inter- 
action of  the  NH  group  with  either  of  the  acidyl  groups,  CO  or  SO,  and 
(6)  the  presence  of  an  ortho-qumoid  complex,  viz., 


The  chromophore  in  thioindigo  was  assumed  to  possess  an  analogous 
structure,  the  only  difference  being  the  replacement  of  the  NH  group 
by  sulphur: 

S 


° 


\/\i 


J.  Lifschitz  and  A.  Lourie  1  then  made  the  discovery  that  the  absorp- 
tion spectrum  of  indigo  is  distinctly  different  from  that  of  substances 
which  are  known  to  possess  a  quinoid  structure.  In  fact  the  type  of 
absorption  which  was  found  to  be  characteristic  of  dyes  of  the  indigo 
group  bore  a  striking  resemblance  to  the  absorption  of  the  halochrome 
derivatives  of  the  ketones.  The  optical  properties  of  these  two  classes 
of  compounds  are,  moreover,  in  striking  agreement  with  their  chemical 
properties  as  is  illustrated,  for  example,  by  the  behavior  of  indigo, 
thioindigo,  etc.,  toward  sulphuric  acid.  Thus  both  investigators  found 
that  the  change  suffered  by  the  spectrum  of  indigo  under  the  action 
of  sulphuric  acid  resembled  in  type  that  suffered  by  halochrome  ketones 
under  the  action  of  the  same  reagent.  These  facts  seemed  to  indicate 
that  some  modification  of  the  Claass'  formula  for  indigo  was  desirable 
and  Lifschitz  and  Lourie,  therefore,  brought  forward  a  new  formula 
which  avoided  the  assumption  of  quinoid  structure  and  attempted  to 

iBer.,  50,897  (1917). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION     475 


explain  the  phenomenon  of  color  in  terms  of  Pfeiffer's  theory  of  halo- 
chromism,  viz., 

,HN 


According  to  this  conception  the  similarity  in  the  behavior  of  indigo 
and  halochrome  ketones  in  the  presence  of  sulphuric  acid  and  other 
reagents  may  be  accounted  for  on  the  basis  of  the  following  formulas: 


/\ 


NH 


CH=CH-C6H5 
4=0 


HX 

Benzal  acetophenone 


Another  important  group  of  organic  dyes  consist  of  substances 
which  like  alizarine  react  with  the  oxides  of  certain  metals  as,  for 
example  Al,  Fe,  Cr,  Cu,  Sn,  Co,  etc.,  to  form  colored  insoluble  salt- 
like  compounds  known  as  color  lakes.  Substances  of  this  type  are 
usually  applied  to  fabric  which  has  first  been  mordanted  or  which,  in 
other  words,  has  first  been  treated  with  a  salt  of  the  particular  metal 
that  is  known  to  give  the  desired  color.  Under  these  circumstances 
the  color  lake  is  usually  deposited  in  a  very  stable  form  upon  the  fabric. 
A  careful  investigation  of  the  different  classes  of  substances  which  possess 
the  power  of  reacting  with  metallic  oxides  to  form  lakes  led  to  the  dis- 
covery that  this  property  is  conditioned  by  certain  definite  configura- 
tions of  the  molecule.  For  example,  the  only  oxyanthraquinones 
capable  of  forming  lakes  are  those  which,  like  alizarine,  contain  two 
hydroxyl  groups  in  the  ortho  positions.1  Later  R.  Mohlau  and  F.  Steim- 
mig2  observed  that  monohydroxyl  derivatives  of  aromatic  hydro- 
carbons in  general  possess  the  power  of  forming  lakes  provided  only 
that  the  hydroxyl  group  occupies  an  ortho-  or  pen-position  with 
reference  to  the  chromophore  CO.  Previous  to  this  time  Kostanecki 3 

1  Rule  of  Lievermann  and  St.  von  Kostanecki,  Annalen  der  Chemie,  240,  245 
(1887). 

2  Zeitschr.  f.  Farbens  u.  Textilchemie,  3,  358  (1904). 

3  Ber.,  20,  3146  (1887);  22,  1347  (1889). 


476  THEORIES  OF  ORGANIC  CHEMISTRY 

had  pointed  out  that  substances  with  certain  isonitroso  and  other 
atomic  groups  in  analogous  positions  with  reference  to  ketone  groups, 
are  able  to  form  color  lakes.  Examples  of  this  kind  continued  to  multi- 
ply but  it  was  not  until  1907  that  L.  Tschugaeff  1  called  attention  to 
the  fact  that  in  the  case  of  the  isonitrosoketones  the  power  to  form 
color  lakes  depends  upon  the  ability  of  the  substance  to  enter  into 
inner  cyclic  salt-like  combinations  such  as  for  example, 

R.C=N—  (X 

>M 
R-C=O  -"' 

According  to  this  conception  lakes  are  to  be  regarded  as  complex  com- 
pounds of  cyclic  structure,  the  stability  of  which  depends  in  part  at 
least  upon  the  presence  of  a  five  membered  ring  in  the  molecule. 

In  order  to  test  the  accuracy  of  this  theory  A.  Werner  2  investigated 
a  large  number  of  organic  compounds  which  possess  the  power  to  form 
so-called  inner  complex  salts  of  the  general  formula: 

R 
C=(\ 

HC<        yu 
^c—  ex 

R 

Salts  of  this  type  are  distinguished  by  the  fact  that  the  metallic  atom 
is  simultaneously  bound  by  a  partial  and  by  a  principal  valence  and  that 
ring  formation  depends  upon  an  intramolecular  saturation  of  partial 
valencies.  The  results  of  Werner's  investigation  showed  that  such 
relatively  simple  substances  as  benzoyl  acetone,  dibenzoyl  methane, 
anisoyl  benzoyl  methane  and  others  are  capable  of  forming  inner  com- 
plex salts  which  belong  to  the  class  of  color  lakes.  P.  Pfeiffer  3  pointed 
out,  moreover,  that  oxy-ketones  and  oxy-quinones  in  general  possess 
this  property  as  do  also  amido-oximes  and  hydroxamic  acids.  In  the 
latter  case  the  inner  complex  salts  belong  to  the  same  general  class  of 
mordant  dyes  and  may  be  assumed  to  possess  formulas  corresponding  to 


NH—  (X  ^N-Ox  /NH—  Ov 

>M      or     RC<  >M      or     RC  > 

••••  \NH2  / 


M 


These  observations  led  Werner  to  formulate  his  "  theory  of  mor- 
dants "  according  to  which  "  substances  which  react  with  mordants  to 

1  Jour,  prakt.  Chemie,  76,  88  (1907). 

2Ber.,  41,  1062  (1908);  also  "Neuere  Anschauungen,  etc.,"  3rd  Ed.,  p.  247. 

3  Ber.,  44,  2653  (1911);  Annalen  der  Chemie,  398,  138  (1913). 


RELATION  BETWEEN  COLOR  AND  CHEMICAL  CONSTITUTION    477 

give  dyes  must  be  so  constituted  as  to  contain  (1)  a  salt  forming  group 
and  (2)  a  second  group  which  is  capable  of  entering  into  coordinative 
union  with  a  metallic  atom.  These  two  groups  must  be  so  placed  with 
reference  to  each  other  that  inner  complex  metallic  salts  may  arise." 
This  characterization  has  been  found  to  be  in  general  agreement  with 
all  the  facts  that  have  been  discovered  up  to  the  present  time.1  In 
conclusion  it  may  be  noted  that  the  number  of  substances  falling  within 
this  category  of  dyes  has  been  increased  by  the  recent  discovery  of 
R.  Mohlau2  who  has  found  that  certain  complicated  monoatomic 
phenols,  as  for  example  a-  and  /3-anthrols,  and  also  certain  multiatomic 
phenols  of  both  simple  and  complex  structure  possess  the  ability  to 
form  mordant  dyes. 

Absorption  phenomena  have  been  accounted  for  by  J.  Stark  on 
the  basis  of  purely  physical  considerations  and  in  conclusion  these  may 
now  be  considered  briefly.  Investigations  in  the  fields  of  the  cathode 
rays  and  radioactivity  have  led  to  the  formulation  of  the  hypothesis  that 
chemical  atoms  are  compounded  exclusively  of  positive  and  negative 
units  of  electricity.  Stark  assumes  that  extended  positively  charged 
areas  exist  on  the  surfaces  of  the  atoms  and  that  above  these  and  in 
definite  positions  in  relation  to  them  are  point-like  negative  electrons. 
The  latter  are  supposed  to  play  the  part  which  has  commonly  been 
assigned  to  valency  and  to  bring  about  the  union  of  two  or  more  atoms. 
They  may,  therefore,  be  referred  to  as  valence-electrons.  From  the 
standpoint  of  the  optical  relationships  of  chemical  compounds  valence- 
electrons  may  be  differentiated  into  three  classes,  namely,  unsaturated, 
saturated  and  semi-detached  ("  gelockert  ").  For  example,  if  an  elec- 
tron is  attached  only  to  its  own  and  to  another  atom,  it  is  unsaturated; 
and  if  for  any  reason  the  attraction  by  which  it  is  held  to  the  positive 
sphere  of  its  own  atom  is  weakened  so  that  it  becomes  more  or  less  free 
to  change  its  position  or  even  to  become  detached,  it  is  said  to  be  semi- 
detached or  labile.  The  latter  condition  is  brought  about  by  the  com- 
bination of  a  given  atom  with  other  atoms,  and  is  possible  only  in  the 
case  of  multivalent  atoms. 

Stark  assumes  that  valence-electrons  are  the  centers  of  the  emission 
and  absorption  of  light  in  band  spectra  and  that  the  semi-detached  or 
labile  electrons  play  a  particularly  important  part  in  connection  with 
these  phenomena.  According  to  this  theory  the  kinetic  energy  involved 
in  the  recurrent  separation  and  union,  which  takes  place  between  an 
electron  and  the  positive  sphere  of  its  atom,  is  transformed  into  the 

Compare  R.  Scholl,  Ber.,  61,  1420  (1918);   62,  565  (1919);   also  O.  Baudisch, 
Ber.,  51,  1058  (1918). 
2  Ber.,  62,  1730  (1919). 


478  THEORIES  OF  ORGANIC  CHEMISTRY 

radiant  energy  of  light.  According  to  calculations,  which  cannot  be 
inserted  here  but  which  will  be  referred  to  again  later,  absorption  in 
the  impenetrable  ultraviolet  and  in  the  infra-red  is  due  to  the  presence 
of  unsaturated  and  saturated  valence  electrons  respectively.  The 
semi-detached  electrons,  on  the  other  hand,  are  responsible  for  band 
spectra  which  lie  beyond  0.0007  mm.  For  example,  benzene  possesses 
electrons  of  this  type  and  their  presence  in  the  following  chromophores 
is  indicated  by  means  of  the  small  circle 

yO-O  >0-0 

HOf=rKf  or 


NO  =  N—  -0;    -N=N-  =  N  -  N  ;  etc. 

A     A  A     A 

It  is  supposed  that  the  relative  degree  of  detachment  of  such  elec- 
trons varies  and  that  it  is  influenced,  for  example,  by  the  substitution 
of  the  above  groups  in  the  benzene  rings,  or  by  the  combination  of  several 
chromophores.  In  the  cases  where  the  lability  of  the  electrons  is 
increased  the  absorption  bands  of  the  substance  may  be  observed  to 
shift  in  the  direction  of  the  red.  It  is  obvious  that  in  this  way  the  old 
idea  of  chromophores  may  be  combined  with  the  more  modern  con- 
ceptions of  the  electron  theory  l  in  interpreting  absorption  phenomena. 
1  Compare  H.  Ley,  "Farbe  und  Konstitution,"  p.  68. 


CHAPTER  XVII 
THE  THEORY  OF  INDICATORS 

AT  the  time  of  the  development  of  the  ionic  theory  Ostwald  1  under- 
took an  investigation  in  regard  to  the  color  of  ions  the  results  of  which 
led  him  to  suppose  that  ions  possess  definite  and  characteristic  absorp- 
tions and  upon  this  assumption  he  proceeded  to  develop  a  new  theory 
of  indicators.2  According  to  this  theory  indicators  are  colorless  sub- 
stances belonging  either  to  the  class  of  weak  acids  or  weak  bases,  which 
readily  dissociate  to  give  colored  ions.  Phenolphthalein,  for  example, 
dissociates  to  give  an  ion  having  an  intensely  red  color  although  its 
undissociated  molecule  is  colorless.  The  color  of  the  ion  thus  becomes 
apparent  whenever  a  highly  dissociated  salt  is  formed  from  the  acid 
which  is  itself  practically  undissociated. 

Later  Stieglitz3  expressed  the  opinion  that  color  changes  in  indi- 
cators are  due  not  to  the  phenomenon  of  dissociation,  but  to  intra- 
molecular rearrangements.  According  to  this  view  the  sodium  salt  of 
phenolphthalein  is  assumed  to  have  a  quinoid  structure  while  free 
phenolphthalein  is  benzoid  and  a  lactone: 

/C6H4OH 

/C6H4OH  C6H4— C< 

O=C6H4=C<  and     I  I  XC6H4OH 

\C6H4COONa  CO O 

Red  solutions  of  the  free  acid  of  methyl  orange  are  accounted  for  in 
an  analogous  manner  and  are  supposed  to  contain  molecules  having 
quinoid  structure  because,  as  compared  with  their  alkaline  solutions, 
they  possess  relatively  deep  colorations: 

0  •  S02  •  C6H4  •  NH  •  N==C6H4=N(CH3)2 

I [ 

Red 

and 

NaOS02  •  C6H4  •  N=N  •  C6H4N(CH3)2  •  HOH 

Yellow 

Zeitschr.  physikal.  Chemie,  9,  579  (1892). 

2  "Die  wissenschaftlichen  Grundlagen  der  analytischen  Chemie,"  1894,  p.  104. 

3  Jour.   Am.   Chem.   Soc.,   26,    1112    (1903);    also   Kremann,   Zeitschr.    anorg. 
Chemie,  33,  87  (1903);    Bredig,  ibid.,  34,  202  (1903),  and  Veley,  Zeitschr.  physikal. 
Chemie,  67,  148  (1907). 

479 


480  THEORIES  OF  ORGANIC  CHEMISTRY 

Hantzsch  had  previously  expressed  views  similar  to  those  of  Stieglitz 
and  after  laying  the  foundation  of  his  theory  of  chromoisomerism  he 
again  returned  to  the  problem.  Realizing  that  his  assumption  that 
"  every  appearance  of  color  developed  during  the  process  of  salt  forma- 
tion with  colorless  metallic  ions  is  due  to  intramolecular  rearrange- 
ments," was  capable  of  experimental  demonstration  in  the  case  of 
indicators,  Hantzsch  undertook  a  critical  examination  of  the  subject. 
The  theory  that  ionization  phenomena  might  in  themselves  be  sufficient 
to  account  for  changes  in  color  seemed  to  him  most  improbable  from 
a  chemical  point  of  view:  (1)  Because  all  colorless  acids  and  salts 
whose  constitutions  are  unalterable  form  colorless  ions.  The  only 
true  acids  which  give  colored  ions  are  those  which  in  undissociated 
condition  are  themselves  colored.  From  this  it  follows  that  color  is 
independent  of  the  presence  or  absence  of  ions.  (2)  Because  numerous 
colorless  compounds  such  as  dinitroethane,  nitroform,  violuric  acid,  etc., 
which  form  colored  ions,  have  been  shown  to  be  pseudoacids  or  sub- 
stances in  which  intramolecular  rearrangements  always  precede  neutral- 
ization reactions.1  These  facts  taken  together  seem  to  indicate  that 
in  the  case  of  indicators  changes  in  color  depend  unconditionally  upon 
changes  in  constitution  and  that  the  formation  of  ions  therefore  repre- 
sents a  secondary  process,  the  ions  being  colored  only  because  the  undis- 
sociated molecules  from  which  they  are  formed  are  also  colored.  The 
salts  of  phenolphthalein  are  in  fact  colored  in  the  solid,  that  is  to  say 
in  the  undissociated  condition. 

Hantzsch  supposes,  in  other  words,  that  indicators  belong  to  the 
class  of  tautomeric  substances.  If  this  is  so  it  should  be  possible  to 
demonstrate  that  phenolphthalein,  for  example,  is  actually  capable  of 
forming  two  series  of  salts,  one  colorless  and  the  other  colored,  corre- 
sponding respectively  to  the  lactoid  and  quinoid  formulas: 

,C6H4OH 

,C6H4OH 


xv^t)XJ.4v/j.x 

C6H4-C<  / 

|   \C6H4ONa  and     0=C6H4=C< 

CO O  x 


C6H4COONa 

All  attempts  to  do  this  have  been  unsuccessful  up  to  the  present  time. 
Two  isomeric  series  of  ethers  and  esters  having  the  required  properties 
have,  however,  recently  been  discovered. 

In  1906  Green  and  King2  prepared  a  colored  methyl  ester  from 
colorless  phenolphthalein  to  which  they  gave  the  formula: 

/C6H4OH 

o=c6H4=c<; 

XC6H4COOCH3 
i  Ber.,  39,  1090  (1906).  2  Ber.,  39,  2365  (1906). 


THE  THEORY  OF  INDICATORS  481 

but  the  act-ual  existence  of  such  a  suostance  was  doubted  by  H.  Meyer  1 
until  Hantzsch  and  K.  H.  Meyer2  verified  these  results.  Later  R. 
Meyer  and  Marx  3  obtained  an  intensely  yellow  diethyl  ester  by  treating 
the  silver  salt  of  tetrabrom-phenolphthalein  with  methyl  iodide  accord- 
ing to  the  method  of  Hantzsch  and  Gorke.  This  substance  readily 
rearranges  to  give  a  colorless  isomer: 


C2H5OCOC6H4< 

X  -L»l  \  JJ1 

-OC2H5 
Br 

Yellow  Colorless 

These  investigators  4  were  able  to  show  later  that  analogous  esters  could 
be  prepared  from  phenolphthalein  itself,  and  that  again  in  this  case  the 
labile  quinoid  modification  was  yellow  and  rearranged  in  the  process 
of  crystallization  to  give  the  colorless,  stable,  lactoid  form. 

The  tautomeric  character  of  phenolphthalein  has  thus  been  clearly 
demonstrated  and  as  a  result  the  theory  of  rearrangement  has  been 
placed  upon  a  sound  experimental  basis.  The  mechanism  of  the  change 
from  a  red  to  a  colorless  solution  by  the  action  of  an  excess  of  concen- 
trated alkali  has,  however,  not  as  yet  been  satisfactorily  accounted  for. 
Hantzsch  and  K.  H.  Meyer  have  suggested  a  possible  solution  of  this 
problem.  The  bleaching  of  phenolphthalein  solutions  by  the  cation 
of  concentrated  alkali  is  not  instantaneous  as  has  been  shown  by  means 
of  conductivity  measurements.  These  investigators,  therefore,  argue 
that  it  is  probably  accompanied  by  rearrangements  in  the  molecule  in 
he  sense  of 

NaOCO-C6H4v  NaOCOC6H4\      /C6H4ONa 

\C=C6H4=0  +  NaOH=  >C< 

NaOCGH4/  NaOC6H4/       \ 


This  would  explain  the  end  changes  in  reactions  where  phenolphthalein 
is  used  as  an  indicator.  To  recapitulate,  free  phenolphthalein  is  a 
lactone  which  first  isomerizes  in  alkaline  solution  and  then  reacts  to 
give  a  quinoid  salt  having  a  red  color  and  capable  of  electrolytic  dis- 

1  Ber.,  40,  2431  (1907). 
2Ber.,  40,  3480  (1907). 

3  Ber.,  40,  1437  (1907). 

4  Ber.,  40,  3603  (1907). 


482  THEORIES  OF  ORGANIC  CHEMISTRY 

sociation  into  colored  ions.  In  the  presence  of  an  excess  of  alkali  this 
salt  hydroly zes  to  give  a  derivative  of  triphenyl  carbinol : 

C6H4COONa  C6H4COONa 

-»  C  C— OH 

HOC6H4  CeEUOH          NaOC6H4    C6H4=O       NaOC6H4    C6H4ONa 

Colorless  lactone  Colored  quinoid  Colorless  carbinol 

Vorlander  had  already  pointed  out  that  the  ionic  theory  of  indica- 
tors was  unable  to  account  for  the  changes  in  color  observed  in  the  case 
of  derivatives  of  aminoazobenzene.1  If,  for  example,  the  intense  violet 
red  color  of  acid  solutions  of  aminoazobenzene  depends  wholly  upon 
the  formation  of  colored  ions  such  as 

C6H5-N=N.C6H4NH/3 

it  follows  that  its  trimethyl  derivatives  should  also  give  intensely  colored 
ions,  viz., 

C6H5  •  N=N  •  CoH4N(CH3)  '3 

but  as  a  matter  of  fact  this  salt  dissociates  to  give  ions  which  are  of 
almost  the  same  color  as  azobenzene  itself.  It  would,  therefore,  seem 
that  color  cannot  in  this  case  be  conditioned  solely  by  ionization. 

Hantzsch  2  has  discovered  recently  that  the  salts  of  aminoazobenzene 
exist  in  two  series  which  are  sharply  differentiated  from  each  other  but 
which  are  nevertheless  readily  transformed  one  into  the  other: 

1.  True  azo  salts  of  orange  color  with  spectra  similar  to  azobenzene: 

C6H5  •  N=N  -  C6H4NR2HX 

2.  Quinoid  salts  of  violet  color  with  characteristic  quinoid  band 
spectra 

C6H5  •  NH  •  N  •  C6H4  -  NR2X 

I 1 

This  discovery  seemed  to  solve  the  problem  of  color  changes  in  the  case 
of  methyl  orange  (helianthine) .  The  orange-colored  solutions  of  this 
indicator  were  assumed  to  represent  solutions  of  true  azo  salts, 

(CH3)2N  •  C6H4]S[=NC6H4S03Na 

1  Ber.,  36,  1485  (1903);'  Annalen  der  Chemie,  320,  116  (1902). 

2  Ber.  der  Chem.  Ztg.  iiber   Naturforscherversammlung  in  Dresden,   1907,  p. 
59;  Ber.,  41,  1187  (1908). 


THE  THEORY  OF  INDICATORS  483 

while  the  free  violet  coloring  matter  was  supposed  to  consist  of  an  inner 
quinoid  salt, 

r  "i 

(CH3)2N  -  C6H4  •  N  •  NH 


This  interpretation  of  the  phenomena  has,  however,  been  modified 
by  Hantzsch  1  as  the  result  of  a  careful  study  of  the  absorption  spectra 
of  these  substances.  Investigations  along  this  line  show  that  the  yellow 
modification  of  helianthine  is  not  azoid  but  quinoid  in  character  and  that 
its  structure  is  almost  identical  with  that  of  the  red  modification.  The 
change  from  the  yellow  to  red  cannot  therefore  be  construed  as  repre- 
senting a  rearrangement  from  a  yellow  azoid  to  a  red  quinoid  salt  but 
must  be  regarded  as  a  rearrangement  of  one  quinoid  valence  isomer 
into  another.  In  other  words  the  yellow  and  red  modifications  of 
helianthine  are  not  structural  but  chromo-isomers  and  the  change  from 
one  into  the  other  must  be  formulated  in  terms  of  partial  valency 
formulas.  For  example  alkaline  solutions  of  methyl  orange  may  be 
supposed  to  contain  the  salt 

C6H4-N— 

SO3Na  I CH3 

Yellow  methyl  orange 

The  addition  of  acid  to  such  a  solution  first  sets  free  the  corresponding 
acid, 


C6H4-N— N 

jH   !— 


V-^O-1--1- 

SO3] 


Yellow  unstable  acid 


but  this  rearranges  immediately  to  give  the  yellow  modification  of 
helianthine  which  may  be  regarded  as  an  inner  complex  salt  having 
the  formula 


C6H4-NH— N= 
Os 


••— -u-«- 

lo 


Yellow  helianthine 

The  latter  then  isomerizes  to  give  the  red  modification 

,CH3 
C6H4.NH— N 


SO 


Red  helianthine 

JBer.,  46,  1537  (1913);  48,  158  (1915). 


484  THEORIES  OF  ORGANIC  CHEMISTRY 

and  this  in  the  presence  of  an  excess  of  acid  passes  into  a  red  acid  salt, 
which  in  the  case  of  hydrochloric  acid  possesses  the  following  formula : 


C6H4-NH.N 


SO31 


Red  hydrochloride 


These  formulas  if  correct  fully  account  for  the  color  changes  which 
accompany  neutralization  phenomena  in  the  presence  of  methyl  orange 
as  an  indicator.  It  will  be  noted  that  the  change  from  yellow  to  red 
depends  upon  an  isomerization  equilibrium  of  the  type 

yellow  helianthine     +±    red  helianthine 

The  appearance  of  the  red  modification  depends  upon  the  presence  of 
free  hydrogen  ions  and  its  formation  is  favored  by  an  increase  in  the 
concentration  of  these  ions.  It  does  not  follow,  however,  that  the 
appearance  of  the  yellow  isomer  depends  in  the  same  way  upon  the 
presence  of  alkali,  since  at  present  it  can  only  be  said  that  while  the  red 
isomer  is  stable  in  the  presence  of  hydrogen  ions  the  yellow  modification 
is  stable  in  the  absence  of  these  ions.  The  fact  that  the  addition  of 
alkali  often  leads  to  the  formation  of  the  yellow  isomer  may,  according 
to  Hantzsch,  be  explained  by  supposing  that  under  such  circumstances 
the  alkali  functions  merely  to  remove  the  hydrogen  ions  which  are 
present  in  the  solution.  Helianthine  when  dissolved  in  absolute 
alcohol  remains  yellow  even  after  acetic  acid  has  been  added  because 
in  the  complete  absence  of  all  moisture  no  hydrogen  ions  are  formed. 
This  shows  how  important  it  is  to  titrate  in  aqueous  rather  than  in 
alcoholic  solutions  when  methyl  orange  is  used  as  an  indicator  in  neu- 
tralization reactions. 

W.  Ostwald  1  has  recently  suggested  that  changes  in  color  in  the 
case  of  indicators  may  be  due  wholly  or  in  part  to  variations  in  the 
degree  of  dispersion  of  the  respective  solutions,  but  Hantzsch2  has 
been  able  to  show  that  this  explanation  cannot  be  applied  to  methyl- 
orange  and  helianthine  since  these  substances  when  perfectly  pure  do 
not  form  colloidal  solutions  with  water. 

1  Zeitschr.  Chem.  Ind.  Kolloide,  10,  97. 

2Ber.,  46,  1541  (1913). 

For  the  more  recent  literature  compare  A.  Thiele,  "Die  Anwendung  neuerer 
Ergebnisse  der  Indikatorenforschung  zu  Quantitative!!  Studien,"  Sitzungsber. 
der  Gesellschaft  zur  Beford.  der  ges.  Naturwissen  zu  Marburg,  1912;  also  N.  Bjerrum, 
"Die  Theorie  der  alkalimetrischen  und  acidimetrischen  Titrierungen,"  Stuttgart, 
Encke,  1914. 


CHAPTER  XVIII 

THE  RELATIONSHIP  BETWEEN   FLUORESCENCE  AND 
CHEMICAL  CONSTITUTION1 

FLUORESCENT  substances  are  those  which  are  luminous  under  the 
direct  action  of  light;  and  such  luminescence  depends  upon  the  fact 
that  certain  of  the  rays  of  light  which  strike  the  substance  are  trans- 
formed by  it  into  others  of  different  wave  length.  The  manifestation 
of  this  phenomenon  does  not  depend  upon  the  state  of  aggregation 
but  is  common  to  solid,  liquid,  and  gaseous  substances.  Fluorspar 
and  uranium  afford  examples  of  the  first  class;  paraffin  oil  and  solutions 
of  eosin,  etc.,  examples  of  the  second;  and  the  vapors  of  sodium,  iodine, 
etc.,  the  third.  If  the  effect  is  prolonged  for  a  relatively  long  period 
after  the  source  of  light  is  withdrawn  the  phenomenon  is  known  as 
phosphorescence.  The  two  phenomena  are,  however,  thought  to  be 
very  closely  related. 

An  important  physical  law  governs  all  such  phenomena — namely, 
that  the  light  which  radiates  from  a  fluorescent  body  is  not  created  by 
it,  but  represents  absorbed  rays  which  have  been  transformed.  Stark  2 
has  recently  discovered,  as  a  result  of  his  investigations  in  regard  to 
line  and  band  spectra,  that  fluorescence  depends  upon  banded  absorp- 
tion, or,  in  other  words  substances  which  show  this  characteristic  type 
of  absorption  also  show  the  property  of  fluorescence.3  Moreover,  if 
a  fluorescing  ray  is  subjected  to  spectral  analysis  it  is  found  to  consist 
of  one  or  more  bands  which  usually  possess  a  maximum  intensity  at 
certain  wave  lengths  and  this  maximum  corresponds  to  the  maximum 
intensity  of  absorption  for  any  given  substance.  This  fact  is  shown  by 
means  of  the  following  diagram  4  in  which  the  ordinates  represent  the 
intensities  of  emitted  and  absorbed  light : 

1  Compare  Kauffmann,  "Die  Beziehungen  zwischen  Fluoreszenz  u.  chemischer 
Konstitution,"  Stuttgart,  1906. 

2  Physikal.  Zeitschr.,  8,  81;  9,  481,  661  (1908). 

3  Ley  and  Fischer,  Ber.,  46,  327  (1913). 

4  Ley,  "Farbe  u.  Konst.,"  p.  132. 


486  THEORIES  OF  ORGANIC  CHEMISTRY 


0.5/;  0.6// 

Innumerable  experiments  seem  to  indicate  that  fluorescence  is  due 
to  rays  of  higher  refraction,  as  for  example  blue,  violet,  and  ultra- 
violet. The  less  refractive  rays  such  as  yellow  and  red  seem  to  be 
unable  to  produce  the  phenomenon.  This  so-called  Stokes  rule  is  not, 
however,  without  exceptions,  and  it  has  been  observed  that  colored 
substances  are  quite  frequently  excited  to  fluorescence  by  waves  of 
shorter  length  than  the  fluorescing  waves  which  they  emit. 

Fluorescence  may  be  most  conveniently  studied  in  the  case  of  sub- 
stances in  solution  since  comparable  conditions  can  most  easily  be 
maintained  under  these  circumstances.  Such  investigations  show  that 
fluorescence  depends  to  a  marked  degree  upon  the  concentration  of  the 
dissolved  substance  and  that  it  is  usually  strongest  and  clearest  at 
great  dilution.  Increase  in  concentration  is  accompanied  not  only  by 
a  decrease  in  fluorescence  but  also  frequently  by  a  change  in  color — due 
possibly  to  the  fact  that  the  solution  itself  reabsorbs  a  part  of  the  emitted 
light.  Temperature  represents  still  another  factor  governing  fluores- 
cence. 

The  mechanism  of  fluorescence  has  been  made  the  subject  of  a  very 
thorough  investigation  by  E.  Wiedemann,1  whose  views  are  very  fully 
discussed  in  "  Zur  Mechanik  des  Leuchtens."  Reasoning  on  the  basis 
of  the  kinetic  theory  of  gases  he  reaches  the  conclusion  that  fluores- 
cence is  due  to  the  movements  of  certain  atoms  or  groups  of  atoms  in 
the  molecule,  which  because  of  their  particular  functions  can  be  called 
fluorogens. 

With  the  exception  of  the  occasional  utterances  of  a  few  investi- 
gators,2 the  first  systematic  study  of  the  relation  of  fluorescence  to 
chemical  constitution  was  undertaken  by  Richard  Meyer.3  He  was 
able  to  show  that  certain  groups  which  he  called  fluorophores  and  which 

i  Annalen  Physik.,  37,  188  (1889);  56,  201  (1895);  also  E.  Wiedemann,  "Fest- 
schrift fur  Se.  Kgl.  Hoheit  des  Prinzregenten  Luitpold  von  Bayern,  dargebracht 
von  der  Universitat  Erlangen."  Philosophische  Fakultat,  2.  Section,  S.  35  (1911). 

2Liebermann,  Ber.,  13,  913  (1880). 

3  Zeitschr.  physikal.  Chemie,  24,  468  (1897). 


FLUORESCENCE  AND  CHEMICAL  CONSTITUTION 


487 


are  present  in  the  molecules  01  such  substances  as  the  fluoresceins, 
xanthens,  acridines,  phenazines,  phenoxazines,  etc.,  are  responsible  for 
fluorescence  in  much  the  same  way  that  chromophore  groups  are  respon- 
sible for  color.  An  enumeration  of  fluorophores  includes  the  pyrone 
ring, 

CO 


Pyrone 

the  azine-,  oxazine-,  thiazol-,  rings,  etc.  Fluorescence  is  not,  however, 
produced  by  the  mere  presence  of  such  a  group.  It  may  be  said  in 
general  that  such  phenomena  are  observed  only  in  cases  where  a  fluoro- 
phore  occupies  a  position  in  the  molecule  between  densely  grouped 
atomic  complexes  such  as  Cells,  etc.  Thus,  for  example,  pyrone  does 
not  itself  exhibit  the  property  of  fluorescence  while  a-a'-diphenylpyrone 


o-a'-Diphenylpyrone 

belongs  to  the  class  of  fluorescing  substances. 

The  effect  of  substitutions  in  the  benzene  ring  is  either  to  weaken 
or  to  destroy  completely  fluorescence,  depending  upon  the  nature  of 
the  substituting  atom  or  group,  as  is  shown  in  the  case  of 

Pht l  Phi  Pht 


Cl 


OH 


Cl 


0 


Fluorane, 
strongly  fluorescent 


Fluoresceine, 
weakly  fluorescent 


Dichlorfluorane, 
only  slightly  fluorescent 


but  depending  also  upon  the  position  of  such  groups.  Position  is,  in 
fact,  a  most  important  item,  since  only  in  the  case  of  substitutions  in 
certain  definite  positions  does  the  fluorescence  of  the  original  substance 

C6H4— CO 
1Pht  =  PhthaLyl— : 

/\ 


488 


THEORIES  OF  ORGANIC  CHEMISTRY 


persist.     Thus,  for  example,  only  the  first  of  the  following  two  isomeric 
dehydroxyfluoranes  shows  the  property  of  fluorescence: 


Pht 


HO 


Pht 


Fluoresceine 


Hydroquinonphthaleine 


Substitution  in  other  than  specific  and  perfectly  definite  positions 
serves  either  to  weaken  greatly  or  else  to  destroy  completely  this  prop- 
erty and  in  every  case  certain  positions  represent  the  maximum  of 
fluorescence  for  any  given  derivative.  This  is  further  illustrated  by 
the  following  set  of  three  isomers: 


OH 


OH 


OH 


CH3         CH3 
Pht 


H 


OH 


Fluorcsces 


In  the  case  of  dissolved  substances  the  nature  of  the  solvent  repre- 
sents another  important  factor  in  fluorescence.  Thus  one  and  the  same 
substance  may  fluoresce  in  certain  solvents  and  not  in  others.  loniza- 
tion  may  also  play  an  important  role  in  certain  instances,  but  in  others 
it  is  definitely  excluded. 

Interesting  experimental  evidence  in  support  of  Meyer's  theory  has 
been  presented  by  F.  Henrich,1  who  has  demonstrated  that  the  oxazol- 
ring 

HC 


Monatsh.  Chemie,  19,  492  (1898). 


FLUORESCENCE  AND  CHEMICAL  CONSTITUTION  489 

may  function  as  a  fluorophore  in  cases  where  the  conditions  of  the  theory 
are  fulfilled.  Thus  in  the  case  of  benzoxazole  and  its  derivatives: 

iN 

JCH 
0 

Benzoxazole  It-Methylbenzoxazole  /i-Phenylbenzoxazole 

I  II  III 

it  has  been  observed  that  the  first  exhibits  the  property  of  fluorescence 
and  that  the  third,  in  which  the  fluorophore  occupies  a  position  between 
two  phenyl  groups,  fluoresces  strongly.  The  second,  on  the  other  hand, 
shows  no  trace  of  fluorescence.  To  what  extent  the  difference  between 
II  and  III  depends  upon  the  unsaturated  character  of  the  substituting 
group  is  shown  by  the  fact  that  /*-hexahydrophenylbenzoxazole 

N 


resembles  the  corresponding  methyl  derivative  in  showing  no  trace  of 
fluorescence.1 

Later  developments  in  the  theory  of  fluorescence  have  taken  place 
along  the  lines  suggested  by  H.  Kauffmann  2  as  a  result  of  his  investi- 
gations in  the  field  of  luminescence.  His  conceptions  of  the  relation 
between  fluorescence  and  chemical  constitution  are  based  upon  certain 
fundamental  considerations  and  presuppose  the  presence  of  two  special 
groups  in  the  molecules  of  all  fluorescing  substances — namely  a  so-called 
luminophore  and  fluorophore. 

According  to  Kauffmann  the  luminophore  must  be  regarded  as  the 
actual  seat  of  fluorescence  in  that  the  fluorescing  rays  are  supposed  to 
originate  there.  The  presence  of  such  a  group  predisposes  the  molecule 
in  which  it  is  found  to  fluorescence,  but  it  need  not  cause  the  mani- 
festation of  this  phenomenon.  For  example,  the  benzene  ring  is  a  lumino- 
phore, but  fluorescence  in  the  case  of  aromatic  compounds  usually 
occurs  only  after  the  addition  of  a  fluorophore  to  the  molecule  and  then 
only  under  certain  conditions.  Thus  aniline  contains  a  luminophore 
but  does  not  fluoresce,  while  anthranilic  acid,  on  the  other  hand,  fluor- 
esces as  the  result  of  the  introduction  into  the  aniline  molecular  of  a 
fluorophore  carboxyl  group  in  the  ortho  position. 

'Ber.,  37,  3108(1904). 

2Ber.,  33,  1731  (1900);  37,  2941  (1904);  38,  789  (1905);  Annalen  der  Chemie, 
344,  30  (1906);  Ahrens  Sammlung,  11,  Vols.  1  and  2  (1906). 


490  THEORIES  OF  ORGANIC  CHEMISTRY 

So  long  as  the  phenomenon  was  studied  only  in  connection  with 
substances  possessing  spectra  in  the  visible  region  progress  was  slow 
but  the  discovery  of  fluorescence  in  the  region  of  the  ultra-violet,  which 
could  be  accurately  measured,  gave  fresh  impetus  to  the  development 
of  the  theory.1  This  discovery  was  due  to  the  investigations  of  J. 
Stark,2  who  observed  that  benzene  possessed  a  fluorescent  absorption 
spectrum  consisting  of  four  bands  in  the  region  of  the  ultra-violet.  He 
concluded  that,  since  the  majority  of  fluorescing  substances  are  benzene 
derivatives,  the  ring  itself  must  be  regarded  as  the  actual  carrier  of 
fluorescence  in  such  compounds,  or,  in  other  words,  that  it  is  a  lumino- 
phore  in  the  sense  in  which  this  term  is  used  by  Kauffmann.  The  effect 
of  substitution  can  be  followed  quantitatively  in  all  such  cases  and  it 
has  been  found  in  general  to  result  in  a  definite  shifting  of  the  fluorescent 
bands  away  from  the  extreme  ultra-violet  in  the  direction  of  the  red. 
Condensation  of  two  or  more  benzene  nuclei  produces  a  similar  result 
so  that  in  passing  from  benzene  — >  naphthalene  — >  anthracene,  for 
example,  it  has  been  found  that  fluorescence  shifts  progressively  from 
ultra-violet  in  the  direction  of  the  longer  wave  lengths.3 

In  studying  the  effect  of  different  substituting  atoms  or  groups  it 
has  been  found  that  the  amido  group  exercises  the  strongest  influence 
and  following  it  the  dimethylamido  and  hydroxyl  groups.  Auxochrome 
groups  in  general,  as  Kauffmann 4  maintains,  represent  an  important 
factor  in  determining  fluorescence.  Ring  formation  represents  another 
such  factor,  as  is  shown  by  the  fact  that  while  the  fluorescent  spectrum 
of  benzophenone 

CO 


is  in  the  ultra-violet,  the  spectra  of  xanthine, 

CO 


O 

and  its  derivatives  are  either  wholly  or  in  part  in  the  visible  5  region. 

1  Physikal.  Zeitschr.,  8,  81  (1908). 

2  Physikal.  Zeitschr.,  9,  481,  661  (1908). 

3  Physikal.  Zeitschr.,  8,  250  (1908). 

4  "Die  Valenzlehre,"  p.  494. 

5  Physikal.  Zeitschr.,  8,  252  (1908). 


FLUORESCENCE  AND  CHEMICAL  CONSTITUTION  491 

The  mechanism  of  the  changes  which  are  induced  by  substitution, 
and  which  in  general  tend  to  shift  fluorescence  from  ultra-violet  toward 
the  visible  may  be  accounted  for  in  terms  of  Wiedermann's  theory. 
Thus  it  may  be  assumed  that,  while  in  general  the  seat  of  fluorescence 
is  the  benzene  ring  as  Stark  and  R.  Meyer  suppose,  the  particular 
manifestation  observed  in  any  given  case  is  determined  by  the  specific 
rate  of  vibration  of  the  luminophore.  If  this  vibration  is  slowed  down 
as  a  result  of  substitution,  the  change  will  be  perceptible  in  fluorescence 
of  longer  wave  lengths.  There  is  no  reason,  moreover,  why  this  con- 
ception should  not  be  extended  to  include  the  possibility  of  fluorescence 
in  the  infra-red  as  well  as  in  the  visible  and  ultra-violet. 

The  influence  of  substitution,  salt  formation,  etc.,  upon  fluorescence 
has  recently  been  made  the  subject  of  special  investigations  by  Ley  1 
and  also  by  H.  Kauffmann.2  According  to  Ley  and  his  co-workers, 
Graefe  and  Englehardt,  the  fluorescent  spectra  of  organic  compounds 
absorbing  in  ultra-violet  is  very  sensitive  to  slight  changes  in  the  con- 
stitution of  the  molecule.  Exact  measurements  in  ultra-violet  show 
that  the  fluorescent  spectrum  of  benzene  changes  in  character  as  the 
result  of  any  and  every  replacement  of  its  hydrogen  by  substituting 
atoms  or  groups,  i.e.,  the  four  separate  absorption  bands  merge  together 
into  a  single  broad  band  and  simultaneously  absorption  shifts  in  the 
direction  of  the  red.  In  general  substituents  which  tend  to  shift  absorp- 
tion in  the  direction  of  the  longer  wave  lengths  are  called  bathoflore 
groups  while  substituents  having  the  opposite  effect  are  called  hypso- 
flores.  Since  intensity  of  absorption,  is  also  affected,  substituents  may 
also  be  classified  as  auxoflores  and  diminoflores. 

Material  differences  in  the  action  of  different  groups  may  be  readily 
accounted  for  on  the  basis  of  differences  in  their  chemical  character. 
Alkyl  groups,  for  example,  increase  the  intensity  of  fluorescent  bands 
while  halogens,  on  the  other  hand,  weaken  the  intensity  in  direct  pro- 
portion to  their  atomic  weights  although  they  have  a  negligible  effect 
upon  the  position  of  the  bands.  This  stands  in  marked  contrast  to 
the  action  of  unsaturated  groups  such  as  OH,  NH2,  CN,  CH=CH,  etc., 
where  the  effect  is  both  auxofloric  and  bathofloric,  as  also  of  the  COOH 
group  where  the  effect  is  diminofloric  and  bathofloric.  The  presence 
of  several  substituents  in  the  same  molecule  frequently  has  an  additive 
effect  although,  of  course,  anomalies  may  result  from  the  interaction  of 
these  groups  upon  each  other. 

An  interesting  case  of  anomaly  has  been  observed  in  connection  with 

iZeitschr.  physikal.  Chemie,  74,  1  (1910);  Ber.,  41,2988  (1908);  Zeitschr.  wiss. 
Phot.,  8,  294;  and  "Farbe  und  Konstitution, "  p.  134. 

2  "Die  Auzochrome,"  1907,  and  also  "Die  Valenzlehre,"  p.  494. 


492  THEORIES  OF  ORGANIC  CHEMISTRY 

certain  nitrocompounds.  Thus  while  it  has  been  pointed  Out  by  R. 
Meyer  1  and  H.  Kauffmann 2  that  nitro  groups  possess  the  power  of 
destroying  completely  the  fluorescence  of  a  substance,  certain  notable 
exceptions  to  this  general  rule  have  been  discovered.  For  example, 
while  nitrobenzene  and  the  nitrotoluene  show  no  trace  of  ultra-violet 
fluorescence,  and  while  the  same  is  true  of  many  radicals,  such  as  picryl, 
C6H2(NO2)s,  which  contain  nitro  groups,  nevertheless,  picryl  guanidine  3 
and  similar  substances  such  as  m-nitrodimethyl  aniline,4  etc.,  show 
strong  fluorescence  even  in  the  visible  spectrum.  This  striking  dif- 
ference between  these  two  classes  of  nitro-compounds  may  be  explained 
on  the  basis  of  anomaly  by  supposing  that  the  specific  action  of  nitro 
groups  upon  fluorescence  is  nullified  by  the  interaction  of  nitro  and 
amino  groups  accompanied  by  a  saturation  of  the  residual  valencies 
within  the  molecule.  As  a  result  of  this  discovery  it  may  be  hoped 
that  the  affinity  values  of  unsaturated  groups  will  in  the  future  be 
advantageously  studied  by  means  of  a  systematic  investigation  of 
fluorescent  spectra.5 

Naphthalene  has,  as  Ley  and  Graefe6  have  discovered,  a  narrow 
band,  almost  line-like  fluorescent  spectrum,  which  changes  to  a  single 
broad  band  and  is  at  the  same  time  shifted  in  the  direction  of  the  red 
as  a  result  of  the  substitution  of  auxochromes  such  as  NH2  and  OH, — 
thus  showing  a  strong  resemblance  to  benzene  in  its  behavior.  Satu- 
rated substituents  such  as  alkyl  groups  and  halides  produce  a  single 
narrow  banded  spectrum  with  only  slight  displacement  in  the  direction 
of  the  red.  In  general  derivatives  having  substitutents  in  the  ^-positions 
fluoresce  much  more  strongly  than  the  corresponding  a-substitution 
products. 

Many  of  the  fluorescing  aromatic  compounds  form  salts  as,  for 
example,  XNH2  -»  XNH2HC1,  XCOOH  -»  XCOONa,  XOH  ->  XONa, 
etc.,  where  X  represents  CeHs,  CioHy,  etc.;  and  it  has  been  observed 
in  such  cases  that  salt  formation  frequently  has  a  marked  and  character- 
istic effect  upon  fluorescence.  Such  changes  in  fluorescence  usually 
run  parallel  to  corresponding  changes  in  absorption  but  possess  an 
advantage  in  that  they  are  apt  to  show  the  character  of  the  resulting 
intramolecular  rearrangements  in  a  much  more  striking  manner.  For 
example,  neutralization  with  hydrochloric  acid  produces  a  strong 

^eitschr..  physikal.  Chemie,  24,  481  (1897). 

2  "  Beziehungen  zwischen  Fluoreszenz  und  chemischer  Konstitution,"  1906, 
p.  80. 

3Ber.  41,  1637  (1908). 

4  Ber.,  40,  2341  (1907);  41,  4396  (1908). 

5  "Farbe  und  Konstitution,"  pp.  136-157. 

6  Ibid. 


FLUORESCENCE  AND  CHEMICAL  CONSTITUTION  493 

diminution  in  fluorescence  in  the  case  of  aniline  while  the  presence  of 
an  excess  of  acid  completely  destroys  this  property.  The  same  is  true 
of  organic  acids  so  that  benzoic,  a-and  /3-naphthoic  acids,  etc.,  gradually 
lose  their  fluorescence  on  the  addition  of  alkali.  In  every  instance 
diminution  in  fluorescence  is  accompanied  by  a  shifting  of  the  absorp- 
tion in  the  direction  of  the  ultra-violet,  thus  indicating  in  general  that 
salt  formation  involves  fundamental  alterations  in  structure. 

The  intense  fluorescence  of  phenol  and  naphthol,  while  considerably 
lessened  by  neutralization,  never  completely  disappears  even  in  the 
presence  of  an  excess  of  alkali,  apparently  showing  that  salt  formation 
in  these  cases  is  different  in  type  from  that  involved  in  the  instances 
previously  cited. 


CHAPTER  XIX 
MOLECULAR  REARRANGEMENTS 

MOLECULAR  rearrangements  have  been  considered  previously  in  con- 
nection with  a  study  of  the  phenomena  of  tautomerism  and  desmotrop- 
ism.  Such  changes,  however,  did  not  involve  the  skeleton  of  carbon 
and  nitrogen  atoms  constituting  the  molecule,  and  dealt  only  with  the 
nature  of  the  union  between  the  atoms,  which  shifted  backward  and 
forward  between  certain  positions  in  the  molecule.  Rearrangements 
must  be  considered  now  where  variations  in  the  molecular  structure 
are  accompanied  by  dissociation  of  the  molecule  and  wandering  not 
only  of  an  individual  atom  but  also  of  whole  groups  of  atoms  or  radicals. 
Transformations  of  this  type  are  very  common  in  organic  chemistry. 
In  fact  they  were  observed  in  the  very  beginning  of  the  scientific  develop- 
ment of  this  subject  for  the  classic  synthesis  of  urea,  which  was  dis- 
covered by  Wohler,  depends  upon  a  molecular  rearrangement  of  ammon- 
nium  cyanate 

/NH2 
CONNH4     -»    C==0 

\NH2 

Molecular  changes  may  be  similar  to  the  transformation  of  ammo- 
nium cyanate  into  urea,  or  ammonium  thiocyanate  into  thiourea — 
i.e.,  where  one  substance  merely  isomerizes  or  passes  over  into  another 
without  loss  of  atoms — or  they  may  be  accompanied  by  the  elimination 
of  such  groups  of  atoms  as  constitute  the  halogen  acids,  H2O,  N2,  etc. 
In  the  latter  instances  the  existence  of  intermediate  substances,  iso- 
meric  with  the  original  substance  or  with  the  product  which  is  formed, 
may  be  assumed.  Such  a  rearrangement  is  illustrated  by  the  trans- 
formation of  pinacone  into  pinacoline: 

CH3v  /CH3         CH3x  /CH3  CH3> 

CH; 


/3  3x  x3  3\ 

— C<          ->  >C— C<  ->    CH-AC— CO— CH3 

I    X?H3         CH3/    \/    \CH3  CH3/ 


OH  OH 


Molecular  rearrangements,  even  at  the  present  stage  of  development 
of  chemistry,  offer  difficulties  of  interpretation  from  the  point  of  view 


494 


MOLECULAR  REARRANGEMENTS  495 

of  the  theory  of  organic  structure;  for  frequently  changes  must  be 
represented  as  taking  place  abruptly  and  involving  a  dissociation  of 
the  molecule,  and  not  in  gradual  stages  where  the  intermediate  steps 
may  be  followed  easily.  It  is,  therefore,  difficult  and  even  impossible 
in  many  cases,  to  express  the  nature  of  the  molecular  transformation 
in  terms  of  our  accepted  structural  theory. 

It  is  to  be  noted  that  the  phenomenon  of  rearrangement  is  always 
associated  with  quite  definite  groups  of  atoms  which  may  represent 
the  whole  or  a  part  of  the  organic  molecule.  In  those  cases  where  two 
isomeric  forms  are  capable  of  existing  simultaneously  and  independently 
under  given  conditions  of  temperature  and  pressure,  the  cause  of  the 
change  from  a  labile  to  a  stable  modification  may  be  accounted  for  by 
supposing  that  one  isomer  possesses  a  higher  energy  content  than  the 
other,  and  so,  under  favoring  circumstances,  physical  or  chemical  in 
nature,  that  transformation  generally  takes  place  which  involves  a 
loss  of  energy.  Such  changes  may  be  brought  about  with  very  great 
ease,  or  may  be  induced  only  by  the  action  of  energetic  chemical  reagents 
catalysts.  At  present  there  is  no  theory  which  covers  every  form  of 
isomeric  change. 

The  most  important  classes  of  molecular  rearrangements  may  now 
be  considered.     It  is  extremely  difficult  to  devise  a  system  of  classifi- 
cation for  the  various  types  of  rearrangement  that  have  been  discovered. 
There  are,  however,   certain  generalities  of  fundamental  importance, 
and  if  these  are  taken  into  consideration  a  partial  classification  can  be 
made  which  enables  one  to  present  this  work  in  a  logical  manner. 
Rearrangements  of  the  Cyanic  and  Thiocyanic  Acid  Series: 
These  transformations  are  brought  about  generally  and  preferably 
by  the  action  of  heat. 

/NH2 

NH4NCO     ->     CO 


Ammonium  cyanate  Urea 


HNCO-NH2-NH2     r»    CO 

^ 

Hydrazine  cyanate  Semicarbazide 

/NH2 
NH4SCN    -*    CS 


Ammonium  thiocyanate  Thiourea 


496  THEORIES  OF  ORGANIC  CHEMISTRY 

^N  x,N-CH2OH  :  CH2 

cr  -»  cr 

\SnTT.PTT  •  PTT.  ^S 


^SCH2CH  :  CH2 

Allyl  thiocyanate  Allyl  isothiocyanate 


L3 
Methyl  thiocyanate  Methyl  isothiocyanate 

/C6H5  2 
N  .xr.r»/YNT/ 


•CON< 

XC6H5 

Diphenylcarbamyl-  Diphenylcarbamyl- 

thiocyanate  isothiocyanate 

N  ^N- (Pyrimidine)  3 

>S-  (Pyrimidine) 

Pyrimidine  thiocyanate  Pyrimidine  isothiocyanate 

Cyanic  esters  corresponding  to  the  thiocyanates  or  rhodanides  have 
not  been  isolated.  Many  cases  are  known  where  the  isothiocyanate 
form  is  the  only  modification  of  the  rhodanide  that  can  be  isolated. 
Also  thiocyanates  are  known  which  cannot  be  rearranged  into  their 
corresponding  isothiocyanates.  These  rearrangements  are  not  rever- 
sible, so  far  as  can  be  concluded  from  experimental  evidence  presented 
up  to  the  present  time. 

Isocyanides  undergo  isomerization  to  give  the  cyanides 


=    -»    C2H5C=N  4 

Phenylisocyanide  Phenylcyanide 

or  acid  nitriles.     To  these  may  be  added  the  rearrangements  of  imido 
esters,  which  will  be  referred  to  latter  in  this  text. 


N-C6H5 


X)CH3 

Methyl  iso-formanilide  Methyl  formanilide 

1  Hoffmann,  Ber.,  13,  1350  (1880);  see  also  Jour,  prakt.  Chemie,  37,  506  (1880). 

2  Johnson  and  Levy,  Am.  Chem.  Jour.,  38,  456  (1907). 

3  Johnson  and  Storey,  Am.  Chem.  Jour.,  40,  131  (1908). 

4  Nef,  Annalen  der  Chemie,  280,  296  (1894). 

6  Wheeler  and  Johnson,  Am.  Chem.  Jour.,  21,  185  (1899). 


MOLECULAR  REARRANGEMENTS  497 

Rearrangements  involving  the  Transference  of  Radicals  from 
Carbon  to  Carbon: 

— CH-CH—    ->    =CH-CHO         CH3.CH-CH2    -»    CH3COCH3 

Y  Y 

Ethylenoxide          — >  aldehyde  Ethylenoxide  — »  ketone 

Changes  of  the  above  type  are  brought  about  by  heat  and  by  the 
action  of  catalysts.  The  so-called  pinacoline  rearrangement  also 
belongs  to  this  class  and  may  be  represented  as  follows: 

CH3\  /CH3  CH3\  /CH3  CH3\ 

>C C<  ->  >C— C<  ->  CHAC-COCH31 

CH-/  I          |  XCH3  CH3/  \S  \CH3  CH3/ 

OH     OH  O 

Pinacone  (Intermediate  ethylenoxide)  Pinacoline 

Conversions  of  this  kind  are  very  common,  and  certain  definite  rules 
have  been  formulated  in  regard  to  the  wandering  of  different  groups  of 
atoms.  Some  of  these  may  be  mentioned  briefly.  The  pinacone, 

/CeH4  •  CH3  (p-) 
-C/  - 


rearranges  to  give  exclusively: 

(p-)CH3.C6H4\ 


es 
(p-)CH3.C6H4/ 


From  which  it  is  obvious  that  the  tolyl  group  shifts  its  position  more 
readily  than  the  phenyl.  With  halogen  substitution  products  the 
course  of  the  raction  is  different  and  a  mixture  of  ketones  is  obtained. 
For  example: 

(p-)ClC6H4 


C6H5 

gives 

(p-)ClCGH4\ 

40  per  cent     ->  CeHs-^C  -  COC6H5 

(p-)C!C6H4/ 

(P-)C1C6H4X 

60  per  cent     ->  C6H5^C  •  COC6H4Cl(p-) 

C6H5/ 

1  Jour.  russ.  physikal.  Chem.  Ges  ,  34,  537;  Biltz  and  Seydel,  Ber.,  46,  138 
(1913);  Meerwein,  Annalen  dcr  Chemie,  396,206(1913);  405,129(1914);  417,255 
(1918);  419,121  (1919), 


498  THEORIES  OF  ORGANIC  CHEMISTRY 

From  this  result  the  conclusion  may  be  drawn  that  the  phenyl  group 
shifts  its  position  more  readily  than  the  p-chlorphenyl.1 

If  all  four  hydrogen  atoms  of  glycol  have  not  been  substituted  by 
carbon  residues,  rearrangement  results  in  the  formation  of  aldehydes. 
The  following  example  illustrates  how  the  wandering  of  the  atoms  takes 
place  in  this  type  of  compounds: 


(p-)ClC6H4CH—  CHC6H4Cl(p-)  -64\ 

-*  >CH-CH02 

OH     OH  (p-)ClC6H4/ 

To  this  series  of  rearrangements  belongs  also  the  unique  cyclic  pinacone 
rearrangement  described  by  Biltz  :  3 


OH 

CO— NHV 
C6H5-C— NH  C6H5-C— NH  C6H5\   |  >CO 

>CO    ->         O<|        >CO    -»  >C NH/ 

C6H5  •  C— NH  C6H5  •  C— NH  C6H5/ 


)H 

Diphenylglyoxalonglykol  Diphenylhydantoin 

An  interesting  rearrangement  closely  related  to  this  series  is  that 
described  by  Wolff4  in  which  a  hydroxyl  group  and  a  methyl  radical 
are  assumed  to  exchange  positions  in  the  molecule  when  the  anhydride 
of  ethyl  diazoacetoacetate  is  subject  to  hydrolysis: 

O 

CH3C     IN 

II     jll  +  H20    -> 
C2H5OOC-C—  |N 

/COOH 
-»    CH3CO.CH(OH)COOC2H5     ->    CH3CH< 

\COOC2H5 

Ethyl  hydroxyacetoacetate  Monoethylester  of  methyl 

malonic  acid 

A  similar  transformation  is  brought  about  by  the  action  of  water  on 
the  anhydride  of  diazobenzoylacetone  : 


,  Am.  Chem.  Jour.,  33,  189  (1905);  Hoogewerff  and  vanDorp,  Rec. 
trav.  chim.  des  Pays-Bas.  9,  225  (1890);  Montagne,  ibid.,  24,  105  (1906);  26,  256 
(1907). 

2  Rec.  trav.  chim.  des  Pays-Bas.,  21,  30  (1902). 

3  Ber.  41,  1379  (1908).      See  also  Montagne,  Rec.  trav.  chim.  des  Pays-Bas.,  21, 
6  (1902);  Ber.,  61,  1479  (1918). 

4  Annalen  der  Chemie,  325,  144  (1902). 


MOLECULAR  REARRANGEMENTS  499 

The  rearrangement  of  halogen  from  carbon  to  carbon  in  the  aliphatic 
series  is  illustrated  in  the  following  case  observed  by  Hantzsch  and 
Conrad.1  This  change  is  brought  about  under  the  influence  of  hydro- 
bromic  acid. 

CH3CO-CHBrCOOC2H5    -»    BrCH2COCH2COOC2H5 

Ethyl  a-bromacetoacetate.  Ethyl  -y-bromacetoacetate. 

A  rearrangement  involving  the  formation  of  a  ring  by  linking  together 
two  carbon  atoms  is  shown  below : 2 


CHaO 


C(CH3)2 
J/ 

Dimethylketazine  3-Methyl-5-dimethylpyrazoline 

Borsche  and  Fels  record  the  following  unique  rearrangement  in 
the  furane  series.3  Here  we  are  dealing  with  the  formation  of  an  inter- 
mediate acyclic  compound,  which  condenses  again  with  formation  of 
a  new  cycle : 

CH— CH  •  COCH3  CH— C  -  COOH 

II          I  HC1  II         II 

C6H5C        CO  >     C6H5C       C-CH3 

No/  \>/ 

Methyl-phenyl-furanecarboxylic  acid 

Rearrangements  involving  the  migration  of  alkyl  radicals  or  halogen 
from  carbon  to  carbon,  and  brought  about  by  the  agency  of  aluminium 
chloride,  belong  to  this  class. 

CH3CH2CH2Br    -4    CH3CHBr.CH3  4 

Sulphonic  acid  groups  also  change  their  positions  in  the  benzene 
ring  under  certain  conditions.5  Such  transformations  have  been  very 
commonly  observed  in  the  aromatic  series. 

SO3H 

/\    /\ 

S03H 


'Ber.,  27,  355,  3168  (1894);  Ber.,  29,  1042  (1896);  Ber.,  36,  2251  (1903). 

2  Curtius  and  Zinkeisen,  Jour,  prakt.  Chemie,  68,  310  (1898). 

3  Ber.,  39,  1809(1906). 

4  Gustavson,  Ber.,  16,  R.  957  (1883);  20,  707  (1887). 
5Schramm,  Ber.,  21,  782  (1888). 


500  THEORIES  OF  ORGANIC  CHEMISTRY 

Several  cases  have  been  recorded  in  the  chemical  literature  where 
a  methyl  group  migrates  from  carbon  to  carbon  during  the  process  of 
reduction: 


—CO  — CHOH  — C=  — CCH3 

;CH3 
CH3 


Benzilic  acid  rearrangements,  which  take  place  when  ortho  dike- 
tones  are  fused  with  potassium  hydroxide,  may  be  regarded  as  belong- 
ing to  this  class,  if  the  course  of  the  transformation  is  formulated  as 
follows  : 

OK     OK 

I          I 
C6H5CO-COC6H5+2KOH    -»    C6H5C  --  C-C6H5    -* 

I          I 
OH     OH 

OK     OK  C6H5\       /OK  2 

I          I  ->  >C< 

CGH5  -  C  --  C  •  C6H5  C6H5X     XCOOK 

\0/ 

In  this  type  of  change,  as  in  the  pinacone  rearrangements,  a  chlorine 
atom  maintains  its  p-position  in  the  benzene  nucleus  in  relation  to  the 
corresponding  carbon  atom  during  the  shifting  of  the  groups.  This  is 
shown  by  the  following  example: 

V      /OK  3 

>C< 
4/     XCOOK 

Nitrogen  alkyl  derivatives  of  isoaldoximes  undergo  rearrangements 
which  are  reversible: 


NO2-C6H4CH  --  N-CH2C6H5     <=±    NOaCf^CHsN  -  CH-C6H5  4 

" 


iBrunner,  Monatsh.  Chemie,  17,  276  (1896);  21,  156  (1900);  Baeyer,  Ber., 
32,  2429  (1899);  Wolff,  Annalen  der  Chemie,  322,  351  (1902);  Knorr,  Ber.,  36,  1272 
(1903). 

2  Tiffeneau,  Revue  gen.  Sci.  pur.  et  appli.,  1907,  p.  585. 

3  Rec.  trav.  chim.  des  pays-Bas,  21,  19  (1902);  see  also  Ber.,  38,  3738  (1905). 

4  Neubauer,  Annalen  der  Chemie,  298,  187  (1897). 


MOLECULAR  REARRANGEMENTS 


501 


The  rearrangement  of  iodine  in  benzene  compounds  is  often  met 
with  as  is  illustrated  in  the  behavior  of  p-iodanisol  towards  nitric  acid: 


OCH3 


HNOj 


OCH3 


HN03 


N02 


Quinole  Rearrangements:  An  exceptionally  reactive  group  of  sub- 
stances was  discovered  as  the  result  of  the  investigations  of  Zincke, 
E.  Bamberger  and  Auwers.2  They  received  the  name  of  "  quinoles  " 
because  of  their  semi-quinoidal  nature.  They  are  obtained  from  para 
substituted  phenylhydroxylamines  and  are  referred  to  in  the  class  of 
rearrangements  involving  migration  of  radicals  from  nitrogen  to  car- 
bon. The  following  types  may  be  taken  as  examples: 


HOV    /Alkyl 


Alky! 


CH2C1 


III 


In  these  compounds  either  of  the  two  groups  bound  to  the  carbon 
atom  indicated  by  *,  may  shift  its  position.  To  illustrate,  the  alkyl 
group  in  I  or  II  may  wander  into  the  nucleus  and  there  assume  another 
position.  This  happens  when  the  substance  is  treated  with  aqueous 
sulphuric  acid  or  sodium  hydroxide: 


Alkyl 


tAlkyl 


Alkyl 


NH 


1  Reverdin,  Ber.,  29,  2595  (1896);  30,  2999  (1897). 

2  Zincke,  Ber.,  28,  3121   (1895);  34,  253  (1901);  Annalen  der  Chemie,  320,  145 
(1901);   322,  174  (1902);   325,  19  (1902);   329,  1  (1903);   330,  61  (1904);   Bamber- 
ger, Ber.,  33,  3600  (1900);   36,  1424,  3886  (1902);  36,  1625,   2028   (1903)   Auwers, 
Ber.,  35,  443,  455,  465,  4207  (1902);  36,  1861,  3902  (1903). 


502 


THEORIES  OF  ORGANIC  CHEMISTRY 


If,  on  the  other  hand,  the  compound  I,  for  example,  is  treated  with 
alcoholic  sulphuric  acid,  the  product  of  rearrangement  consists  of  the 
alkyl  derivative  of  a  substance  which  now  contains  an  hydroxyl  group  in 
the  ortho  position  of  the  benzene  nucleus.  In  other  words,  either  the 
hydroxyl  or  alkyl  group  can  be  rearranged  at  will. 

HOX    /Alkyl 


Rearrangements    Involving    the    Transference    of   Radicals   from 
Oxygen  to  Carbon:     Several  rearrangements  of   this   type  have  been 
observed  in  the  aliphatic  series. 


x, 

c6H5c<; 

XOC2H5 

a-Ethoxystyrol 


CH- 


V) 

Phenylpropylketone 


OH 


0-COCH3 

CH2 
\)-COC6H5 


COOC2H5  2 


C6H5C 


Ethyl  diacetoacetate 

CH2-COC6H5  3 

Dibenzoylmethane 


A  rearrangement  of  similar  type  in  the  cyclic  series  is  that  described 
by  Biilow:4 


C=CH-COCH3 


H-COCH3 


Acetonyl-phthalide 


Phthalylacetone 


1  Ber.,  29,  2931  (1896). 

2Claisen  and  Haase,  Ber.,  33,  1242  (1900);   Ber.,  36,  3674,  3778  (1903). 

3  Claisen,  Ber.,  36,  3674  (1903). 

4  Ber.,.  37,  4380  (1904). 


MOLECULAR  REARRANGEMENTS 


503 


To  this  class  belong  a  number  of  the  rearrangements  which  phenol 
ethers  and  esters  undergo  when  acted  upon  by  heat  or  by  catalytic 
agents,  and  which  give  rise  to  phenols  with  substitution  in  the  benzene 
nucleus: 

OS03H       OH 

/\S03H  i 


O-COCHs 


OH 


CHs-l^y  CH3-( 

COCH3 

0-COCH3 

) 
Jo-COCHs 


aOCH32 


0-COCH3 


CH3CO 


COCH3 


COCH3 


-\COCH3 


Rearrangements  of  phenol  ethers  into  alkylated  phenols  belong  to  this 
class,  and  are  brought  about  by  application  of  heat  or  by  the  action 
of  acids: 


/- 
R-CH< 

X)- 


/io 
R-CH< 

X:IOHGOH 


1  Baumann,  Ber.,  9,  55,  1715  (1876). 

2Eijkman,  Chem.  Centralbl.,  1904,  I,  1597;  1905,  I,  814;  also  Heller,  Ber.,  42, 
2736  (1909);  46,  418  (1912). 

3R.  Benedikt,  Annalen  der  Chemie,  199,  127  (1879). 
4  Claisen,  Annalen  der  Chemie,  237,  261  (1887). 


504  THEORIES  OF  ORGANIC  CHEMISTRY 

0-CH2CH  :  CH2  OH 


H2CH  :  CH2 


Allylphenylether.  Allylphenol. 


,CONH2  /OH 

C6H50-CH<  -»    C6H4<         /CONH2  2 

XC6H5  \CH< 

XC6H5 

Kolbe's  synthesis  of  salicylic  acid  from  phenol  and  carbon  dioxide 
is  based  on  a  metameric  change  involving  the  transfer  of  a  carboxyl 
group  from  oxygen  to  carbon: 

.0-COONa  /\OH 

Hcat) 

Sodium  phenyl  carbonate.  Sodium  salicylate. 

Rearrangements  Involving  the  Transference  of  Radicals  from 
Nitrogen  to  Carbon:  Transformations  of  this  type  are  brought  by 
heat,  by  catalytic  agents,  and  by  the  action  of  mineral  acids.  They 
are  observed  very  frequently  in  the  case  of  aromatic  compounds  and 
heterocyclic  combinations. 

To  this  class  belong  the  rearrangements  of  alkyl  anilines,  discovered 
by  A.  W.  Hofmann  and  Martius.3  It  was  observed  that  the  halogen 
salts  of  secondary  and  tertiary  aromatic  amines,  as  well  as  quaternary 
ammonium  salts,  when  heated  at  200°  to  350°  in  the  presence  of  alcohol 
suffer  an  intramolecular  change,  whereby  the  substituting  groups  wander 
into  the  benzene  nucleus,  assuming  o-  and  p-,  but  never  m-,  positions. 

C6H5NHC2H5-HC1    -»    C2H5C6H4NH2.HC1 

In  this  type  of  rearrangement  there  is  very  good  reason  for  supposing 
that  the  original  substance  breaks  down  at  the  high  temperature  to 
form  aniline  and  the  alkyl  halide,  which  then  recombine  to  form  an 
isomeric  product  with  the  substituting  alkyl  group  in  the  benzene 

i  Claisen  and  Eisleb,  Annalen  der  Chemie,  401,  21  (1913);  Ber.  45,  3157. 

2Bucherer  and  Grolee,  Ber.,  39,  1012  (1906). 

3  Hofmann,  Ber.,  4,  742  (1871);  6,  704,  720  (1872);  7,  526  (1874);  18,  1821 
(1885);  Noltingand  Baumann,  Ber.,  18,  1149  (1885);  Nolting  and  Forel,  Ber.,  18, 
2680  (1885);  Limpach,  Ber.,  21,  640,  643  (1888);  Hodgkinson  and  Limpach, 
Jour.  Chem  Soc.,  61,  420  (1892);  Benz,  Ber.,  15,  1646  (1882). 


MOLECULAR  REARRANGEMENTS  505 

nucleus,  but  in  the  following  instances  genuine  intramolecular  rearrange- 
ments involving  dissociation  of  the  molecule  seem  fairly  certain: 


NH-SO3H  NH2  NH2 


Phenyl  sulphaminic  acid 

Another  arrangement  of  this  class  is  that  described  by  Senier  and 
Shepheard,2  and  also  that  of  the 


CH2 
N-C6H5 


aromatic  amine-oxides,  which  have  been  shown  to  undergo  rearrangement 
to  hydroxyanilines  as  is  represented  below: 


OH  3 

(CH3)2N 


Phenyl  sulphaminic  acid  rearranges  to  give  first  the  o-,  and  then 
p-sulphonic  acid  derivative  of  aniline.  The  former  reaction  takes  place 
at  low  temperatures  in  the  presence  of  dilute  acids;  and  the  latter  on 
the  addition  of  concentrated  acids.  If  phenyl  sulphaminic  acid  is 
treated  directly  with  concentrated  sulphuric  acid,  it  passes  into  the 
p-sulphonic  acid  (sulphanilic  acid)  and  no  o-sulphonic  acid  is  formed. 
The  complete  transformation  involves,  therefore,  a  migration  of  the 
sulphonic  acid  radical  from  nitrogen  to  carbon  and  finally  a  second 
change  from  carbon  to  carbon. 

Aromatic  nitroamines  rearrange  to  form  nitro-anilines.4 

1  Bamberger  and  Kunz,  Ber.,  30,  654,  1261,  2277  (1897). 

2  Jour.  Chem.  Soc.,  95,  494  (1909). 

3  Bamberger  and  Tschirner,  Ber.,  32,  1889  (1899). 

4  Bamberger,  Storch  and  Landsteiner,  Ber.,  26,  471,  485  (1893);  27,  584  (1894); 
28,  401  (1895);  30,  1252  (1897);  Stoermer,  Ber.,  31,  2528  (1898). 


506  THEORIES  OF  ORGANIC  CHEMISTRY 

NH2 


NO2 


Here  the  nitro  group  wanders  independently  to  two  different  positions 
in  the  benzene  ring,  since  o-nitroaniline  cannot  be  rearranged  into 
the  p-modification.  In  these  rearrangements  the  nitro  group  in  migrat- 
ing will  even  force  another  group  from  its  position  in  the  benzene  ring: 


NH-NO2 


aj-nitro-a-methyl-0-phenylurea  rearranges  by  the  action  of  cold  con- 
centrated sulphuric  acid  to  give  a  mixture  of  ct-methyl-fi-ortho-  and 
para-nitrophenylureas  : 


/  /NHCH3  2 

C0<      \CH3       -»      C0< 

\NHC6H5  XNH  •  C6H4NO2  (o  and  p) 

Aromatic  nitrosamines  rearrange  to  give  p-nitroso  derivatives  of  aniline,3 
(Otto  Fischer-Hepp  rearrangement)  : 

C6H5N-NO 


1  Orton  and  Smith,  Jour.  Chem.  Soc.,  87,  389  (1905). 

2  Scholl  and  Nyberg,  Ber.,  39,  2491  (1906). 

3  O.  Fischer  and  Hepp,  Ber.,  19,  2991  (1886);  20,  1247  (1887);  O.  Fischer,  Ber., 
46,  1098  (1912). 


MOLECULAR  REARRANGEMENTS 


507 


Brombenzene  diazonium  chlorides  undergo  molecular  transforma- 
tions with  change  of  position  of  the  halogen,  and  the  diacylanilides, 
on  heating,  pass  over  into  acylamidoketones.1 


COC6H5 


COCHs 
N— COCHs 


COCH3 
N-H 


COC6H5 
N-COC6H5 


NH 


COC6H5 


NH-COC6H5 


OC6H5 


Although  in  the  case  of  these  anilides,  only  one  substituting  group 
enters  the  benzene  ring,  there  are  instances  where  two  or  more  groups 
may  enter  the  benzene  nucleus  as  the  result  of  rearrangements.  Thus 
N-chlor-acetanilides  rearrange  to  form  p-and  o-chloracetanilides  :2 


COCH3 
N-C1 


COCHs 
NH 


The  p-chloracetanilide  formed  may  now  be  changed  into  N-chlor-p- 
chloracetanilide,  and  this  in  turn  may  rearrange  so  that  the  chlorine 
atom  shifts  from  nitrogen  and  replaces  a  second  hydrogen  atom  in  the 
ring.  The  process  may  be  repeated  until  the  hydrogen  atoms  occupy- 
ing the  p-  and  the  two  o-positions  in  the  benzene  ring,  as  well  as  the 
hydrogen  in  union  with  nitrogen,  have  been  replaced  by  chlorine. 
Chlorine  does  not  shift  to  the  ra-position. 

1  Chattaway,  Jour.  Chem.  Soc.,  85,  388  (1904). 

2Chattaway  and  Orton,  Ber.,  32,  3573,  3635  (1899);    Blanksma,    Rec.    trav. 
chim.  des  Pay-Bas,  21,  329  (1902);  Hantzsch  and  Smythe,  Ber.,  33,  505  (1900). 


508 


THEORIES  OF  ORGANIC  CHEMISTRY 

COCH3  COCH3  COCH3 

N-C1  NH  N-C1 


— Cl 


ci 


31  Cl 

Phenylhydroxylamines  rearrange  to  form  p-amidophenols : 1 
NH-OH  NH2 


When  the  hydrogen  in  the  para  position  is  substituted  in  these  hydroxyl- 
amine  combinations,  rearrangemekt  takes  place  with  formation  of 
quinoles,  since  the  hydroxyl  radical  does  not  migrate  to  an  ortho  or  meta 
position  in  such  cases : 


CH3 


NHOH 


OH 


H2O 


NH3 


CH3 


Diazoamido  compounds  rearrange  to  form  amidoazo  compounds: 
NH-N=NC6H5  NH2 


N=NC6H5 

iGattermann,  Ber.,  26,  1845  (1893);  Wohl,  Ber.,  27,  1432  (1894);  Lagutt,  Ber, 
31,  1501  (1898), 


MOLECULAR  REARRANGEMENTS  509 

In  the  case  of  this  reaction  also,  it  has  not  yet  been  possible  to  establish 
the  fact  that  a  genuine  intramolecular  rearrangement  takes  place,  and 
to  exclude  the  possibility  of  a  decomposition  of  the  compound  by  heat, 
accompanied  by  secondary  processes  involving  recombinations  of  the 
groups  thus  formed.  The  following  rearrangements  of  diazo  compounds 
are  also  known  to  take  place : l 

Br3C6H2N:NCl    ->    Br2ClC6H2N  :  N-Br 

When  heated  with  sulphuric  acid  azoxybenzenes  rearrange  according 
to  the  following  scheme: 

C6H5-NV 

|  >0    ->    C6H5N  :  NC6H4OH  2 
C6H5-N/ 

Azoxybenzene  Hydroxyazobenzene 

Pyrrol  rearrangements  should  be  mentioned  at  this  point. 


CH— CH  CH— CH 

II         II          ->       I         II 
CH    CH  CH— C- 


V 

N-COCH3  NH 


COCH3 


C4H4NCH3    ->    CH3-C4H3-NH4 

As  is  well  known  A.  Pictet  was  able  to  carry  through  successfully  a 
synthesis  of  nicotine  on  the  basis  of  such  a  pyrrol  rearrangement. 

In  addition  to  these  rearrangements  which  have  been  recorded  the 
semidine  and  also  the  benzidine  rearrangements  should  be  mentioned. 
The  benzidine  change  is  one  of  great  commercial  importance.  There 
are  innumerable  instances  of  changes  of  both  types  to  be  found  in  the 
chemical  literature,5  but  one  illustration  in  each  case  will  suffice  here, 
thus: 

1  Hantzsch,  Schleissing  and  Jager,  Ber.,  30,  2337  (1897);  Hantzsch  and  Smythe, 
Ber.,  33,  505  (1900). 

2  Wallach  and  Kiepenheuer,  Ber.,  14,  2617  (1881). 

8  Ciamician  and  Silber,  Ber.,  18,  881, 1828  (1885) ;  19,  1962  (1886) ;  20,  698  (1887) ; 
22,  659,  2518  (1889). 

4  Pictet,  Ber.,  38,  1951  (1905). 

5  Jacobson  and  Fischer,  Ber.,  25,  992  (1892);  26,  681,  688,  699,  703  (1893^;  27, 
2700  (1894);  Annalen  der  Chemie,  287,  97  (1895);  Tauber,  Ber.,  25,  1019  (1892); 
Witt  and  Schmidt,  Ber,  25,  1013  (1892);  27,  2351,  2358  (1894). 


510 


THEORIES  OF  ORGANIC   CHEMISTRY 


<^      \3C2H5 


C2H5 


NH2 


p-Substituted  Hydrazobenzenes 


and 


NH-NH< 

Hydrazobenzene 


-NHs 


NH2 


(o-Semidine) 


>NH< 

(p-Semidine) 


Benzidine 


•NH 


Stieglitz2  interprets  this  change  as  taking  place  through  dissociation 
into  free  radicals  while  Wieland  3  disagrees  with  this  conclusion. 

Aromatic  ammonium  hydroxides,  sulphonic  acids,  and  cyanides  of 
quinoid  structure  rearrange  to  form  tertiary  amino  combinations: 


OH(S03H)(CN) 
=NR2 


OH(S03H)(CN) 
>NR2 


The  rearrangement  of  methylene  dianilines  into  derivatives  of  diphenyl- 
methane  is  brought  about  by  the  action  of  acids : 4 


NH-CH2NHC6H5 


NH-CH2NH 

ON02 


NHs 


H2NHC6H5 


NH2 


N02 


,  Jour,  prakt.  Chemie,  36,  93  (1863). 
2Ber.,  46,  911  (1913);  Jour.  Amer.  Chem.  Soc.,  35,  1143  (1913). 
3Ber.,  48,  1100  (1915). 

4Eberhardt  and  Welter,  Ber.,  27,  1810  (1894);    Meyer  and  Rohmer,  33,  250 
(1900);  Von  Braun  and  Krules,  Ber.,  46,  2977  (1912). 


MOLECULAR  REARRANGEMENTS 


511 


Rearrangements  involving  intramolecular  oxidation  have  also  been 
observed  in  the  aromatic  series: 


NO2  NO2 


NO  N02 


H2S04 


1,  8-Dinitronaphthalene         l-Nitro-8-nitroso-5-hydroxynaphthalene 


cHO 


/NcooH 

UN° 


Ethyl  phthalimidoacetate  also  loses  its  identity  when  acted  upon  by 
sodium  ethylate: 


C6H4<         >N-CH20     >C2H5 

Ethyl  phthalimidoacetate 


/CO— NH  3 

CJ'<co-i 


HCOOC2H5 

Ethyl  hydroxyisocarbostyril-3- 
carboxylate 


Mention  should  also  be  made  here  of  the  rearrangement  of  hydroxy- 
formamidine  combinations  into  ureas.4 


;NC6H5 


Diphenyloxyformamidine 


HOC  

\NHC6H5 

(Pseudo  form  of  urea) 


C6H5N;C:NCVH5 

Carbo6Tij)heriyldiimidev 


Diphenylurea. 


This  rearrangement  is  brought  about  by  the  action  of  acetic  anhydride. 
Whether  it  involves  a  molecular  exchange  of  the  hydrogen  and  hydroxyl 
groups,  or  a  dehydration  of  the  molecule  with  intermediate  formation 
of  carbodiphenyldiimide  has  not  been  established. 

1  Friedlander  and  Scherzer,  Chem.  Centralbl.  (1900),  I,  409. 

2  Weigert  and  Kummerer,  Ber.,  46,  1207  (1913). 

3  Gabriel  and  Colman,  Ber.,  33,  980  (1900);  35,  1358  (1902). 

4  Bamberger,  Tschirner  and  Destraz,  Ber.,  36,  720,  1874  (1902). 


512  THEORIES  OF  ORGANIC  CHEMISTRY 

Rearrangements  involving  the  Transference  of  Radicals  from  Nitro- 
gen to  Nitrogen:  To  this  series  belong  the  interesting  changes  which 
were  discovered  and  elucidated  by  M.  Busch  and  his  students,  i.e., 
those  involved  in  the  reaction  between  mono  alkylated  hydrazines 

R-NH-NH2 

and  isocyanates,  or  isothiocyanates  (R-NCO  and  R-NCS).  Contrary 
to  views  previously  held,  these  investigators  have  demonstrated  experi- 
mentally that  the  primary  products  of  these  reactions  are  respectively 
semicarbazides  and  thiosemicarbazides,  the  formation  of  which  may 
be  expressed  by  the  equation  : 


RNH-NH2  +  (S)OCNR"  =  RN-NH2 

>(S)NHR' 


U±.  1  J. 

CO( 


If,  now,  R  =  alphyl  (i.e.,  an  aliphatic  alkyl  group),  the  resulting  cx-semi- 
carbazides  and  a-thiosemicarbazides  are  stable  bodies.  If,  however, 
R  =  aryl  (i.e.,  an  aromatic  alkyl  group),  the. primary  product  is  unstable 
and  the  substituted  carbamyl  or  thiocarbamyl  radical  will  shift  from 
the  a-nitrogen  to  the  0-position  in  the  molecule.1  This  will  happen 
in  alcohol  solution  or  upon  fusion: 

a        0  a         0 

RN-NH2  RNH-NH 

CONHR'  CONHR' 

and 

RN-NH2  RNH-NH 

CSNHR'  CSNHR' 

Further,  M.  Busch  and  O.  Limpach2  have  discovered  a  remarkable 
rearrangement  which  takes  place  when  phenyl  mustard  oils  react  with 
esters  of  phenylcarbazinic  acid  and  which  involves  a  shifting  of  a  car- 
bethoxy  group  in  an  opposite  direction  (0  — >  a)  to  that  shown  above : 

COOC2H5 
raCOOC2H5+SCNC6H5    -»    C6H5N-NH 

Ethyl  phenylcarbazinate 


iRer.,  34,  320  (1901);  36,  1362  (1903);  37,  2318  (1904);  42,  4596,  4602  (1909). 
2Ber.,  44,  1573  (1911). 


MOLECULAR  REARRANGEMENTS  513 

Other  rearrangements  of  this  series  are  as  follows: 


CH2N<  /C] 

\COCH3     -»    C6H4< 
NH2  \NH-COCH3 

Nitriles  of  o-aminocinnamic  acids  rearrange  to  quinoline  compounds. 
An  analogous  change  is  met  with  in  the  transformation  of  cyanacetyl- 
ureas  into  pyrimidine  combinations:  2 

NH— CO  NH— CO 

II  II 

CO      CH2    -i    CO     CH 

NH2    CN  NH— C-NH2 

4-Aminouracil 


R 

OCH=C-CN 
NH2 


Acyl  derivatives  of  these  nitriles  undergo  similar  transformations  with 
migration  of  the  acyl  group  to  a  different  nitrogen  atom. 

R 

/\CH=C—  CN 
I     jNH-COCHs 


Thiocyanacetanilide    rearrangements    belong    in    this    series.     Thio- 
cyanacetamide  isomerizes  to  a  cyclic  combination.4 

CH2  --  S  CH2  --  S 

CO         CN     -»    CO          C=NH 
\NH2 


1  Widman,  Jour,  prakt.  Chemie,  47,  354  (1893). 

2  W.  Traube,  Ber.,  33,  3035  (1900). 

3  Pschorr  and  Wolfes,  Ber.,  32,  3399  (1899). 

4  Miolati,  Gazz.  chim.  ital.,  23,  90  (1893);  Chem.  Centralbl.  (1893),  I,  640. 


514  THEORIES  OF  ORGANIC  CHEMISTRY 

Substituted  thiocyanacetamides  or  anilides  undergo  similar  transforma- 
tions giving  unstable  pseudothiohydantoins,  which  can  rearrange  further 
into  stable  modifications.  These  transformations  can  be  brought 
about  by  heating  with  or  without  solvents,  and  in  many  cases  the 
labile  modifications  of  the  hydantoin  cannot  be  isolated.  Isothio- 
cyanacetamide  and  the  corresponding  anilides  have  not  been  pre- 
pared. 


CH2 S  CH2 — S  CH2 — S 

II  II  II 

CO         CN    ->     CO      C  :  NH    -»    CO      C  :  NC6H5 


NHC6H5  N-C6H5  NH 

Thiocyanacetanilide  Unstable  form  of  phenyl  Stable  form  of  phenyl 

pseudothiohydantoin  pseudothiohydantoin 

In  this  series  belong  also  the  following  important  types  of  molecular 
rearrangements:  imst/w-acylthioureas  are  rearranged  easily  by  heat 
into  their  stable  s^/m-modifications :  2 


XCOCH3     Heat      </ 

\NH-COCH3 


t/nsj/m-acetylphenyl-  Sj/m-acetylphenyl- 

thiourea  thiourfin. 


and  this  is  also  true  of  the  pseudothioureas:  3 


C-SCH3 
^N-COCH3 

t/nsym.-Acetylphenyl  /Sj/m.-Acetylphenyl- 

pseudomethylthiourea  pseudomethylthiourea 

/COCeHs 
/N<f  ,NHCOC6Hi 

C-SCH3COC6H5     Heat,      C  SCH3 


1  Wheeler  and  Johnson;  Am.  Chem.  Jour.,  28,  121  (1902),  Johnson,  Jour.  Am. 
Chem.  Soc.,  26,  482  (1903);  Walther  and  Stenz,  Jour,  prakt.  Chemie,  61,  575  (1900). 

2  Wheeler,  Am.  Chem.  Jour.,  27,  270  (1902);  Hugershoff,  Ber.,  32,  3649  (1899). 

3  Wheeler  and  Johnson,  Am.  Chem.  Jour.,  27,  274  (1902). 

4  Johnson  and  Jamieson,  Am.  Chem.  Jour.,  35,  297  (1906). 


MOLECULAR  REARRANGEMENTS 
Unsym.  acylamidines  likewise  rearrange  to  their  sym.  forms : 1 


515 


C6H5C 


/N< 

\COC6H5 


. 


>NH 


Rearrangements  involving  a  change  in  the  structure  of  the  ring, 
as  shown  in  the  case  of  pseudothiohydantoins,  have  also  been  observed 
in  the  hydantoin  series.  Diphenylimidohydantoin  in  the  presence  of 
sodium  ethylate  changes  into  unsym  oxalyl  diphenylguanidine.2 


C6H5N— CO 

oi 

C6H5N— C  :  NH 

1,  2-Diphenyl-4-imidohydantoin 

C6H5N— CO 
NH:C 
C6H5-N— CO 

(Sym.-Oxalyl  diphenylguanidine 


-CO 


C6H5N- 
C6H5N  :  C 

NH— CO 

Oxalyl  diphenylguanidine 

C6H5N CO 

C6H5N  :  C 

NH— CO 

Unsym. -Oxalyl  diphenyl- 
guanidine 


Phenylmelamines  also  undergo  similar  molecular  changes : 


CGH5N  :  C— 


:  NC6H5 


C6H5N  :  Cl     JC  :  NC6H5 
NH 


The  interesting  transformation  discovered  by    Schrader4  should    also 
be  recorded  in  this  series: 

1  Wheeler,  Johnson  and  McFarland,  Jour.  Am.  Chem.  Soc.,  25,  787  (1903). 
2Dieckmann,  Ber.,  38,  2980  (1905);   Ber.,  40,  3738  (1907). 

3  Rathke,  Ber.,  20,  1071  (1887);  21,  867  (1888). 

4  Ber.,  50,  777  (1917). 


516 


THEORIES  OF  ORGANIC  CHEMISTRY 


N02 

Picrylchloride 


/ 
NaN/  || 

N 


N02 


Heat 


NO2 

Picrylazide 


_NO 

N02/~     NNO  +  N2 

"N02 


Rearrangements  involving  the  Transference  of  Radicals  from 
Nitrogen  to  Oxygen:  An  illustration  of  this  class  is  that  represented  by 
the  transformation  of  quinone-phenylhydrazone  combinations  into 
hydroxy-azo  compounds,1  which  was  discovered  by  Willstatter: 


OC6H5 


This  type  of  change  involves  the  migration  of  an  acyl  group  and  also 
takes  place  when,  instead  of  benzoyl,  the  acyl  radicals  COCHs  and 
COOC2Hs  are  in  union  with  the  nitrogen. 

Betaine  rearrangements  are  to  be  recorded  in  this  group.  Betaine 
is  transformed  by  heat  into  the  methylester  of  dimethylamino-acetic 
acid.  The  reaction  is  reversible  and  is  apparently  a  general  one  for 
all  a-betaines,  although  in  the  case  of  /3-betaines  analogous  transfor- 
mations cannot  be  brought  about.  Betaine  and  its  isomeric  ester — 
methyl  dimethylaminoacetate — are  both  stable  below  135°;  between 
135°  and  293°  the  oetaine  is  stable  and  the  ester  is  the  unstable  modi- 
fication. Above  293°  the  betaine  cannot  exist.  In  the  case  of  mixed 
a-betaines  containing  both  ethyl  and  methyl  groups  on  the  nitrogen 
it  is  the  methyl  or  smaller  radical  which  migrates  from  nitrogen  to 
oxygen.2 

.  i  Willstatter  and  Veraguth,  Ber.,  40,  1432  (1907). 
2  Willstatter,  Ber.,  36,  587  (1902). 


MOLECULAR  REARRANGEMENTS  517 

/CH3  CH3\ 

CH2—  Nf-CH3  t>  >N-CH2COOCH3 

\CH3  CH«/ 

-O 


^s  j-  j-^         j 

CO — ( 


25  2s\ 

CH2—  Nf-C2H5  -*  V-  N-CH2COOCH3 

1  |  \CH3  C2H5/ 

O  —  O 


v^. 

i 


This  same  type  of  change  has  also  been  observed  in  the  aromatic  series  : 
CO  -  O  COOCH3 


Rearrangements  Involving  the  Transference  of  Radicals  from  Oxy- 
gen to  Nitrogen;  one  of  the  first  reactions  of  this  kind  to  be  recognized 
was  that  described  by  Bottcher,3  who  found  that  the  benzoyl  derivative 
of  o-nitrophenol  is  converted  by  reduction  into  benzoylaminophenol  : 

/O-COC6H5        H  /OH  4 

C6H5<  -_  ->     C6H4< 

\  XNH-COCGH5 


Or^o-aminophenylcarbonates  also  undergo  a  similar  transformation; 
the  carbethoxy  group  migrating  to  nitrogen.  The  mechanism  of  the 
reaction  has  not  been  established,  but  Ransom  assumes  the  formation 
of  an  intermediate  addition  product  as  shown  below: 

/O-COOC2H5  /  O  v       /OH 

C6H4<  -»    C6H4< 


Ethyl  o-aminophenylcarbonate  (Not  isolated) 

,OH  5 


>NH-COOC2H5 
'Griess,  Ber.,  6,  585  (1873). 
2Griess,  Ber.,  13,  246  (1880). 
3  Ber,  16,  630  (1883). 

«Auwers,  Annalen  der  Chemie,  332,  159  (1904);  Ber.,  37,  2249    (1904);  Ber., 
33,  1923  (1900);  Einhorn  and  Pfyl,  Annalen  der  Chemie,  311,  34  (1900). 

5  Ransom,  Am.  Chem.  Jour,  23,  11  (1900);  Ber,  31,  1060  (1898);  33,  199  (1900). 


518 


THEORIES  OF  ORGANIC  CHEMISTRY 


Acyl  groups  may  also  migrate  from  a  phenolic  oxygen  to  a  nitrogen 
substituted  in  a  side  chain, 


L     JCH2Br+C6H5NH2 
COCH3 


COCHs 


CTT         1 
6A15 

\COCH3 


and  Hantzsch  assumes  that  a  rearrangement  of  an  acyl  group  is  involved 
in  the  formation  of  anfo'-diazoacetates  from  diazonium  salts, 


RN 

N 


KO 


CHaCO 


RN 
-ON 


RN 


O-COCH3 


RN-COCHs  2 
NO 


Rearrangements  involving  migration  of  a  radical  from  an  oxygen 
atom  substituted  in  the  ring  to  a  nitrogen  within  the  cycle  or  in  a  side 
chain  belong  in  this  class: 


Methoxykynurine 


N-Methylkynurine 


Analogous  changes  are  brought  about  in  anil  combinations  by  the  action 
of  alkyl  halides  giving  amine  oxides: 


CH3I 


cH=NC6H5 
\X 


XCH3  * 
ICH=N< 

\n- 


OCH3 

CH:NC6H5 

CH=CH2 

Anil  of  cotarnone 


CH2—  0 

\  CH3 

O-l      JCH=CH2 


iAuwers,  Ber.,  33,  1923  (1900). 

2  Hantzsch,  and  Wechsler,  Annalen  der  Chemie,  325,  229  (1902). 

3  Meyer,  Monatsh.  Chemie,  27,  260  (1906)  ;   Haitinger  and  Lieber,  Ibid.,  6,  323 
(1885);  Knorr,  Annalen  der  Chemie,  236,  104  (1886). 

4  Freund  and  Becker,  Ber.,  36,  1524,  1537  (1903)  ;  Briihl,  Die  Pflanzenalkaloide, 
pp.  317  (1900). 


MOLECULAR  REARRANGEMENTS  519 

The  imidoester  rearrangements  should  be  considered  at  this  point. 
Isomerization  of  these  combinations  into  acid  amides  by  the  action  of 
heat  or  of  alkyl  halides  has  been  observed  by  several  investigators, 

I     CeH5 

NC6H5  i/C2H5 

HC^  2  ,5>     HC^  or 

X)C2H5  XOC2H5 

Ethyl  isoformanilide  (Intermediate  product) 


,N— C2H5 
2H5    ->    HC^  +C2H5I 

Ethyl  formanilide 

,NH 

C6H5Cf  -+    C6H5CONHC2H5  l 

\OC2H5 

Benzimido  ethylether  Ethyl  benzamide 

Imidoacid-anhydride  rearrangements  may  also  be  included  here : 


C6H5COOAg  =  >O 


C6H5C=0 

(Not  stable)  Dibenzanilide 

This  type  of  change  is  also  met  with  in  cyclic  combinations, 

CeHsN—  N  npfl        CeHsN—  N-COCHs3 

I       II  —  ^  I       I 

CH3CO  •  OC     C  •  O  •  COCH3  CO  CO 

\/  \/ 

N  N-COCHg 

1  Wheeler  and  Johnson,  Ber.,  32,  35  (1899);   Am.  Chem.  Jour.,  21,   186  (1899); 
23  135  (1900). 

Literature  references  on  imidoester  rearrangements  :  Wislicenus  and  Goldschmidt, 
Ber.,  33,  1467  (1900).  Hofmann  and  Olshausen,  Ber.,  3,  272  (1870).  Hofmann, 
Ber.,  19,  2061  (1886).  Meyer  and  Pinner,  "Die  Imidoather  und  Ihre  Derivate," 
pp.  215.  Andreocci,  Ber.,  24,  R.  205  (1891).  Knorr,  Ber.,  30,  922,  937  (1897); 
Annalen  der  Chemie,  293,  1  (1896).  Wislicenus  and  Korber,  Ber.,  36,  1991,  164 
(1902)  .  Gabriel  and  Neumann,  Ber.,  25,  2383  (1892)  .  Lander,  Jour.  Chem.  Soc.,  83, 
406  (1903).  Meyer  and  Beer,  Monatsh.  Chemie,  34,  1173  (1913). 

2  Wheeler  and  Johnson,  Am.  Chem.  Jour.,  30,  24,  31  (1903). 

3Hoogewerff  and  Van  Dorp,  Rec.  trav.  chim.  des  Pay-Bas.,  12,  12  (1893);  13, 
93  (1894).  Kuhara  and  Fukui,  Am.  Chem.  Jour.,  26,  454  (1901);  van  der  Meulen, 
Rec.  trav.  chim.  des  Pays-Bas.,  15,  282  (1896). 


520  THEORIES  OF  ORGANIC  CHEMISTRY 

C=CHC5H4N  C=CHC5H4N  1 

C6H4/  )>0  ->    C6H4/  )>NH 

XC=NH  XC=0 

and  in  thioimido-acid  anhydride  rearrangements, 

C6H5(C2H5)N.C=NC6H5  Cells (C2H5)NC=S  2 

\S  at  lor  >N.C6H5 

(CH3)2NC=S  (CH3)2NC=S 

o-Dimethylethylphenylpseudo-  Dimethylethyldiphenyldithio- 

dithiobiuret  biuret 

S S  HBr  S-S3 

CH3N=C       C=S     Heat'    CH3N=C   C=NCH3 


The  change  of  thiodiazoles  into  triazoles,  which  was  discovered  by 
M.  Busch,4  may  now  be  considered.  Thus  thiodiazolonanile  (I) 
rearranges  in  solution,  or  upon  fusion,  to  give  an  endoxytriazol-thiol 
(II): 

R-N NH  R-N N 


R'N  :  C       CO         ->    HSC      C 

S  NR' 

I  II 

If  the  hydrogen  atom  of  the  imido  group  in  I  is  replaced  by  CH3  the 
course  of  the  reaction  is  as  follows : 

R-N N-CH3  RN — NCH3 

R'N  :  C      CO          ->     S  :  C      CO 

V  V 

S  NR' 

III 

ivon  Huber,  Ber.,  36,  1664  (1903). 
'Billeter,  Ber.,  26,  1688  (1893). 
3Freund,  Annalen  der  Chemie,  285,  166  (1895). 
4  Busch  and  Limpach,  Ber.,  44,  560  (1911). 


MOLECULAR  REARRANGEMENTS  521 

Also  in  the  case  of  substances  belonging  to  a  class  represented  by 
formula  IV  and  isomeric  with  III,  an  analogous  rearrangement  takes 
place : 

R  •  N N  •  CH3  RN — N  •  CH3 

OC      C  :  NR'  -»    OC       C  :  S 


Y 


NR' 
IV 

According  to  Busch  and  Limpach  these  transformations  of  thiodiazole 
compounds  into  triazoles  are  decidedly  not  reversible  processes  as 
Nirdlinger  and  Acree  assume.  Aliphatic  nitrites  can  be  rearranged 
to  nitro  compounds.1 


C2H50-N=O 


Rearrangements  Involving  the  Transference  of  Radicals  from  Oxy- 
gen to  Oxygen;  Dibenzhydroximic  acids,  which  are  imidoacid-anhydride 
combinations,  rearrange  spontaneously  to  dibenzhydroxamic  acids 
instead  of  giving  diacyl  derivatives  of  hydroxylamine:  2 

NOH  C6H5C=NOH 

C6H5Cf  +    C6H5COOAg    ->  No  -> 

XC1  C6H5.C=0 

Dibenzhydroximic  acid 


CeHsCONH-O-COCeHs    -*  >N-OH 

C6H5CO  / 

Dibenzhydroxamic  acid  Dibenzoyl  hydroxylamine 

Rearrangements  Involving  the  Transference  of  Radicals  from  Oxy- 
gen to  Sulphur :  These  molecular  transformations  are  generally  brought 
about  by  the  agency  of  alkyl  halides,  and  have  been  investigated  very 
thoroughly  by  Wheeler  and  his  co-workers.  In  the  presence  of  alkyl 
halides  thioncarbamates  isomerize  to  give  the  corresponding  thiol  com- 
binations, the  halide  acting  in  all  probability  as  a  catalytic  agent.  In 

1  Neogi  and  Chowdhari,  Jour.  Chem.  Soc.,  109,  701  (1916);    Gaudion,  Annalen 
chimie  et  phys.,  (1912),  25,  125. 

2  Lessen,  Ann.  186,  42  (1877) ;  Werner  and  Skiba,  Ber.,  32,  1654  (1899) .     Werner 
and  Buss,  Ibid.,  27,  2198  (1894). 


522  THEORIES  OF  ORGANIC  CHEMISTRY 

the  following  equations  the  reaction  is  interpreted  as  involving  the  for- 
mation of  an  intermediate  addition  product: 


or 


v  X 

Ethyl  thioncarbamate.  .     ^IscH  Ethyl  thi<>lcarbamate. 


Addition  product. 


C:S  +CH3I  -  >I-C^CH3  -  ^CO  -t-C2H5I 

^OC2H5  ^OC2H5          ^SCH3 

Ethyl  thioncarbamate.      Addition  product.    Methyl  thiolcarbamate. 

Knoor  has  recently  supported  Wheeler's  interpretation  of  this  change, 
but  he  represents  the  reaction  as  taking  place  in  three  stages  as  is  repre- 
sented by  the  following  scheme  :  2 

I 
NH2CSOC2H5+CH3I     -+      H2N—  C—  SCH3     -» 

OC2H5 
OC2H5 

•  HI     ->    NH2COSC2H5  +  C2H5I 


Imidoether  stage 


Thioncarbanilates  may  rearrange  to  thiolcarbanilates;  but  the 
change,  however,  is  not  brought  about  as  easily  as  in  the  case  of  the 
thioncarbamates.  Here  also  alkyl  halides  act  as  the  catalytic  agents: 


. 

CS  +     C2H5I     -*     CO 

\OC2H5  \3C2H5 

Ethyl  thioncarbanilate  Ethyl  thiolcarbanilate 

R-CSOR/    ->    R-COSR'  4 

1  Wheeler  and  Barnes,  Am.  Chem.  Jour.,  22,  143  (1899). 
^Ber.,  50,  767  (1917). 

3  Wheeler  and  Barnes,  Am.  Chem.  Jour.,  24,  60  (1900). 

4  Bettschart  and  Bistrzycki,  Helv.  chim.  Acta.,  2,  118  (1919). 


MOLECULAR  REARRANGEMENTS  523 

Thioncarbazinic  esters  rearrange  to  their  thiol  isomers. 

,NH  •  NHC6H5  xNH  •  NHC6H5 

cs  °2H5*  do 


Substituted  thioncarbanilates  rearrange  more  easily  than  the  carbani- 
lates  themselves. 

C^6H5 

CS 


Acylthioricarbamates  show  little  tendency  to  isomerize  to  the  isomeric 
thiol  combinations. 

/NH-COCHs  /NH-COCHs  2 

CS  ™ 


Methyl  acetyl  thioncarbamate  Methyl  acetyl  thiolcarbamate 

Alkyl  sulphites  can  be  isomerized  to  their  isomeric  alkyl  sulphonic 
esters. 

/OC2H5  /OC2H5  3 

0=S<  02S< 

XOC2H5  XC2H5 

Alkyl  sulphite  Alkyl  sulphonic  ester 

Rearrangements  Involving  the  Transference  of  Radicals  from  Iodine 
to  Carbon:  Intramolecular  halogenation  is  illustrated  by  the  trans- 
formation of  phenyliododichloride  into  p-iodochlorbenzene  and  of 
o-methoxy phenyliododichloride  into  l-methoxy-2-iodo-5-chlorbenzene . 
These  unique  changes  are  accelerated  by  the  action  of  sunlight: 


1  Wheeler  and  Dustin,  Am.  Chem.  Jour.,  24,  425  (1900). 

2  Wheeler  and  Johnson,  Am.  Chem.  Jour.,  24,  189  (1900). 

3  Rosenheim  and  Sarow,  Ber.,  38,  1300  (1905). 

4  Keppler,  Ber.,  31,  1136  (1898);  Jannasch,  Hinterskirch  and  Naphtali,  Ber.,  31, 


524  THEORIES  OF  ORGANIC  CHEMISTRY 

Rearrangements  Involving  the  Transference  of  Radicals  from  Car- 
bon to  Nitrogen  :  A  type  of  change  which  should  be  recorded  in  this 
series  is  that  involving  the  rearrangement  of  azo  compounds  into 
hydrazones: 

AcyK  AcyK  /C6H5 

AcyAc-N= 

AcyK 


O.  Dimroth  and  Hartmann,  who  studied  this  reaction,  found  that 
it  took  place  upon  heating  the  dry  substance  to  the  fusion  temper- 
ature or  upon  warming  in  indifferent  solvents.1 

The  Beckmann  Rearrangement.  This  consists  of  an  interchange  of 
an  organic  radical  and  hydroxyl  in  oxime  combinations.  A  simple 
illustration  is  the  transformation  of  acetophenone  oxime  into  acetanilide 
and  may  be  expressed  by  the  following  equation  in  which  the  CeHs 
group  is  represented  as  migrating  from  carbon  to  nitrogen: 

C6H5.C.CH3  HO-C-CH3 

-»  -*    CH3CO.NH.C6H5 

HO-N  C6H5N 

Acetophenoneoxime  Acetanilide 

This  type  of  change  is  very  general  in  both  the  cylic  and  acylic  series 
and  is  brought  about  by  the  action  of  acid  dehydrating  agents,  such 
as  phosphorus  pentachloride,  organic  acid  chlorides  and  anhydrides, 
acetic  acid,  sulphuric  and  hydrochloric  acids,  and  phosphorus  pent- 
oxide.  The  reagent  first  used  by  Beckmann  to  produce  the  change  was 
phosphorus  pentachloride.  Although  the  mechanism  of  the  change  has 
received  very  careful  study  by  numerous  investigators,  it  is  still  very 
obscure.  Several  theories  have  been  proposed,2  but  none  has  as  yet 
finally  supplanted  Beckmann's  original  interpretation,  which  is  still 
strongly  supported  by  experimental  evidence. 

Beckmann3  assumed  that  a  direct  interchange  of  radicals  took 

*Ber.,  40,  4460  (1907). 

2  Beckmann,  Ber.,  27,  300  (1894).     Baeyer,  Ber.,  32,  3627  (1899).     Nef,  Annalen 
der  Chemie,  298,  308  (1897);  318,  39,  227  (1901).     Stieglitz,  Am.  Chem.  Jour.,  18, 
751  (1896);  29,   49  (1903);  Ber.,  43,  782  (1910).  Slosson,  Am.  Chem.  Jour.,  29,  289 
(1903).      Werner  and  Piguet,  Ber.,  37,  4295  (1904).       Sluiter,  Rec.  trav.  chim.  des 
Pays-Bas,  24,  372  (1905).      Wallach,  Annalen  der  Chemie,  346,  272  (1906).     Diels 
and  Stern,  Ber.,  40,  1631  (1907).    Schroeter,  Ber.,  42,  2136  (1909).     Montagne,  Rec. 
trav.  chim.  des  Pays-Bas,   25,    376(1906);   Ber.,   43,   2014   (1910).    Kuhara    and 
Co-workers,  Memoirs  College  Science  and  Engineering  Kyoto  Univ.  (Japan),  1907- 
1916.     Hantzsch,  Ber.,  35,  3579  (1902). 

3  Ber.,  19,  988  (1886);  27,  300  (1894). 


MOLECULAR  REARRANGEMENTS  525 

place  between  carbon  and  nitrogen,  and  that  the  reagent  used  acted 
merely  as  a  catalyst.  Kuhara,  on  the  other  hand,  postulated  that  the 
action  of  the  acid  chloride  or  anhydride,  when  these  bodies  were  used 
as  reagents,  interacted  with  formation  of  an  acyl  derivative,  and  that 
this  then  rearranged  with  an  interchange  of  radicals  to  give  imidoacid 
anhydride  combinations  as  secondary  products.  These  are  then 
broken  down  by  hydrolysis  with  the  formation  of  the  anilide.  Accord- 
ing to  Kuhara,  the  more  negative  the  acid  radical  the  greater  will  be 
the  tendency  to  rearrange.  This  view  is  practically  identical  with 
Beckmann's  first  interpretation  of  the  reaction  which  bears  his  name, 

R'C-R'        (CH3CO)20    R-C-R' 


N-OH  N-O-COCHs 

R-C-O-COCHs 


N-R' 


RCO-NHR' 


l'£'R'      C6H5S02C1 


NOH  N-O- 

R-C-O-SOaR 


NR' 


RCONHR 


According  to  the  investigations  of  Wheeler  and  Johnson  1  imidoacid 
anhydride  combinations  corresponding  to  the  acetyl  derivative  above 
are  unstable  compounds  and  readily  rearrange  to  diacyl  anilides.  The 
latter  substances  would  give  on  hydrolysis  the  product  of  rearrange- 
ment or  the  wono-acylanilide. 

According  to  Hantzsch  and  others  the  transformation  of  an  oxime 
to  an  anilide  by  the  action  of  phosphorus  pentachloride  is  to  be  repre- 
sented as  follows: 


pQ5     Cells-  C-CoHs  CeHsC-Cl  HoO 

N-OH  N-C1 


Stieglitz  and  Peterson,2  who  later  synthesized  several  chlorimido  com 
pounds  of  this  type,  were  unable  to  effect  a  rearrangement  of  such  com 
binations. 

1  Am.  Chem.  Jour.,  30,  24,  31  (1903). 

2  Ber,  41,  782  (1910);   Am.  Chem.  Jour.,  46,  325  (1911). 


526  THEORIES  OF  ORGANIC  CHEMISTRY 

The  rearrangement  of  ketonic  oxides  into  acid  esters  recently  dis- 
covered by  A.  von  Baeyer  and  Villiger,1  has  been  interpreted  by  them 
as  analogous  to  the  Beckmann  rearrangement: 


O 


c=o 


I  xo  ->      I      / 
c  o— c- 


Wieland's  interesting  peroxide  rearrangement  may  be  recorded  here: 2 


/C6H5 

/_ —  ^^  •  \^/  •  \_/  •  V>^       v^QA-L*) }  ^-'\J-m--m-o        S 

\CCH5  C6H5/ 

Triphenylmethylperoxide 


X 

>C— OC6H5 
C6H5X 

>C— OC6H5 
C6H/  i 

— OC6H5 


An  interesting  rearrangement,  the  mechanism  of  which  is  not  under- 
stood and  which  was  originally  described  by  Claisen,3  is  that  of  oxalyl 
dibenzylketone  into  an  isomeric  lactone  combination: 

OH 
C6H5CH— CO  C6H5C=C— C=CHC6H5 

CO  J^  CO O 

C6H5CH— CO 

Oxalyl  dibenzylketone 

Stieglitz  considers  that  three  important  rearrangements  may  be 
arranged  in  the  same  general  group  and  are  therefore  capable  of  much 

iBer.,  32,  3625  (1899). 
2Ber.,  44,  2550  (1911). 

3  Claisen  and  Ewan,  Annalen  des  Chemie,  284,  290  (1895);  Bamberger,  Jour, 
prakt.  Chemie,  51,  588(1895). 


MOLECULAR  REARRANGEMENTS  527 

the  same  interpretation.     These  are  the  Hofmann  rearrangement,  the 
Curtius  process  of  passing  from  acylazides  to  urethanes  and 

N 

RCO-N<  -4    N2  +  RCON<        ->    R-NCO 


/ 

-N</|| 

C2H5°H)        RNH-COOC2H5 


from  alkylazides  to  imides;  and  the 

XN 
C6H5CH2N     ||      -»    N2+CH2  :  NC6H5 


Beckmann  change.  In  order  to  develop  a  general  explanation  of  the 
mechanism  of  these  transformations  he  supposes  that  in  all  such  cases 
the  transformation  results  from  the  formation  of  univalent  nitrogen 
derivatives  or  unsaturated  radicals  as  intermediate  products,  and 
assumes  further  that  the  free  valences  of  the  univalent  nitrogen  are 
powerful  enough  to  detach  the  radical  R  from  carbon  and  bring  about 
a  rearrangement  to  a  stable  molecule.  In  other  words,  the  presence  of 
univalent  nitrogen  in  the  intermediate  compounds  is  assumed  to  be 
responsible  for  all  rearrangements  in  this  entire  group.2 

/Cl 
R-CO-N/ 

/H  _H  0 

R-CO-N<          _?:;  R-NCO  4 

X)H 

C&H.5  \  NaOH 

C6H5-^C-NHC1     1Na      >     (C6H5)2C=N -Cells  5 

C6H5/ 

Triphenylmethylchloramine  Phenylimidobenzophenone 

CeHs  ^CNHOH    _£2!i»     (C6H5)2C  :  NC6H5  6 
C6H5/ 

1  Curtius,  Jour,  prakt.  Chemie,  63,  428  (1901);  Ber.,  35,  3229  (1902). 

2  Stieglitz  and  Leech,  Jour.  Am.  Chem.  Soc.,  36,  272  (1914);  Stieglitz,  Ber.,  43, 
782  (1910);  46,  2149  (1913);  Am.  Chem.  Jour.,  46,  327  (1911). 

3  Stieglitz  and  Leech,  Jour.  Am.  Chem.  Soc.,  36,  272  (1914). 

4  Lessen,  Annalen  der  Chemie,  161,  359  (1872). 

6  Stieglitz    and   Vosburgh,    Ber.,    46,  2151  (1913);   Jour.  Am.  Chem.  Soc.,    38, 
2081  (1916). 

*  Stieglitz  and  Leech,  Ber.,  46,  2147  (1913) ;  Jour.  Am.  Chem.  Soc.,  36,  272  (1914). 


528  THEORIES  OF  ORGANIC  CHEMISTRY 

In  reviewing  the  various  types  of  change  classified  together  under 
Hofmann's  rearrangement,  it  is  obvious  that  a  univalent  nitrogen 
derivative  cannot  be  formed  when  an  oxime  of  the  type  of  /3-triphenyl- 
methyl-/3-methymydroxylamine  undergoes  rearrangement: 


--  _*  - 

C6H5/          XOH  C6H5/\       X>>H5 


sv  /CH3     ppi  es^  / 

C6H5-)C-N<  _!l*  C-N 


0-Triphenylmethyl-|8-methyl-  OH 

hydroxylamine 

Whether  we  are  dealing  here  with  a  mechanism  other  than  a  direct  inter- 
change of  radicals  as  postulated  by  Beckmann  remains  to  be  established: 

N       Tipof 

ne    >     (C6H5)2C:NC6H5+N22 
C6H5  N 

Triphenylmethylazide 

C6H5  •  C  •  OC2H5      CH3CO  •  O  •  C  •  OC2H5  w  o 

'*+  Zi;  C6H5NHCOOC2H5  3 

CHsCO-ON  C6H5N 

%n-acetylethylbenzhydrox-  Phenylurethane. 

imic  ester 

An  interesting  rearrangement  brought  about  by  the  catalytic  action 
of  methyliodide  is  that  of  3,4,4,5-tetramethylpyrazol  into  1,3,4, 
5-tetramethylpyrazol. 

N  --  N  CH3N  --  N4 

CH3C—  C—  CCH3     °H3l>     CH3N=C—  CCH3 
CH3  CH3  CH3 

Rearrangements  which  are  accompanied  by  an  Elimination  of 
Groups  of  Atoms  :  The  decompositions  of  diazonium  compounds  and 
the  Sandmeyer  5  reaction  may  be  regarded  as  instances  of  rearrange- 
ments belonging  in  this  series  : 

C6H5N2OH    ->    N2  +  C6H5OH 
C6H5N2Cl(Br)     -»    N2  +  C6H5Cl(Br) 
C6H5N2(CN)     ->    N2  +  C6H5CN,  etc. 

^tieglitz  and  Stagner,  Jour.  Am.  Chem.  Soc.,  38,  2049  (1916). 

2  Senior,  Jour.  Am.  Chem.  Soc.,  38,  2718  (1916). 

3  Kuhara,  Jour.  Chem.  Soc.,  106,  538  (1914). 
4Oettinger,  Annalen  der  Chemie,  279,  247  (1894). 

"Griess,  Annaleri  der  Chemie,  137,  67  (1866);  Sandmeyer,  Ber.,  17,  1633,  2650 
(1884);  23,  1880(1890). 


MOLECULAR  REARRANGEMENTS  529 

as  may,  also,  the  transformation  of  unsymmetrical  monohalogen-ethyl- 
ene  derivatives  into  tolanes:  1 


->    C6H5C=CC6H5 

CoH-5'  \Halogen  Tolane 


5x  y 

>C=C<  -»    C6H5CE=C-CH3 

CH3/  \Halogen 

and  the  transformation  of  phenylhydroxypivallic  acid  esters  into  esters 
of  a-phenyl  -/3,  /3-dimethylacrylic  acid  :  2 

/CH3  /CH3 

^CH3        -»    C6H5C=C< 
\COOX  XCH3 

COOX 


The  conversion  of  acid  amides  into  amines  by  the  action  of  bromine 
in  alkaline  solution  belongs  to  this  same  group  of  rearrangements  and 
was  discovered  by  A.  W.  Hofmann: 3 

R-CONH2    -»    RNH2 
The  course  of  the  change  is  explained  as  follows : 4 

CH3-C=O          CH3C=O          CH3C=O 
HNH  BrNH  -N- 

CH3N  CH3NH2+C02 

II  -^^ 

c=o 

1  Annalen  der  Chemie,  279,  328,  335  (1894);  Tiffeneau,  Compt.  rend.,  135,  1348 
(1902). 

2  Tiffeneau,  Rev.  ge"n.  Sci.  pur.  et  appli.  (1907),  p.  587. 
3Ber.,  15,  762(1882). 

4Hoogewerff  and  van  Dorp,  Rec.  trav.  chim.  des  Pays-Bas,  16,  107  (1896); 
Stieglitz,  Am.  Chem.  Jour.,  18,  752  (1896);  Graebe,  Ber.,  35,  2747  (1902); 
Hantzsch,  Ber.,  35,  3579  (1902);  Lapworth,  Proc.  Chem.  Soc.,  19,  22  (1903);  Mohr, 
Jour.,  prakt.  Chemie,  72,  297  (1905);  Schroeter,  Ber.,  42,  2337,  3356  (1909);  44, 
1201  (1911). 


530  THEORIES  OF  ORGANIC  CHEMISTRY 

The  transformation  of  oximes  into  acid  amides,  which  was  dis- 
covered by  E.  Beckmann,1  and  which  has  already  been  referred  to,  is 
analogous  to  the  preceding: 

R\ 
\C=NOH    ->    RCONHR' 

R'/ 

The  reaction  takes  place  most  readily  under  the  influence  of  acid 
reagents,  and  plays  an  important  role  in  the  determination  of  the  con- 
stitution of  stereoisomeric  nitrogen  compounds.  At  present  it  is 
regarded  as  a  special  case  of  the  Hofmann  rearrangement,  which  will 
be  discussed  more  fully  at  the  end  of  this  chapter. 

When  these  various  reactions  involving  molecular  rearrangements 
are  considered  merely  with  reference  to  the  results  obtained,  it  is 
apparent  that  in  all  cases  an  interchange  of  atoms  or  groups  of  atoms 
takes  place,  and  that  this  is  accompanied  by  a  more  or  less  pronounced 
change  in  molecular  constitution,  often  resulting  in  the  elimination  of 
H2O,  HC1,  N2,  etc.  Frequently  the  course  of  such  transformations 
is  represented  in  the  simplest  empirical  and  mechanical  manner,  and 
the  facts  are  described  without  any  attempt  to  explain  them.  For 
example,  in  the  case  of  the  rearrangement  of  phenylhydroxylamine  into 
p-amidophenol : 

C6H5NHOH    -*    HOC6H4-NH2 

the  statement  is  made  that  the  hydroxyl  group  changes  place  with  the 
hydrogen  atom  occupying  the  p-position  in  the  benzene  nucleus.  At 
other  times,  however,  and  this  is  especially  true  in  the  case  of  those 
rearrangements  involving  complicated  molecular  changes,  explanations 
are  attempted  in  an  effort  to  avoid  the  assumption  of  such  sudden  and 
direct  interchanges  of  groups  of  atoms  and  the  theory  is  advanced  that 
intermediate  products  are  formed  in  the  course  of  all  such  transfor- 
mations. The  fate  of  many  explanations  of  this  kind  is  illustrated  in 
the  case  of  the  pinacoline  rearrangement.  E.  Erlenmeyer,  Sr.,2  con- 
sidered it  probable  that  in  this  transformation  water  was  split  off  and 
a  trimethylene  derivative  formed.  Since  water  was  added  again  at 

1  Ber.,  19,  988  (1886) ;  20,  1507,  2580  (1887) ;  21,  766  (1888) ;  Annalen  der  Chemie, 
252,  1  (1889);  Ber.,  27,  300  (1894);  Stieglitz,  Am.  Chem.  Jour.,  18,  751  (1896);  29, 
49  (1903);   Nef,  Annalen  der  Chemie,  318,  227  (1901);   Slosson,  Am.  Chem.  Jour., 
29,  289  (1903);  Werner,  Ber.,  37,  4295  (1904);  Sluiter,  Rec.  Trav.  chim.  des  Pays- 
Bas.,   24,  372    (1905);     Wallach,  Annalen  der  Chemie,  346,  272  (1906);   O.  Diels 
and  Stern,  Ber.,  40,  1631  (1907). 

2  Ber.,  14,  322  (1881). 


MOLECULAR  REARRANGEMENTS  531 

another  point  the  rearrangement  could  be  interpreted  structurally  in 
the  following  manner: 

CH3v  /OH  CH3\  /OH 

>C— CeCH3    =  H20  +  >C C< 

./  i        \nTTo  nw^/     \    /    Nr1! 


CH3/   |        \CH3  CH3/     \y   \CH3 

OH 

+H20 


CH;/     \S     \CH3  CHa/ 

CH2  CH 


CH3-C-CO-CH3+H20 
CH3/ 


Since,  now,  tetraphenyl  pinacone  rearranges  to  a  corresponding 
pinacoline,  the  mechanism  of  this  reaction  must  be  the  following,  if 
the  above  interpretation  is  right  in  principle: 

C6H5\  (I)     (II)  /OH  C6H5v  (I)     (II)  /OH 

>C  --  C<  -»  >C  -  C< 

\  /  |  \ 


C6H5v  (I)       (II) 
CeHs 


It  is  obvious  that  in  this  process  of  condensation  the  benzene  nucleus, 
which  was  originally  bound  to  the  carbon  atom  II  by  its  valence  1, 
becomes  ultimately  attached  to  the  carbon  atom  I  by  its  valence  2,  or 
at  least  by  some  valence  other  than  its  valence  1.  In  other  words,  a 
change  in  the  point  of  union  of  one  of  the  benzene  rings  must  take  place 
if  this  is  the  mechanism  of  the  reaction. 

Since  it  has  not  been  possible  to  isolate  the  intermediate  products 
of  this  change,  Montague  l  devised  a  means  for  proving  experimentally 
the  course  of  the  rearrangement  in  the  case  of  p-tetrachlorphenyl- 
pinacone.  If  Erlenmeyer's  interpretation  is  correct  this  substance 

^ec.  trav.  chim.  des  Pays-Bas,  24,  105  (1905);  26,  413  (1906). 


532  THEORIES  OF  ORGANIC  CHEMISTRY 

sh'ould  pass  into  a  pinacoline  in  which  one  of  the  chlorine  atoms  occupies 
not  the  para-position  but  the  meto-position  in  the  ring. 

(p)  C1C6H4X  /C6H4C1  (p)  (p)  C1C6H4\ 

\C C<  ->    (p)  ClCeH^ 

(p)  C1C6H4/  |         |  XC6H4C1  (p)  (m)  C1C6H4/ 

OH    OH 

This,  however,  was  found  not  to  be  the  case  as  the  resulting  pinacoline 
was  shown  to  have  all  four  chlorine  atoms  in  the  para-position. 

(p)  ClC6H4v 

(p)  ClC6H4-^C-COC6H4Cl(p) 

(p)  C1C6H4/ 

This  discovery  was  verified  later  by  Acree,  and  thus  Erlenmeyer's 
whole  scheme  became  in  the  highest  degree  improbable. 

Other  investigators  l  have  postulated  that  an  oxide  or  cyclic  ether 
is  formed  in  the  process  of  the  rearrangement  which  is  isomeric  with 
the  resulting  ketone  or  pinacoline. 

CH3\                 /CH3  CH3\              /CH3           CH3\ 

yc C<  -+             yc— C<  ->   CH3-)C-COCH3 

CH-/  I          I  XCH3  CH3/  \/  XCH3          CH3/ 

OH     OH  O 

Since,  however,  organic  oxides  are  relatively  stable  in  the  presence  of 
dehydrating  agents,  Montagne  and  Meerwein  came  to  the  conclusion 
that  characteristic  intermediate  products  were  not  formed,  and  that 
the  pinacoline  conversion  represents  a  genuine  case  of  intramolecular 
rearrangement  accompanied  by  the  elimination  of  water.  The  question 
then  arose  as  to  whether  the  loss  of  water  takes  place  before  or  after 
the  rearrangement.  Tiffeneau,2  after  studying  the  reaction,  was  of 
the  opinion  that  the  interchange  of  groups  took  place  as  a  result  of  the 
action  of  the  hydrating  agents;  and  he  gives  an  expression  for  the  reac- 
tion which  implies  only  these  facts.  This  explanation  assumes  the 
intermediate  formation  of  trivalent  carbon  combinations. 


/^TT  f^TT  f^TT  r^TJ  ^TT 

L^±13\  x>Ly±l3  L^±l3y    |  /I_yil3  Orl3\ 

>C C<  -*  >C C^-CH3  -+  CH3-9C-COCH3 

|   \CH3         CH3/  |  \O CH3/ 


|          | 
OH     OH 


1  Erlenmeyer,    Jr.,    Annalen  der  Chemie,   316,   84    (1901);    Nef,    Annalen  der 
Chemie,  335,  243  (1904). 

2  Revue  gen.  Sci.  pur.  et  appli.,  1907,  591. 


MOLECULAR  REARRANGEMENTS  533 

It  seems  to  have  been  established  conclusively  by  the  work  of 
Montagne  l  and  Meerwein  2  that  ethylene  oxide  or  trimethylene  com- 
binations are  not  intermediate  products  in  pinacoline  rearrangements. 
According  to  their  interpretation  the  first  stage  of  these  transformations 
involves  a  loss  of  water  with  formation  of  an  unsaturated  combination 
as  is  represented  below: 

OH     OH  ..O 

R 


R'  XR  W  XR  4-  H2O 

A  condition  of  stability  is  then  established  in  the  unsaturated  radical 
by  migration  of  one  of  the  groups  R  from  carbon  to  carbon  with  for- 
mation of  a  ketone.  If  R  represents  a  cyclic  group  this  becomes 
linked  in  the  ketone  by  the  same  carbon  atom  of  the  cycle,  which 
served  to  hold  the  grouping  in  its  original  position  in  the  glycol.3 

In  his  study  of  the  mechanism  of  these  rearrangements  Meerwein  4 
has  revealed  some  very  interesting  and  important  data  regarding  the 
intensity  of  molecular  attraction  or  affinity  of  atoms  and  radicals  in 
glycol  combinations.  For  example,  in  the  splitting  off  of  water,  he 
assumes  that  the  most  loosely  bound  hydroxyl  and  hydrogen  atom 
function  in  this  change;  and  that  the  reactivity  of  the  hydroxyl  group 
is  dependent  directly  upon  the  nature  of  the  respective  radicals  joined 
to  the  same  carbon  atom  in  the  glycol.  Interesting  relations  were 
brought  out  by  a  study  of  the  rearrangements  of  unsymmetrical  pinacone 
combinations : 

O 

->    R^C— CO-R' 
R'/ 


/R 

->    R-CO-C^R' 
\R' 

The  introduction  of  a  benzene  radical  into  a  glycol  increased  strongly 
the  reactivity  of  the  adjoining  hydroxyl  group  or,  in  other  words,  this 

iRev.  gen6r.  (1907),  591. 

2  Annalen  der  Chemie,  396,  200  (1913). 

8  Montagne,  Ber.,  61,  1482  (1918). 

4  Annaleu  der  Chemie,  419,  921  (1919). 


534  THEORIES  OF  ORGANIC  CHEMISTRY 

cycle  neutralized,  so  to  speak,  a  larger  proportion  of  the  valency  of  the 
glycol  carbon.  This  results  in  a  loose  combination  of  hydroxyl  and 
consequently  an  increased  tendency  for  this  group  to  split  off  and  form 
water.  A  study  of  molecular  rearrangements  in  glycol  combinations 
containing  phenyl  and  phenylene  radicals  led  to  the  interesting  result 
that  the  phenyl  radical  absorbs  a  greater  proportion  of  the  carbon 
valency  in  the  glycol  than  a  phenylene  radical,  and  consequently  renders 
more  reactive  its  adjoining  hydroxyl  group.  He  found,  for  example 
that  asi/ra-diphenyl-phenylene-glycol  I  undergoes  a  rearrangement  to 
9,  9-diphenyl-phenanthrone  II  as  is  represented  below: 


OH     OH  /  O  \ 

H<K     |          |  XC6H4  C&R5.    |          |  XC6H4  \ 

--  ->  --        |         I 

C6H4/ 


CO—  C6H4 
6H4 
II 


—   6 
CeHsy 

>C  --  C 
C6H5/ 


In  aliphatic  combinations  a  methyl  radical  neutralizes  or  absorbs  more 
valency  energy  than  an  ethyl  group.  For  example,  dimethyl  diethyl- 
glycol  rearranges  to  ethyl  amylketone: 

OH     OH  O 

C2H5 


CH3        C2H5     CH3         C2H5 


CH3> 

CH3  >C— CO— C2H5 
C2H5" 


Dimethyl  di-normalpropyl-glycol  and  dimethyl  di-normalbutyl 
glycol  rearrange  to  give  mixed  ketones,  and  an  analysis  of  the  products 
of  reaction  revealed  the  interesting  fact  that  a  methyl  and  n-propyl 
group  neutralize  about  an  equal  amount  of  affinity  of  a  carbon  atom 
in  glycol  combinations.  The  ability  of  radicals  to  neutralize  affinity 
is  not  constant  and  directly  proportional  to  the  size  or  molecular  magni- 
tude of  the  respective  radicals  but  changes  periodically,  and  n-alkyl 


MOLECULAR  REARRANGEMENTS 


535 


groups  containing  an  uneven  number  of  carbon  atoms  absorb  more 
affinity  than  those  containing  an  even  number  of  carbon  atoms: 


OH     OH 
H3CV  |          |  /C3H7 

>C C< 

H3C/  XC3H7 

I 


OH     OH 


H3Cv 

H3C-)C— CO— C3H7 
H7C3/ 

la 

/CH3 

H3C— CO— C^C3H7 
\C3H7 
Ib 


H3C\ 
\/~i 

/ 
Ha 

H3C— CO— 


lib 


CH3 
C4H 
\C4H9 


In  both  the  pinacone  and  the  Beckmann  rearrangement  a  phenyl 
group  always  attaches  itself  after  migration  through  a  common  carbon 
atom.  For  example,  p,  p-dichlorobenzophenone  oxime  rearranges 
smoothly  to  p-chlorbenzoyl-p-chloranilide.1 

Reactions  which  involve  the  shifting  of  atoms  and  radicals  from  a 
side  chain  of  the  benzene  ring  into  the  nucleus  are  of  special  impor- 
tance to  an  understanding  of  the  processes  of  substitution  in  the  benzene 
ring.  In  considering,  for  example,  the  change  of  N-chloracetanilide 
into  p-chloracetanilide : 

COCH3  COCH3 


C1C6H4NH 


it  may  be  supposed  either  that  one  molecule  acts  as  a  chlorinating 
agent  with  reference  to  a  second,  or  that  there  is  simply  an  exchange 
of  a  chlorine  for  a  hydrogen  atom  inside  one  and  the  same  molecule. 
In  the  first  case  the  change  belongs  to  the  order  of  bi-  or  polymolecular 
reactions.  Blanksma  actually  found  by  following  the  rate  of  reaction 
experimentally  that  it  was  monomolecular.  Reasoning  by  analogy, 
other  migrations  of  atoms  and  groups  from  the  side  chain  into  the 
benzene  nucleus  might  be  regarded  as  exchanges  taking  place  within 

1  Montagne,  Ber.,  51,  1479  (1918). 


536  THEORIES  OF  ORGANIC  CHEMISTRY 

a  single  molecule.  Certainly  the  splitting  off  of  certain  substituents 
of  side  chains  have  been  observed  frequently.  In  the  rearrangement 
of  phenylnitramine  into  o-  and  p-nitroanilines,  for  example,  the  presence 
of  nitrous  acid  has  been  demonstrated  often,  though  in  such  small 
quantities  as  to  be  almost  negligible  as  far  as  theoretical  considerations 
are  concerned.  E.  Bamberger,  who  made  the  observation,1  says  in 
regard  to  it:  "it  is  not  strange,  in  view  of  the  great  amount  of  chemical 
energy  which  is  stored  up  in  phenylnitramine,  that  a  small  part  of  its 
nitrogen  should  separate  in  the  form  of  nitrous  acid,  even  if  it  were  not 
present  originally  in  the  form  of  nitroso  groups." 

Hantzsch,  in  cooperation  with  his  students  Schleissing  and  Jager, 
has  shown,  by  a  study  of  diazo-compounds,  that  brominated  benzene 
diazonium  chlorides,  rearrange  readily  into  chlorinated  benzene  diazo- 
nium  bromides.  This  rearrangement  has  already  been  referred  to  in 
this  chapter: 

Br3C6H2N.Cl    -*    ClBr2C6H2N.Br 


i 


"  The  conditions  under  which  this  very  remarkable  2  rearrangement  takes 
place  are  the  following:  the  tendency  to  rearrangement  increases  with 
the  increase  in  the  number  of  bromine  atoms  present  in  the  benzene 
ring.  Only  bromine  atoms  in  the  o-  and  p-,  never  those  in  the  m- 
position  may  be  exchanged  for  chlorine.  The  change  takes  place  very 
rapidly  in  mixed  solutions  of  ether  and  alcohol  and  only  slowly  in 
aqueous  solutions.  It  is  slightly  accelerated  by  the  presence  of  free 
hydrochloric  acid,  and  very  greatly  accelerated  by  an  elevation  in 
temperature." 

Now  it  was  conceivable  that  this  transformation  took  place  by  the 
action  of  one  molecule  upon  another,  and  belonged,  therefore,  to  the 
order  of  bimolecular  reactions.  The  rate  of  reaction  in  methyl  alcohol 
measured  by  Hantzsch  and  Symthe,  gave,  however,  a  constant,  agreeing 
not  with  the  calculated  for  a  bimolecular,  but  for  a  mono-molecular 
reaction.  This,  also,  must  be  regarded,  therefore,  as  an  intramolecular 
change  or  one  which  takes  place  within  a  single  molecule. 

Cases  are  known,  however,  where  rearrangements  take  place  which 
involve  the  formation  of  intermediate  products  and  where,  under 
favorable  conditions  these  intermediate  products  may  be  isolated.  The 
transformation  of  imidoethers  into  acid  amides  will  serve  as  an  illus- 

^er.,  30,  1248  (1897). 
2Ber.,  33,  505  (1900). 


MOLECULAR  REARRANGEMENTS  537 

tration.     According  to  W.  Wislicenus  this  reaction  takes  place  in  some 
cases  more  or  less  easily  upon  heating.1 


/OC2H5  ,£> 

HC/  ->    HCf      /CaHs 

^N-C6H5  XN 


< 
XC2H5 

And  Wheeler  and  Johnson  2  found  that  certain  representives  of  mono- 
substituted  imidoether  combinations  go  over  into  substituted  acid 
amides  in  the  presence  of  ethyl  or  methyliodide  at  a  temperature  below 
100°.  In  this  case  it  is  necessary  to  assume  one  of  two  courses  for  the 
reaction.  The  alkyl  halide  first  adds  to  form  an  intermediate  product 
which  may  have  theoretically  two  formulas  depending  upon  the  manner 
of  addition: 

(1)  Addition  with  formation  of  a  pentavalent  nitrogen  derivative: 


IC2H5 


C6H5C/  +  C2H5I 

\NH-C2H5 

(2)  Addition  to  both  carbon  and  nitrogen  with  saturation  of  the 
double  bond  in  the  imidoester: 

/OC2H5  /OC2H5 

C/  +  C2H5I   =  C6H5Cf-I  -> 

^  \NHC2H5 


^ 

C2H5I+C6H5af 
X 


The  rearrangement  of  benzimido-/3-chlorethyl  ester  into  /S-chlor- 
ethylbenzamide  takes  place  in  a  manner  analogous  to  the  interpreta- 
tion of  Wheeler  and  Johnson,  and  has  been  formulated  by  Gabriel 
and  Neumann  3  as  follows  : 


/OCH2-CH2C1  .0 

-  < 

\ 


NH-CH2CH2C1 

JBer.,  33,  1467  (1900). 

'Ber.,  32,  41  (1899);  Am.  Chem.  Jour.,  21,  185  (1899);  23,  140  (1900). 

3Ber.,  25,  2386  (1892). 


538  THEORIES  OF  ORGANIC  CHEMISTRY 

Wislicenus  1  has  since  demonstrated  that  this  change  takes  place  in 
the  sense  of  Wheeler  and  Johnson's  interpretation,  or  by  means  of  so- 
called  intramolecular  alkylation,  and  was  able  to  isolate  the  inter- 
mediate product: 


/OCH2CH2C1  /OCH2CH2C1 

2C6H5C<  - 


X)CH2 


OCH2 

C6H5c 
\ 


2.    C6H5C<:      \          +     HC1    ->    C6H5C/ 

•CH2  \NH-CH2CH2C1 


It  is  obvious  from  this  and  other  transformations  that  there  are 
several  possible  ways  according  to  which  rearrangements  may  take 
place;  and  it  becomes  necessary  to  inquire  into  each  individual  case 
as  to  whether  a  genuine  intramolecular  exchange  of  atoms  or  groups 
of  atoms  has  occurred,  or  whether  an  interaction  has  taken  place  between 
molecules.  In  answering  this  question  physico-chemical  methods  are 
of  the  greatest  importance  in  supplementing  purely  chemical  investi- 
gation. 

It  is  far  from  easy  to  explain  satisfactorily  at  the  present  time,  in 
terms  of  structural  chemistry,  the  mechanism  of  such  rearrangements 
as  take  place  entirely  within  an  organic  molecule,  especially  when  the 
intermediate  products  cannot  be  isolated.  In  such  cases  violent  and 
sudden  displacements  both  of  atoms  and  of  organic  radicals  seem  to 
occur.  Some  investigators  believe  that  a  direct  dissociation  of  unsatu- 
rated  radicals  from  the  molecule  is  the  only  assumption  left  open  in 
explaining  these  changes,  and  Nef  s  theories  consequently  have  found 
frequent  application  in  such  cases  though  in  a  somewhat  modified 
form.2 

Rearrangements  of  this  type  are  often  represented  as  resulting  from 
the  mobility  of  the  atoms  inside  the  molecule,  and  assumptions  are  made 
regarding  the  form  and  movements  of  certain  atoms.  Thus  the  system  of 
atoms  in  a  compound  capable  of  undergoing  rearrangement,  may  be 
imagined  as  being  in  a  condition  of  mobile  equilibrium  between  two  or 
more  stable  phases.  In  the  course  of  a  series  of  changes  molecular  aggre- 

iBer.,  35,  164,  1991  (1902). 

2  Nef,  Annalen  der  Chemie,  298,  307  (1897);  318,  137  (1901);  Jour.  Am.  Chem. 
Soc.,  26,  1564  (1904);  J.  von  Braun,  Ber.,  38  (1905),  44  (1911). 


MOLECULAR  REARRANGEMENTS  539 

gates  are  passed  through  which  are  not  capable  of  existing  as  chemical 
compounds.  A.  Lapworth,1  more  than  any  other,  has  succeeded  in  repre- 
senting the  phenomena  of  rearrangements  in  general  terms,  that  are 
in  harmony  with  the  prevailing  conceptions  in  regard  to  the  carbon 
atom.  His  elaborations,  which  have  faults  common  to  all  explanations 
which  are  based  upon  the  conception  of  the  mobility  of  the  atoms,  may 
be  summarized  briefly.  Lapworth  2  points  out,  in  connection  with 
Armstrong's  3  theoretical  deductions,  that  molecular  rearrangements 
(inclusive  of  tautomerism  and  desmotropism)  take  place  only  in  the 
case  of  certain  well  denned  groupings  of  the  atoms.  Of  these  groups 
atoms,  two  may  be  considered  first,  because  they  stand  out  as  pre- 
eminent in  the  tendency  of  each  to  oass  readily  into  the  other,  i.e., 


R 


I  •  E2   •  E3        <=^       EI   I  E2  •  E  •  3 

I  R' 


where  EI,  E2,  E3,  signify  elements  as  C,  N,  0,  S,  etc.,  and  RI,  R2,  etc., 
organic  substituents  bound  to  these  elements. 

If  now,  as  is  usually  the  case,  a  mobile  atom  or  radical  R2  as  for 
example,  a  hydrogen  atom,  is  bound  to  E3,  the  scheme  represented 
in  I  assumes  the  following  form  : 

Ei-E2:E3     <±    EI  :  E2E3     «=*    EiE2  :  E3 
RI  R2  II      RI    R2         R2        RI 

This  rearrangement  may  proceed  in  one  direction  quite  as  readily  as 
in  the  other,  and  in  the  case  of  tautomeric  substances  generally  operates 
in  both  ways. 

Lapworth  4  represents  the  mechanism  of  this  process  in  the  special 
•case  Ei=E2=E3  =  C  by  means  of  a  diagram.  In  this  scheme  the 
carbon  atoms  are  assumed  to  lie  at  the  centers  of  tetrahedra,  and  the 
complete  molecule  is  imagined  as  projected  upon  the  plane  of  the  paper 
in  such  a  manner  that  the  characteristic  unions  lie  in  this  plane.  Under 
these  circumstances  only  the  upper  surfaces  of  the  tetrahedra  are  shown, 
the  incidence  of  two  sides  signifying  a  union  by  means  of  double  bonds, 
and  the  incidence  of  two  corners,  a  union  by  means  of  single  bonds. 

1  Jour.  Chem.  Soc.,  73,  445  (1898). 

2  Jour.  Chem.  Soc.,  73,  446  (1898). 

3  Jour.  Chem.  Soc.,  61,  258  (1887). 

4  Jour.  Chem.  Soc.,  73,  445  (1898). 


540 


THEORIES  OF  ORGANIC  CHEMISTRY 


The  following  figure  applies  to  the  second  equation,  II,  arrows  indicat- 
ing the  direction  in  which  the  atoms  move: 


Ill, 


YL 

It  is  not  necessary  to  assume  that  the  labile  groups,  R,  and  R2  (repre- 
sented in  the  diagram  by  O  and  •),  are  in  reality  free  groups;  but  rather 
that  each  is  at  all  times  under  the  influence  of  the  specific  attraction 
of  Ci,  or  C2,  while  coming  at  given  moments  within  the  spheres  of 
attraction  of  both  of  these  carbon  atoms. 

If  such  a  process  is  reversible,  we  have  the  phenomenon  of  tautomer- 
ism;  if  it  has  a  definite  tendency  to  proceed  in  either  one  direction  we 
have  the  phenomenon  of  intramolecular  rearrangement  (including 
desmotropism).  The  process  may  be  only  partially,  or  wholly  complete. 
The  following  reactions  adapt  themselves  to  this  scheme: 


Ei-E2  :  E3 
(R)ffS-C  ;  N 

Thiocyanic  acid  and 
derivatives 

(R)#0-C;N 

Cyanic  acid 

#N  :  C  :  NH 

Carbodiimide 

#0-C=N 

Lactim 

#C-N  :  O 

Nitroso 

tfC-N:  0 


EI  :  E2  •  ES 
S  :  C  :  N#(R) 

Isothiocyanic  acid  and 
derivatives 

O  :  C  :  N#(R) 

Isocyanic  acid 


Cyanamide,  eto- 

O  :  C  :  N# 

Lactam 

C  :  N-O# 

Isonitroso 

C:N-0# 


A 


Nitro 
E2  * 


Isonitro 


:  E2-E3 


MOLECULAR  REARRANGEMENTS  541 

HO  •  C  :  CH  -  COOC2H5  «=±  0  :  C  •  CH#  -  COOC2H5 
CH3                                          CH3 

Esters  of  2-hydroxycrotonic  Acetoacetic  acid  esters 

acid 

EI  •  E2  •  E3  — >  EI  i  E2  •  E3 

C2H5O.C:CH2  ->  0:C.CH2.C2H5 


If,  now,  other  unsaturated  groups,  as  £4  :  Es,  are  joined  to  the  chain 
R-Ei-E2  :  ES  at  the  point  E3,  analogous  oscillations  of  the  tetrahedra 
give  rise  to  the  rearrangements: 

Ei-E2  :  E3-E4  :  E5   ^    EI  :  E2-E3-E4  :  E5    <=±    EI  :  E2-E3  :  E4-E5 


III 

The  following  reactions  adapt  themselves  to  this  scheme: 


CN  :  0         -*    0  :  C<f         ^C-N  :  0 

CH 


N:0 

0:C 


and 

C_C-  C-     C- 

p  -/        \p 

(^  •  sT  /^  *  — * 

\ /  \ / 

HOC      C-N:N-C6H5  0=C      C-N  :  NC6H5 

C-     C- 


O=C 


In  those  cases  where  hydrogen  atoms,  or  other  labile  substituents, 
are  in  union  with  E3  and  ES  rearrangements  of  the  system  may  be  repre- 
sented thus: 

ETHI      .    T7i       TTi      •    TP         k        T7^       TJ^       •    "C^       TT'      •    T^  \        T7^       TT^      •    t^       "C^      •    "C^ 

l*jii2  •  H/3'Ji(4  .  His     ^      ±!ji  •  Ji/2  .  Jcj3'±L<4  .  Ji/s     <=£     ±!ji*Ji/2  .  li/8'^  .  Ji/s 

III        III         III 

Ri          R2          R3  R2          Ri      R3  R2          R3          Ri 

IV 


542  THEORIES  OF  ORGANIC  CHEMISTRY 

Rearrangements  of  this  type  are  especially  conspicuous  among  the 
derivatives  of  benzene;  and  all  reactions  analogous  to  the  sulphona- 
tion,  nitration,  etc.,  of  aniline  adapt  themselves  to  this  scheme: 

CH=CHV  /CH=CHV 

HS03-NHC<  >CH    ->    H2N-C<  >CH 


E3       E4  | 

SO3H 


CH=CH 


also  the  formation  of  toluidines  from  monomethylaniline  : 

V 

CH    -> 


X 

C-CH3 


and,  further,  the  semidene  and  benzidine  rearrangements: 
HN-NHC6H5  NH  NH 

i  4 


\ 

HC     C 


;H                HC    c<  HC    C^-H 

-        II       IXH  || 

HC     CH                       HC     CH  HC     CH 

\/  \s 

CH  C-NH-C6H5 
NH2                            NH2 


i  A 

/\  /\ 

HC     CH  HC     CH 

II       I  ^        II       I 

HC     CH  HC     CH 

\/ 

C-C6H4NH2,etc. 


N^/  V> 

Xci--  ^ 


Having  explained  the  formation  of  o-  and  p-derivatives  of  benzene 
by  means  of  a  general  scheme,  Lap  worth  next  sought  to  interpret  sub- 
stitution in  the  m-position.  To  this,  and  to  those  cases  of  rearrange- 


MOLECULAR  REARRANGEMENTS  543 

ment  which  involve  the  elimination  of  a  simple  molecule  from  the  com- 
plex, it  is  impossible  to  do  more  than  to  refer  in  passing. 

Views  in  regard  to  the  mechanism  of  intramolecular  rearrangements 
which  depend  upon  the  ability  of  multivalent  atoms  to  change  their 
position  inside  the  molecule,  have  also  been  developed  by  E.  Erlen- 
meyer,  Junior.1 

Recently  M.  Tiffeneau,2  in  a  treatise  well  worth  reading,  has  con- 
sidered those  rearrangements  which  may  be  regarded  as  genuinely 
intramolecular  in  character.  He  divides  them  into  two  large  groups, 
comprising,  first,  those  rearrangements  which  are  complete  in  them- 
selves without  addition  or  elimination  of  atomic  complexes  and  where 
the  product  of  the  reaction  has  the  same  per  cent  composition  as  the 
original  substance,  as  for  example,  the  rearrangement  of  phenylhy- 
droxylamine  into  amidophenol,  and  of  phenyl  sulphaminic  acid  into  sul- 
phanilic  acid.  According  to  Tiffeneau  the  mechanism  of  reactions  of 
this  type  consists  first  in  the  cleavage  of  certain  atomic  linkages,  followed 
by  a  reorganization  of  the  molecular  structure  which  results  in  a  change 
in  the  relative  position  of  atoms  or  groups  of  atoms. 

In  the  second  group  are  included  all  rearrangements  which  involve 
the  elimination  of  hydrobromic  or  hydrochloric  acid,  water,  nitrogen, 
etc.  Previously  it  had  been  assumed  that,  with  the  loss  of  these  simple 
compounds  and  elements,  intermediate  products  were  formed  which, 
while  not  necessarily  stable  substances,  could,  nevertheless,  be  repre- 
sented structurally  as  having  saturated  valencies  between  all  of  the 
atoms.  In  such  cases  it  was  usually  supposed  that  the  exchange  of 
groups  took  place  at  the  very  beginning  of  the  process  and  was  due  to 
a  kind  of  contact  action  produced  by  the  particular  reagent  employed 
to  induce  the  change.  Tiffeneau  came  to  quite  a  different  conclusion 
as  the  result  of  his  observations.  He  believes  that  in  all  intramolecular 
rearrangements  the  shifting  of  groups  takes  place  not  at  the  beginning 
of  the  operation  but  at  a  later  period.  Instead  of  intermediate  products 
which  are  structurally  possible  and  which  are  formed  as  the  result  of 
initial  changes,  he  assumes  intermediate  systems  which  possess  free 
valencies,  and  which  are  not,  as  such,  capable  of  an  independent  exist- 
ence, but  of  necessity  rearrange  to  give  structurally  possible  compounds. 

According  to  Tiffeneau  every  rearrangement  takes  place  in  two 
phases.  There  is  first  formed,  as  a  result  of  the  action  of  the  reagent 
used  to  induce  the  change,  a  phase  of  disorganization  resulting  in  systems 
of  atoms  possessing  free  valencies.  In  the  second  phase  or  that  of 
reorganization,  the  atoms  progressively  rearrange  to  form  stable 

1  Annalen  der  Chemie,  316,  75  (1901). 

2  Revue  gen.  Sci.  pur.  et  appli.  (1907),  583. 


544  THEORIES  OF  ORGANIC  CHEMISTRY 

chemical   compounds.      Both  phases  probably  take  place   simultane- 
ously. 

In  an  effort  to  elucidate  the  chemistry  of  intramolecular  rearrange- 
ments, G.  Schroeter  has  recently  tried  to  consider  from  a  common  point 
of  view  those  transformations  which  have  been  referred  to  in  this  text 
as  the  Hofmann,  Curtius,  Beckmann  and  benzilic  acid  rearrangements. 
Previous  to  this  time,  Stieglitz,1  had  attempted  to  explain  the 
rearrangements  of  Hofmann  and  Curtius  by  assuming  the  formation 
of  intermediate  products  which  contain  univalent  nitrogen  and  which 
subsequently  rearrange  to  give  esters  of  isocyanic  acid. 

R-CO-N/    ->     RN=C=O 


As  a  matter  of  fact,  even  though  these  intermediate  esters  have  never 
been  isolated,  their  presence  has  frequently  been  detected  in  the  course 
of  the  Hofmann  rearrangement  because  of  their  penetrating  odors. 
They  must  also  be  formed  in  the  course  of  the  Curtius  rearrangement  :  2 

/N 

C6H5CON<J|    ->    N2+C6H5CON=    -4 

C6H5N=C=O     °2H5QH)     C6H5NHCOOC2H5 

Urethane 

According  to  Stieglitz  the  Hofmann  rearrangement,  which  is  brought 
about  by  the  action  of  alkaline  reagents  is  to  be  interpreted  as  following 
the  course  indicated  below: 

/H  /Na 

R-CO-N<        +NaOH=RCO-N<  +  H2O     -» 

\  \ 


NaBr    ->     RN=C=O 

Isocyanates  have  been  isolated  as  products  of  this  rearrangement. 
Stieglitz  3  has  shown  also  that  the  dry  salts  of  the  halogenated  acid 
amides  decompose  when  heated  giving  isocyanates: 


XNa 
\ 


RCO-N<f  Hea\  NaBr+RN=C=0 


i  Am.  Chem.  Jour.,  18,  751  (1896);  29,  49  (1903). 

2Ber.,  45,  1058  (1912). 

3  Jour.  Am.  Chem.  Soc.,  36,  273  (1914). 


MOLECULAR  REARRANGEMENTS  545 

This  last  change  is  perfectly  analogous  to  that  which  takes  place  when 
acylazides  are  heated. 

Schroeter l  has  shown  that  the  azides  of  organic  acids  are  decom- 
posed almost  quantitatively  when  heated  into  free  nitrogen  and  isocyanic 
acid  esters: 


<N        TTpof  / 

||         ea  >  N2  +  C6H5CON/     ->    C6H5N=C=0 
N  ^ 


According  to  Stieglitz  when  the  formation  of  univalent  nitrogen 
is  interfered  with,  as  in  the  case  of  alkyl  derivatives  of  acid  amides, 
no  rearrangement  is  brought  about  by  the  reagents  which  favor  such 
rearrangements  in  the  case  of  non-alkylated  compounds. 

In  order  to  apply  his  theory  to  the  Beckmann  rearrangement, 
Stieglitz  was  forced  to  postulate  that  hydrochloric  acid  is  the  catalytic 
agent  which  effects  this  change.  According  to  his  interpretation  this 
reagent  adds  at  the  double  bond  giving  an  intermediate  product  cor- 
responding to  Formula  I,  which  then  loses  water  in  the  presence  of 
the  dehydrating  agent  (phosphorus  pentachloride)  and  passes  into  the 
derivative,  II,  containing  univalent  nitrogen.  This,  being  unstable, 
then  rearranges  to  III,  which  finally  interacts  with  water  to  form  the 
anilide  IV,  according  to  the  following  scheme : 2 

Rv  rrn       Rx      /Cl  Rx      /C\ 

\C=NOH  f^;      >c<     ->        >c<        -» 

R'/  R'/     \N-OH    R'/     \N_ 

I        H  II 


< 


tll*    RCONHR' 
NR' 
III  IV 


Objections  to  this  interpretation  have  been  brought  forward  by 
Montagne;3  but  the  observations  of  Schroeter  would  seem,  neverthe- 
less, to  support  it.  Since  the  intermediate  products  between  I  and  IV 
could  not,  in  the  first  instance,  be  isolated  Schroeter  tried  out  the  reac- 

'Ber.,  42,  2337,  3356  (1909);  44,  1201  (1911). 

2  Stieglitz,  Am.  Chem.  Jour.,  18,  754  (1896);  46,  327  (1911). 

'Ber.,  43,  2014  (1910). 


546  THEORIES  OF  ORGANIC  CHEMISTRY 

tion  with  compounds  which  possessed  a  certain  similarity  in  constitution 
to  I,  i.e.,  with  the  azide-chlorides  or  diazides  of  ketones: 

/N 

Cl  N\H 

Ra  :  C/     /N       and      R2  :  C/       S 


He  reasoned  that  such  compounds  were  capable  of  decomposing  in  a 
manner  analogous  to  that  which  has  just  been  given  in  the  scheme 
representing  the  Beckmann  transformation,  i.e., 

/Cl  /Cl  /Cl 

R2C<      /N    -»    N2+R2C<         -*    RC/ 

\Ncll  XN=          ^N-R 


N 
and  R2C</       £    -*    N2 

N/  N-R 

\N 

In  the  second  case  it  was  to  be  expected  that  the  product  would  undergo 
the  further  change: 

/N 
N< 


N 
R-C 


Schroeter  and  his  co-workers  actually  succeeded  in  transforming  di 
phenylmethane  diazide  (V)  into  N-a-diphenyltetrazol  (VI) 

N 
N 


(C6HS)2C  ^    C6H6C          ||    +  N2 


VI 


MOLECULAR  REARRANGEMENTS  547 

In  the  meantime  F.  Henrich  and  Ruppenthal 1  had  discovered  hydro- 
chloric acid  addition  products  of  the  oximes  in  the  Beckmann 
rearrangement,  and  had  proved  that  these  were  definitely  the  halogen 
salts  of  the  oximes.  On  treatment  with  phosphorus  pentachloride 
these  halides  pass  immediately  into  yellow  chlorides  as  represented 
in  III;  and  which  interact  with  water  to  give  the  acid  anilides. 
These  investigators,  therefore,  concluded  that  the  formula  for  the  inter- 
mediate product  I,  as  given  in  the  above  scheme  of  Stieglitz's  for  the 
Beckmann  rearrangement,  must  be  changed  to  R2C=N  •  OH.  Accord- 


Cl 

ing  to  Henrich  and  Ruppenthal,  so  far  as  present  evidence  goes,  only 
those  oximes  which  give  hydrogen  chloride  addition  products  are  cap- 
able of  undergoing  rearrangement  in  the  presence  of  phosphorus  penta- 
chloride. Hydrochloric  acid  must,  therefore,  be  regarded  as  having  a 
definite  chemical  action  in  the  Beckmann  rearrangement. 

A  salt  having  the  constitution  assigned  by  Henrich  would  undergo 
dehydration  giving  as  an  intermediate  product  the  chlorimido  com- 
bination : 


R\ 
H20  +       >C  :  NCI 

R/ 


Halogen  combinations  of  this  type,  however,  do  not  undergo  the  Beck- 
mann rearrangement  according  to  Stieglitz  and  Peterson.2  Therefore, 
the  view  that  they  form  intermediate  products  seems  to  be  untenable. 

The  observation  by  Stieglitz  and  Leech3  that  a  rearrangement  of 
/3-triphenylmethyl-/3-methylhydroxylamine  is  effected  by  the  action  of 
phosphorus  pentachloride,  has  forced  new  speculations  regarding  the 
mechanism  of  the  Beckmann  rearrangement: 


OH  HO/         XC6H5 

Here  the  formation  of  a  simple  univalent  nitrogen  derivative  RaCN 
is  impossible.     This,  therefore,  represents  a  case  where  the  facts  weigh 

^er.,  44,  1533  (1911);  Schroeter,  Ibid.,  44,  1205  (1911). 

2  Am.  Chem.  Jour.,  46,  329  (1911);  Ber.,  43,  782  (1910). 

3  Jour.  Am.  Chem.  Soc.,  36,  272  (1914). 


548  THEORIES  OF  ORGANIC  CHEMISTRY 

in  favor  of  Beckmamrs  theory  of  the  direct  exchange  of  radicals.1 
The  latest  development  in  this  field  of  speculation  is  the  presentation 
of  a  new  interpretation  of  the  rearrangement  of  ketoximes  based  on 
the  application  of  the  electronic  conception  of  organic  structure.  The 
rearrangement  is  expressed  by  this  theory  as  follows: 2 


R 

R  R 

++  +      -  +   -+++  +- 

C  :  N— Cl    -»    R  . .  C  I  N_—    Cl > 

I 

RCC1(:NR)       H2°>     RCONHR 

The  conclusions  of  Stieglitz  regarding  the  mechanism  of  these 
transformations  are  summarized  in  the  following  abstracts  from  one 
of  his  recent  papers:3 

(a)  Unless  conclusive  evidence  is  brought  that  salts  leading  to  the 
intermediate  formation  of  salts  of  univalent  nitrogen  derivatives,  are 
involved  in  their  rearrangement,  the  rearrangement  of  /3-triphenyl- 
methyl  /3-methylhydroxylamine,  established  in  this  paper,  the  rela- 
tion of  stereoisomerism  of  oximes  to  their  rearrangement  products, 
established  by  Beckmann,  Werner  and  Kuhara  4  would  be  inconsistent 
with  the  theory  of  the  intermediate  formation  of  univalent  nitrogen 
derivatives  in  the  rearrangement  of  hydroxylamines  and  at  present 
these  facts  agree  better  with  Beckmann's  theory  of  a  direct  exchange 
of  radicals  or  with  a  modification  of  this  theory  (electronic  explanation). 
Intermediate  salt  formation,  if  established,  would  harmonize  these 
facts  with  the  univalent  nitrogen  theory. 

iBer.,  27,  300  (1894). 

2  Stieglitz  and  Leech,  Jour.  Am.  Chem.  Soc.,  36,  281  (1914). 

3  Stieglitz  and  Stagner,  Jour.  Am.  Chem.  Soc.,  38,  2064  (1916). 

4  Mem.  Coll.  Sci.  Eng.  Kyote,  1,  254  (1903-8);  2,  367  (1909-10);   6,  1  (1913); 
7,  25  (1914). 


MOLECULAR  REARRANGEMENTS  549 

(b)  Other  facts,  such  as  the  rearrangements  of  azides  and  especially 
the  non-rearrangement  of  chlorimido-benzophenones,  of  chlorimidoesters, 
and  /3-triphenylmethyl-j3-methylchloramine,  are  inconsistent,  without 
further   specific   assumptions,    with   Beckmann's   theory   of   a   direct 
exchange  of  radicals;    but  are  in  striking  agreement  with  Stieglitz's 
theory  of  the  intermediate  formation  of  unsaturated  nitrogen  deriva- 
tives in  the  rearrangements  of  chloramines,  hydroxylamines  and  azides. 

(c)  Unless  further  experimentation  should  modify  these  facts  or 
their  bearing,  one  should  consider  that  both  types  of  rearrangement 
may  take  place. 

(d)  Common  to  both  theories,  and  the  most  important  feature  in 
their  modern  forms,  is  that  the  rearrangements  originate  from  the 
tendency  of  unstable  positive  atoms  Cl+,  ~O+,  =N+,  etc.,  to  go  over 
into  their  stable  negative  forms  Cl~,  ~O~,  N=,  by  a  capture  of  electrons 
from  other  atoms  in  the  same  molecule,  a  change  which  is  effected  in 
the  rearrangements  in  question.     It  is  quite  consistent  with  this  funda- 
mental relation  that  the  rearrangement  should  go  over  one  or  the  other 
path,  the  one  over  the  unsaturated  nitrogen  derivatives  forming  prob- 
ably the  path  of  least  resistance. 

The  most  important  theories  advanced  in  explanation  of  the 
"  Beckmann  rearrangement  "  previous  to  1903  have  been  discussed 
by  Stieglitz  in  the  American  Chemical  Journal.1 

J.  U.  Nef2  has  explained  the  benzilic  acid  rearrangement  by  the 
following  scheme: 

,OH 
C6H5CO 


|  +  H20  =      XOH 
C6H5CO         C6H5CO 

Benzil 


/~1    TT  /"VTT 

V^6Al5\  \  yUll 

>CO  +    >C<  ->    HCOOH 

c6H5/          /    XOH 


^OH 

,OH 
+  HCOOH    =  >C<C 

CoHs/     XCOOH 

Benzilic  Acid 

According  to  this  interpretation  benzophenone  and  formic  acid  should 
react  to  give  benzilic  acid,  but  this  has  not  as  yet  been  accomplished. 

1  Am.  Chem.  Jour.,  29,  49  (1903). 

2  Annalen  der  Chemie,  298,  372  (1897) 


550  THEORIES  OF  ORGENIC  CHEMISTRY 

G.  Schroeter,  therefore,  modified  Nef's  scheme,  on  the  basis  of  his 
experimental  investigations,  to  the  following: 


OK 

+  KOH   =  XOH 


C6H5CO  C6H5C/ 

|    +  KOH   = 
C6H5CO  C6H5CO 


OK        C6H5C= 
OH        C6H5CO 

Diphenylketene 
I 

C6H5x  OH  C6H5x      /OH 

\c=c=o  +  |      -*          >c< 

C6H5/  OK  C6H5/     \COOK 

II 

While  the  energy  involved  in  the  reaction  of  diphenylketenes  makes 
the  isolation  of  intermediate  products  impossible,  Schroeter  and  Wachen- 
dorf  were,  nevertheless,  able  to  obtain  diphenylketenes  from  benzil  by 
starting  with  azo-benzil.  This  substance,  when  dissolved  in  an  indif- 
ferent solvent  and  gently  heated,  gave  a  good  yield  of  diphenylketene 
together  with  nitrogen  gas. 


I    N   ->  N2  +  -»  c==c==o 

C 


C6H5CO  C6H5CO 

These  views  of  Schroeter  have  been  tested  recently  by  Nicolet  and 
Pelc  l  and  found  to  be  untenable.  Tiffeneau  2  recognized  an  analogy 
between  the  benzil  rearrangement  and  that  of  pinacone  into  pinacoline, 
but  Michael  3  has  recently  criticized  severely  his  explanations  and 
draws  the  conclusion  that  the  benzil  rearrangement  is  caused  by  "  the 
great  positive  energy  of  the  alkali  employed  as  reagent."  Lachman4 
has  recently  produced  experimental  evidence  that  tends  to  rule  out 
the  hypotheses  of  Nef,  Tiffeneau  and  Michael.  According  to  him  no 
one  of  the  explanations  of  the  benzil  rearrangement  is  capable  of  account- 
ing for  the  actual  facts.  The  rearrangement  of  dihydroxy-tartaric  acid 
into  tartronic  acid  5  by  the  action  of  alkali  and  that  of  benzil  into 

Uour.  Am.  Chem.  Soc.,  43,  935  (1921). 

2  Rev.  Gen.  Sci.,  18,  584  (1907) 

3  Jour.  Am.  Chem.  Soc.,  42,  812  (1920). 

4  Jour.  Am.  Chem.  Soc.,  44,  330  (1922). 

6  Jour.  Am.  Chem.  Soc.,  43,  2091  (1921). 


MOLECULAR  REARRANGEMENTS  551 

benzilic  acid  are  considered  by  him  as  transformations  of  a  similar  type, 
and  he  views  the  changes  as  not  only  rearrangements  but  also  as  intra- 
molecular oxidation-reduction  processes.  He  finds,  for  example,  that 
benzil  can  be  rearranged  to  benzilic  acid  by  heating  with  water  alone 
and  that  the  transformation  is  accelerated  by  alkali.  His  interpreta- 
tion of  the  change  calls  for  an  addition  of  water  to  each  carbonyl  group 
of  benzil  giving  an  unstable  product  corresponding  to  formula  I.  This, 
then  undergoes  a  rearrangement  with  exchange  of  a  mobile  hydroxyl 
group  for  a  phenyl  radical  giving  the  compound  II.  This,  then  looses 
water  with  formation  of  the  final  product  of  rearrangement  namely, 
benzilic  acid. 

2H20 
C6H5'CO.CO.C6H5  »  C6H5C(OH)2.C(OH)2CGH5 

Benzil  j 

-»     (C6H5)2C(OH).C(OH)3    -*    H20+(C6H5)2C(OH).COOH 

TT  Benzilic  acid 


CHAPTER  XX 
THE  BASIC   PROPERTIES   OF   OXYGEN 

IN  1869  Blomstrand  1  and  in  1871  Mendelejeff  2  pointed  out,  on  the 
basis  of  purely  theoretical  considerations,  that  oxygen  could  react  as 
an  element  possessed  of  more  than  two  valencies.  Four  years  later 
this  assumption  was  confirmed  experimentally  by  Friedel,3  who  was  able 
to  show  that  dimethylether  combines  with  hydrochloric  acid  to  give 
a  well-defined  chemical  compound  having  the  formula  (CHa^O-HCl. 
In  discussing  the  constitution  of  this  body  Friedel  came  to  the  con- 
clusion that  oxygen  functioned  as  a  tetravalent  element.  A  year  later 
Baeyer  and  Fischer  4  discovered  addition  products  of  fluorescein  with 
sulphuric  acid  and  of  orcinphthalein  with  hydrochloric  acid.  In  1877 
van't  Hoff  in  his  "  Ansichten  iiber  die  organische  Chemie  "  pointed 
out  the  analogy  between  oxygen  and  sulphur  as  elements  belonging 
to  the  same  group  in  the  periodic  system,  and  indulged  in  speculations 
regarding  the  additive  properties  of  oxygen.  Although  addition  reac- 
tions are  not  met  with  so  frequently  in  the  case  of  oxygen  as  in  the  case 
of  sulphur,  nevertheless,  four  valencies  appear  in  a  great  many  oxygen 
compounds.  According  to  van't  Hoff  the  second  pair  of  bonds  differ 
in  their  chemical  properties  from  the  first  pair.  While  the  first  two 
valencies  are  distinctly  negative  in  character  the  third  and  fourth, 
although  weaker,  are  positive  and  impart  a  basic  character  to  oxygen. 
In  the  succeeding  years  still  other  compounds  representing  new  com- 
binations of  acids  with  substances  containing  oxygen  were  discovered 
and  the  results  of  physico-chemical  investigations  demonstrated  the 
tetravalence  of  oxygen.5 

The  idea  of  the  quadri valency  of  oxygen  was  not  generally  accepted, 
however,  until  about  1899.  In  this  year  Collie  and  Tickle  6  discovered 

1  Chemie,  der  Jetztzeit,  1869. 

2  Ostwald's  Klassiker,  No.  68,  pp.  108. 
s  Bull.  soc.  chimie,  24,  166,  241  (1875). 
4  Annalen  der  Chemie,  183,  1  (1876). 

5Bruhl,  Ber.,  28,  2847,  2866  (1895);  30,  163  (1897);  33,  1710  (1900);  Walden, 
Ber.,  34,  4185  (1901);  35,  1764  (1902). 

6  Jour.  Chem.  Soc.,  75,  710  (1899);  85,  971  (1904);  compare  Baeyer  and  Piccard, 
Annalen  der  Chemie,  384,  208  (1911);  407,  332  (1915). 

552 


THE  BASIC  PROPERTIES  OF  OXYGEN  553 

that  dimethyl  pyrone  (I)  showed  a  very  close  similarity  to  dimethyl 
pyridone  (II)  in  its  behavior  toward  acids : 

CO  CO 

HC/NCH 

CH3cl     Jc-CH3 


I 

Like  the  latter  it  combines  with  mineral  and  other  acids  in  aqueous 
solution  to  form  salts  as,  for  example,  (CH3)2C5H2O2-HC1.  These 
salts  combine  with  the  chlorides  of  the  heavy  metals,  etc.,  to  form 
complex  double  salts  in  a  manner  closely  analogous  to  the  behavior 
of  the  hydrochlorides  of  the  amines.  Collie  and  Tickle  assumed,  there- 
fore, that  oxygen  resembles  nitrogen  in  possessing  unsaturated  residual 
affinities,  although  in  a  less  pronounced  degree.  Since  almost  all  organic 
compounds  which  contain  oxygen  react  with  acids  and  alkalies,  it  seemed 
quite  possible  that  the  cause  for  this  reactivity  might  be  found  in  the 
residual  affinities  of  the  oxygen.  In  order  to  explain  salt  formation 
in  terms  of  this  conception  it  was  supposed  that  oxygen  is  capable  of 
replacing  nitrogen,  phosphorus,  and  sulphur  in  the  basic  compounds 
of  these  elements.  Compounds  arising  in  this  way  are  to  be  regarded 
as  derivatives  of  a  hypothetical  base,  oxonium  hydroxide,  HsO-OH, 
which  belongs  to  the  same  general  group  as  EUN-OH,  EUPOH,  HaSOH, 
H2IOH,  H^As-OH,  and  whose  salts  can  consequently  be  designated 
as  oxonium  salts.  Collie's  assumptions  in  regard  to  the  structural 
formulas  of  dimethyl  pyrone  and  its  salts  are  supported  by  the  work 
of  Homfray  on  molecular  refraction  1  and  also  by  the  spectroscopic 
measurements  of  Baly.2 

In  1899  F.  Kehrmann3  arrived  independently  at  the  same  con- 
clusions as  Collie  and  Tickle  as  the  result  of  a  careful  study  of  the  prop- 
erties of  three  dyes  which  were  at  that  time  represented  structurally 
by  means  of  the  following  formulas : 
NH2 


HC1  I  I         -HC1 


S 

(Oxazine  derivative)  (Thiazine  derivative) 

I  II 

1  Jour.  Chem.  Soc.,  87,  1443  (1905). 

2  Jour.  Chem.  Soc.,  96,  144  (1909). 

3  ROT-       39     9RO1     HSQQV      Armalon    Aor    PVmrm'p     379     9»7   (1 Q1  frt 


554  THEORIES  OF  ORGANIC  CHEMISTRY 


H2N 


C6H5       Cl 

(Safranine  derivative.) 
Ill 

These  three  combinations  showed  such  close  similarity  in  physical  and 
chemical  properties  that  Kehrmann  concluded  that  the  above  formulas 
I  and  II  did  not  represent  the  true  constitution  of  the  salts  of  the  oxazine 
and  thiazine  derivatives.  When  after  very  thorough  investigation  it 
was  finally  established  that  the  chloride  III  is  to  be  regarded  as  an 
orthoqumoid  azoxonium  compound,  Kehrmann  decided  that  all  three 
combinations  were  salts  of  the  same  order  and  changed  the  formulas 
of  I  and  II  to  conform  with  this  constitution,  representing  them  as  the 
oxonium  salts  la  and  Ha: 


H2N/    V  j/   v  H2N 


According  to  this  conception  these  salts  may  therefore  be  regarded  as 
derived  from  the  hypothetical  oxonium  bases: l 


and 

H2N 

OH 

1  Kehrmann  and  Lowy,  Ber.,  44,  3006  (1911). 


THE  BASIC  PROPERTIES  OF  OXYGEN  555 

The  most  recent  investigations  of  Kehrmann l  and  of  Pummerer2 
seem,  however,  to  favor  a  return  to  the  original  p-quinoid 
formulas. 

Since  in  addition  to  oxygen  and  sulphur,  nitrogen  is  also  present 
in  all  of  the  compounds  investigated  by  Kehrmann  his  interpretation 
is  open  to  the  objection  that  nitrogen,  rather  than  oxygen  and  sulphur, 
functions  in  salt  formation  and  is,  therefore,  responsible  for  the  basic 
properties  of  these  substances. 

In  order  to  meet  this  objection  Werner  3  undertook  the  investigation 
of  cyclic  oxygen  compounds  in  which  nitrogen  was  not  present.  He 
started  with  xanthone  (I),  a  derivative  of  pyrone,  which  on  reduction 
passes  readily  into  the  corresponding  alcohol  xanthydrol  (II). 


CO 


vxOH 
C 


II 


He  found  that  this  latter  compound  forms  salts  with  acids  but  that  in 
so  doing  it  does  not  react  according  to  the  above  formula  (II)  but  in 
a  tautomeric  manner  corresponding  to  the  isomeric  formula  (III). 


To  distinguish  between  these  two  compounds  Werner  calls  the  former 
(II)  the  pseudoxanthonium  base  and  the  latter  (III)  the  true  xanthon- 
ium  base.  According  to  this  conception  the  yellow  solutions  fonned 

iBer.,  47,  1889  (1914). 
JBer.,  46,  2310  (1913). 

3  Ber.,  24,  3300  (1901);  also  "Neuere  Anschauungen  auf  dem  Gebiete  der  anor- 
ganischen  Chemie,"  3d  Ed.,  p.  255,  Braunschweig,  1913. 


556  THEORIES  OF  ORGANIC  CHEMISTRY 

by  xanthydrol  and  its  derivatives  in  the  presence  of  acids  must  contain 
oxonium  salts  such  as,  for  example, 


While  salts  of  this  type  have  not  themselves  been  isolated  they  have, 
nevertheless,  been  obtained  in  the  form  of  well-defined  double  salts 
with  FeCl3,  PtCU,  HgCl2,  etc.1 

The  interpretation  of  reactions  in  terms  of  tetravalent  oxygen2 
became  more  frequent  from  this  time  on.  The  work  of  Baeyer  and 
Villiger  3  in  1901  and  1902  was,  however,  the  first  to  give  the  problem 
a  really  broad  and  general  significance.  They  made  an  exhaustive 
study  of  many  substances  which  might  possibly  contain  quadrivalent  oxy- 
gen and  isolated  and  examined  a  number  of  salt  derivatives  which  were 
formed  by  the  action  of  weak  acids  upon  ethers,  alcohols,  acids,  esters, 
aldehydes  and  ketones.  They  were  able  to  demonstrate  experimentally 
that  nitrogen  and  oxygen  show  perfect  analogy  in  the  phenomenon  of 
salt  formation  since  (1)  oxygen  salts  cannot  be  assumed  to  result  from 
the  addition  of  acids  to  the  double  bond  between  oxygen  and  carbon, 
since  (2)  the  same  influences  which  tend  to  increase  or  decrease  the 
basicity  of  nitrogen  operate  similarly  in  the  case  of  oxygen,  as  for 
example  : 

H3N  (base)  H2O  (neutral) 

(strong  base)  (C2Hs)2O  (weak  base),  etc. 


and  since  (3)  oxygen  in  every  form  of  chemical  combination  shows 
basic  properties.  Whether  present  in  organic  molecules  in  the  form 
of  an  oxide  —  as  for  example  in  an  ether  —  or  as  hydroxide  or  as  car- 
boxyl,  oxygen  always  exhibits  an  inclination  to  form  salts  with  acids 
and  thus  to  bring  into  play  its  two  residual  valencies.  It  was  difficult 
at  first  to  find  suitable  acids  with  which  to  demonstrate  the  basicity 
of  oxygen  compounds,  but  Baeyer  and  Villiger  finally  discovered  that 

1Ber.,  44,  3505  (1911). 
2Ber.,  33,  1636  (1900). 
3Ber.,  34,  2679,  3616  (1901);  36,  1801  (1902). 


THE  BASIC  PROPERTIES  OF  OXYGEN  557 

complex  acids — such  as  ferrocyanic,  ferricyanic,  chloroplatinic,  and 
even  oxalic  acid — could  be  used  to  advantage  to  prove  the  existence 
of  even  weak  basicity. 

Further  impetus  was  given  to  the  study  of  oxonium  compounds 
through  the  discovery  by  K.  A.  Hofmann  and  his  students 1  that  a  con- 
centrated solution  of  perchloric  acid  will  react  to  give  oxonium  salts 
'  (=O  •  C1O4)  which  are  only  slightly  soluble  and  which  crystallize  well. 
Hofmann's  results  seem  to  show  that  perchloric  acid  is  superior  to  all 
other  acids  which  have  as  yet  been  examined  in  this  respect  and  is,  there- 
fore, preeminently  the  reagent  to  be  used  in  the  case  of  weakly  basic 
oxonium  compounds.  Innumerable  alcohols,  ethers,  aldehydes,  ketones, 
acids  and  esters  give  salt-like  derivatives  which  for  the  most  part  are  well- 
defined  crystalline  bodies  and  which  appear  to  differ  in  their  chemical 
behavior  in  no  way  from  the  salts  derived  from  weak  bases  containing 
nitrogen.  It  thus  came  to  be  generally  recognized  that  the  property 
which  had  long  been  regarded  as  characteristic  of  nitrogen — namely 
the  power  of  passing  under  certain  conditions  from  a  trivalent  to  a 
pentavalent  condition — was  possessed  also  by  oxygen,  the  only  dif- 
ference being  that  the  latter  element  passed  from  a  bivalent  to  a  tetra- 
valent  condition. 

The  analogy  between  oxygen  and  nitrogen  is  shown  in  yet  another 
way.  The  investigations  of  E.  Bamberger2  have  demonstrated  that 
in  the  process  of  sulphonating  aromatic  amines,  and  in  particular  aniline, 
the  amino  group  is  attacked  first.  The  sulphate  which  is  formed  in 
this  way  readily  splits  off  water,  passing  into  a  sulphamine  combina- 
tion which  finally  undergoes  molecular  rearrangement  giving  ortho- 
and  para-sulphanilic  acids: 

NH2-H2S04      NH-SO3H         NH2  NH2  3 

;o3H 


JSO3H 


Baumann  4  observed  a  perfectly  analogous  reaction  for  phenols  as 
early  as   1878  when  he  discovered    that    potassium  phenyl-sulphate 

1  Ber.,  43,  178,  183,  1080,  2630  (1910);  42,  4856  (1909). 
2Ber.,  30,  654,  2274  (1897). 
1  H.  Wieland,  Ber.,  40,  4269  (1907). 

4  Ber.,  11,  1909  (1878);  Schmitt,  Jour,  prakt.  Chemie,  31,  409  (1885);  Claisen, 
Ber.,  33,  3780  (1900),  etc. 


558  THEORIES  OF  ORGANIC  CHEMISTRY 

rearranges  upon  heating  to  give  the  potassium  salt  of  phenol-p-sulphonic 
acid: 

OSO3K 


5O3K 

In  the  process  of  sulphonating  phenol  it  may  be  assumed  that  salt 
formation  represents  an  intermediate  stage  in  the  reaction  and  that 
it  results  in  the  formation  of  an  unstable  addition  product  (I).  '  Under 
the  conditions  of  the  reaction  this  then  loses  water  to  give  the  phenyl 
ester  (II)  and  this  in  turn  isomerizes  to  form  the  sulphonic  acid  deriva- 
tive (III). 

OH .  HO  •  S02OH       0  •  S02OH 


I  II 

Kendall,  who  has  recently  investigated  the  ability  of  sulphuric  acid 
to  add  to  phenol  combinations,1  postulates  that  the  sulphonation  of 
a  phenol  is  preceded  by  the  formation  of  an  addition  product  of  the 
nature  of  an  oxonium  salt.  Whether  this  involves  a  tautomeric  change 
of  phenol  I  into  its  ketonic  modification  II  2  followed  by  addition  on 
the  oxygen  III  as  in  the  case  of  alcohols  3  is  not  as  yet  known. 


X)SO2OH 


H 

(Enol  form)  (Keto  form)  (Oxonium  salt) 

I  II  III 

1  Jour.  Am.  Chem.  Soc.,  36,  2507  (1914). 

2  Gomberg  and  Cone,  Annalen  der  Chemie,  376,  220  (1910), 

3  Maass  and  Mclntosh,  Jour.  Am.  Chem.  Soc.,  34,  1284  (1912). 


THE  BASIC  PROPERTIES  OF  OXYGEN  559 

The  objection  has  been  raised  that  these  so-called  oxonium  salts 
are  really  molecular  compounds  and  that  they  are,  therefore,  analogous 
in  structure  to  such  substances  as,  for  example,  are  formed  by  the  reac- 
tion of  the  picrates  with  aromatic  hydrocarbons.  This  objection  has, 
however,  been  met  by  Walden  1  who  has  demonstrated  by  means  of 
physico-chemical  measurements  that  the  compounds  formed  by  the 
union  of  dimethyl  pyrone  with  acids  are  actually  true  salts,  and  that 
solutions  of  such  salts  have  the  properties  that  are  to  be  expected  in 
salt  combinations  arising  from  the  union  of  a  weak  base  and  a  relatively 
strong  acid.  Walden's  experiments  included  the  determination  of 
(1)  mutarotation  of  grape  sugar  in  the  presence  of  dimethyl  pyrone 
due  to  the  formation  of  hydroxyl  ions,  (2)  the  coefficient  of  distribu- 
tion of  picric  acid  between  water  and  benzene  as  compared  with  that 
of  picric  acid  plus  dimethylpyrone,  (3)  the  lowering  of  the  freezing 
point  of  pure  hydrochloric  acid  in  aqueous  solution  as  compared  with 
that  of  hydrochloric  acid  solution  plus  dimethyl  pyrone,  and  (4)  the 
conductivity  of  dimethyl  pyrone  alone  as  compared  with  that  of 
dimethyl  pyrone  plus  an  acid  (picric  acid)  determined  in  different 
solvents  such  as  (a)  liquid  sulphur  dioxide,  (b)  acetonitrile  and 
(c)  water. 

Walden  found  as  a  result  of  these  experiments  that  dimethyl  pyrone 
is  a  weak  base  slightly  stronger  than  urea  but  much  weaker  than  aniline. 
At  the  same  time  he  was  able  to  show  that  under  certain  conditions  it 
may  be  slightly  acidic  in  character.  The  substance  may  be  regarded, 
therefore,  as  an  amphoteric  body  showing  a  tendency  to  form  hydrogen 
as  well  as  hydroxyl  ions  in  solution. 

In  connection  with  the  acid  properties  of  pyrone  derivatives  it  is 
to  be  noted  that  Willstatter  and  Pummerer2  made  the  independent 
observation  that  pyrone  itself  possesses  the  properties  of  a  weak  acid 
and  has  the  ability  to  add  sodium  and  potassium  alcoholates  in  alcohol 
solutions.  They  regard  the  change  as  perfectly  analogous  to  that  which 
Claisen3  concludes  takes  place  when  esters  of  organic  acids  interact 
with  alcoholates,  and  which  he  represents  as  addition  to  the  unsaturated 
C=O  group  with  formation  of  carbonium  compounds: 

/CH=CHX      /ONa  /ONa 

r\/  \C*s  n-nA       C^  TT      C*/    C\C*   TJ 

vjc  /  ^\  and     vy6ri5  •  \^- — ui^2ri5 

\CH=CH/     XOC2H5  \OC2H5 

(Pyrone  addition-product)  (Ethyl  benzoate 

addition-product) 

^er.,  34,  4189  (1901);  35,  1764  (1902);  Sackur,  Ber.,  35,  1242  (1902). 
2Ber.,  37,  3740  (1904). 
3  Ber.,  20,646(1887). 


560  THEORIES  OF  ORGANIC  CHEMISTRY 

Willstatter  differs,  however,  from  Claisen  in  his  interpretation  of  the 
mechanism  of  the  reaction  and  assumes  that  sodium  ethylate  adds  to 
the  oxygen  of  ethyl  acetoacetate  to  give 

NaO  OC2H5 

CH3C—  CH2COOC2H5 

as  an  intermediate  product  of  the  reaction. 

Oxygen  has  thus  been  shown  to  possess  basic  properties  and  it  is 
now  assumed  by  many  chemists  that  its  salts  with  acids  are  of  the  type 
of  ammonium  salts.  Combinations  in  which  oxygen  functions  as  a 
tetravalent  element  are  called  oxonium  compounds,1  and  may  be  formu- 
lated in  such  a  way  as  either  to  correspond  to  the  salts  of  the  tertiary 
nitrogen  bases  as  in  (I),  or  to  the  quaternary  nitrogen  bases  as  in  (II). 

c  c  c 

\/ 

Ac  (acid  residue)     O 


i 


LC 
I  II 

The  possibility  of  two  types  corresponding  to  the  above  formulas 
was  first  recognized  when  Kehrmann  2  discovered  a  well-defined  addition 
product  of  methyl  iodide  with  dimethyl  pyrone.  While  a  number 
of  different  structural  formulas  are  posssible  in  the  case  of  this  substance 
only  two  were  seriously  considered  by  Kehrmann,  viz.: 

CH3I 

I 
O  O 

CH3  H2C 

and 


and  of  these  the  first  seemed  to  him  to  be  the  most  probable.  Baeyer  3 
was,  however,  able  to  show  later  that  the  second  formula  expresses  the 
chemical  properties  of  the  substance  better  than  the  first  and  that  it 
must,  therefore,  be  regarded  as  a  substitution-product  of  a  cyclic  com- 
bination to  which  Baeyer  assigned  the  name  pyroxonium  iodide,  or  in 
other  words  as  dimethyl-methoxy-pyroxonium  iodide 

*E.  Wedekind,  Ber.,  38,  421  (1905). 

sfier.,  39,  1299  (1906). 

^  Ber.,  43,  2337  (1910);  Hofmann,  ibid.,  43,  2630  (1910). 


THE  BASIC  PROPERTIES  OF  OXYGEN 


561 


Soon  after  Werner's  discovery  of  the  xanthonium  bases,  the  salts 
of  the  phenylated  xanthonium  compounds  and  of  the  substituted  phenyl 
derivatives  were  investigated  by  H.  Decker  and  Biinzly l  and  by 
Kehrmann  2  respectively.  This  work  was  followed  by  the  discovery 
of  colored  pyryllium  salts  by  Decker  and  von  Fellenberg.3  The  latter 
obtained  a  number  of  typical  compounds  which  could  not  be  regarded 
either  as  acid  addition  products  or  as  derivatives  of  basic  carbon.  These 
substances  correspond  to  the  following  formulas  in  which  X  represents 
the  acid  radical  and  which  must  be  assumed  to  contain  tetravalent 
oxygen: 


Salt  of 
benzopyryllium 


Salt  of 
naphthopyrylliu  m 


Salt  of 
diphenylpyryllium 


As  a  result  of  these  investigations  Decker  concludes  that  "  tetravalent 
basic  oxygen  must  in  the  future  take  its  place  beside  pentavalent  nitro- 
gen, tetravalent  sulphur,  and  trivalent  iodine  and  must  be  regarded  as 
functioning  in  much  the  same  way  as  these  elements  in  ring  structures." 
According  to  Willstatter  5  the  colored  anthocyanins  and  their  hydro- 
lytic  products  the  anthocyanidins  must  be  regarded  as  o-quinoid  oxonium 
compounds  and,  therefore,  belong  to  the  class  of  benzopyryllium  deriv- 
atives : 


1  Ber.,  37,  2931  (1904). 

2Ber.,  41,  3440  (1908);   42,  870  (1909);  44,  3505  (1911);  47,  3052  (1914);  also 
compare  von  Braun,  Ber.,  49,  191  (1916). 

3  Annalen  der  Chemie,  356,  281  (1907);  364,  1  (1909). 

4  Jour.  Prakt.  Chemie,  94,  54  (1916). 

6  Annalen  der  Chemie,  401,  189  (1913);  408,  1  (1915);  412,  113  (1916). 


562 


THEORIES  OF  ORGANIC  CHEMISTRY 


These  substances  occur  in  the  coloring  matter  of  plants  and  have  been 
made  the  subject  of  an  exhaustive  investigation  by  Willstatter  and  his 
students.  They  all  possess  the  characteristic  property  of  losing  color 
gradually  in  aqueous  and  alcoholic  solutions  and  of  regaining  it  again 
upon  the  addition  of  acid.  Earlier  investigators  who  observed  this 
change  in  color  interpreted  it  as  due  to  the  reduction  of  the  coloring 
matter,  but  Willstatter  has  been  able  to  show  that  the  reaction  is  some- 
what more  complicated.  He  explains  it  by  supposing  that  the  salt  is 
first  hydrolyzed  to  an  oxonium-  or  color-base  and  that  this  then  iso- 
merizes  to  give  a  pseudobase  or  carbinol : 

OH 


OH 


HO 


H 


HO 


OH 

Colored  salt 


in 

Violet  colorbase 


OH 


/-\OH 
CH 


OH 


Colorless  pseudo  base 


Von  Baeyer1  subsequently  prepared  a  series  of  simple  derivatives 
of  pyryllium 

A 

/1\ 

— c    c— 


6        2 


5       3 
— C         C— 


in  which  methyl,  methoxy,  phenyl  and  other  groups  replace  the  hydro- 
gen of  the  ring.     This  work  was  carried  out  in  co-operation  with  J. 
'Ber.,  43,  2337  (1910). 


THE  BASIC  PROPERTIES  OF  OXYGEN  563 

Piccard  1  and  others  and  demonstrated  beyond  a  doubt  that  oxygen 
possesses  decidedly  basic  properties  and  is  able  to  form  well-defined 
salts. 

In  comparing  the  perchlorates  of  alkyl  and  aryl  derivatives  of 
pyryllium  it  was  observed  that  the  former  are  colorless  while  the  later 
are  colored  compounds.  For  example  2,  4,  6-trimethylpyryllium  per- 
chlorate  (I)  is  colorless,  while  2,  6-dimethyl-4-phenylpyryllium  per- 
chlorate  (II)  is  as  yellow  as  sulphur. 

C104  C104 

A  i 

/\  A 

H3C—  C      C—  CH3  H3C—  C     C—  CH3 

II       I  II       I 

HC     CH  C     C 

v  v 

C  C 

CHa  CeHs 

2-4-6-Trimethylpyryllium  2-6-Dimethyl-4-phenyl- 

perchlorate  pyryllium  perchlorate 

Triphenyl-,  phenylanisyl-,  and  trianisyl-pyryllium  have  been  pre- 
pared by  W.  Dilthey  2  and  have  been  found  to  possess  greater  basicity 
than  the  corresponding  trimethyl  derivative.  This  is  illustrated  by 
the  fact  that  the  acetate  of  trianisylpyryllium  is  stable  in  aqueous 
solution.  These  salts  are  all  brightly  colored  and  exhibit  the  phenom- 
enon of  halochromism.  For  example,  the  triphenyl  derivative  of 
pyryllium  reacts  with  picric  acid  to  give  a  red  monopicrate  and  a  yellow 
dipicrate  : 


and 


N02 

N02  +  C6H2(N02)3OH 


1  Annalen  der  Cheniio,  384,  208  (1911);  407,  332  (1915). 

2  Jour,  prakt.  Chemie,  94,  53  (1916);  95,  107  (1917). 


564  THEORIES  OF  ORGANIC  CHEMISTRY 

Interpreted  in  the  sense  of  Pfeiffer's  theory  this  phenomenon  may  be 
explained  by  supposing  that  the  addition  of  the  first  molecule  of  acid 
acts  to  increase  greatly  the  unsaturation  of  the  carbon  atom  in  position  2. 


OC6H2(N02)3 
O 
H5C6C      C- 


y 


CeHs 


This  would  account  for  the  red  color  of  the  salt  and  also  for  its  ability 
to  add  a  second  molecule  of  acid  in  this  position : 

OC6H2(N02)3 


•[CCH30(N02)3] 


Y 

CeH5 


The  formula  of  the  dipicrate  indicates  that  it  is  more  highly  saturated 
than  the  monopicrate  and  that  a  priori  it  might,  therefore,  be  expected 
to  show  a  weaker  coloration. 

It  is  evident 1  that  the  conception  of  tetravalent  oxygen  affords  a 
satisfactory  and  an  exact  interpretation  of  many  reactions  and  it  fre- 
quently happens  that  such  explanations  are  more  plausible  than  those 
which  they  have  superseded.  Ester  formation,  for  example,  is  usually 
expressed  in  the  following  way: 

CH3COOH  +  C2H5OH    -»    CH3COOC2H5  +  H20 

But  this  offers  no  explanation  as  to  why  the  rate  of  esterification  is  so 
greatly  accelerated  by  the  presence  of  acids  or,  in  other  words,  by  the 
presence  of  hydrogen  ions.  H.  Goldschmidt 2  has  recently  shown,  as 
a  result  of  his  researches  on  the  rate  of  esterification,  that  the  reaction 

*H.  Kauffmann,  "Die  Valenzlehre,"  p.  185. 
2  Zeitschr.  Elektrochemie,  14,  581  (1908) 


THE  BASIC  PROPERTIES  OF  OXYGEN  565 

consists  primarily  in  the  addition  of  a  hydrogen  ion  to  a  molecule  of 
alcohol  with  the  formation  of  the  complex  ion  (C2HsOH2).  In  terms 
of  the  quadrivalence  of  oxygen  it  may  be  assumed  that  the  formation 
of  the  complex  positive  ion  takes  place  as  the  result  of  the  addition  of 
the  acid,  as,  for  example  HC1,  to  the  alcohol,  followed  by  the  immediate 
dissociation  of  the  products.  Thus: 


C2H5OH  +  HC1     -»         >0<  ->  >0  +  Cl' 

CK     \H  \H 

The  positive  ion  which  is  formed  in  this  way  then  reacts  with  a  mole- 
cule of  acid  in  the  following  manner: 

C2H5OH2-  +  CH3COO'    -»    CH3COOC2H5  +  H2O 

The  formation  of  ether  by  the  interaction  of  ethyl  iodide  and  sodium 
alcoholate  may  be  regarded  as  an  analogous  transformation  : 


+  C2H5I     -»  >0<  -»     C2H5OC2H5  +  Nal 

V     XNa 

For  other  examples,  the  reader  is  referred  to  the  original  papers 
mentioned  in  this  chapter  and  also  to  the  work  of  E.  Fromm.1 

A  study  of  the  mechanism  of  the  Grignard  reaction  has  given  further 
confirmation  of  the  hypothesis  of  oxonium  salt  formation.  Combi- 
nations which  contain  quadrivalent  oxygen  are  apparently  formed  as 
intermediate  products  in  the  application  of  this  reaction  in  ether  solu- 
tion and  evidence  of  heterosposis  has  been  obtained.  For  example 
an  oxonium  salt  of  this  type  can  decompose  in  either  of  two  ways:  2 

R\          *R'X+ROR 
ROR'+RX      ->    R-^OX< 

R'/       ^"RX+R'OR 

The  American  investigator  Gomberg  is  very  strongly  opposed  to 
the  prevailing  ideas  in  regard  to  the  tetra  valency  of  oxygen  in  organic 
combinations  containing  this  element.  He  does  not  believe  that  the 
arguments  in  favor  of  the  oxonium  constitution  of  dimethyl  pyrone 
salts  and  related  compounds  are  sound,  and  has  put  forward  the  view 
that  the  compounds  generally  considered  as  oxonium  salts  should 
be  regarded  as  carbonium  compounds.3  Gomberg  advocates  a  quinonoid 

1  Annalen  derChemie,  396,  75  (1913). 

2Stadnikoff,  Ber.,  44,  1157  (1911);  Tschelinzeff,  Ber.,  37,  4534  (1904). 

»  Gomberg  and  Cone,  Annalen  der  Chemie,  370,  142  (1909);  376,  183  (1910). 


566  THEORIES  OF  ORGANIC  CHEMISTRY 

constitution  for  the  colored  salts  of  triphenyl  carbinol  and  related  com- 
pounds, and  explains  the  unique  behavior  of  such  salts  by  supposing  that 
they  are  capable  of  existing  in  two  isomeric  modifications  the  one 
aromatic  and  colorless  (I),  and  the  other  quinonoid  and  colored  (II). 


CeHs/ 
I 

Gomberg  is  of  the  opinion  that  a  large  number  of  the  organic  derivatives 
which  contain  oxygen  possess  a  quinocarbonium  configuration  and  he 
considers  it  unnecessary  to  assume  an  oxonium  structure  for  these  com- 
binations. He  has  furthermore  shown  that  a  number  of  the  carbinols 
are  capable  of  existing  in  both  the  quinoid  and  benzoid  state.1 


(C6H5)3C-OH  (C6H5)2C 

OH 

Carbinol  (pseudo-base)  Quinocarbonium  base 

Gomberg  and  his  collaborators  have  not  only  made  a  number  of 
applications  of  their  quinocarbonium  hypothesis,  but  they  have  also 
succeeded  in  bringing  the  salts  of  dimethyl  pyrone  into  line  with  those 
which  have  just  been  considered.  Gomberg  and  Cone  2  emphatically 
deny  the  formation  of  an  oxonium  salt  when  dimethyl  pyrone  com- 
bines with  an  acid,  and  interpret  the  reaction  as  taking  place  by  addi- 
tion according  to  the  scheme: 

O  0 

CH3/NCH3+HC1 


HOC— Cl 

In  other  words,  the  reaction  depends  upon  the  presence  of  the 
carbonyl  group,  and  addition  results  in  a  carbonium  salt.  The  behavior 
of  picric  acid  towards  hydrochloric  acid  has  been  explained  in  a  similar 
way  by  Stepanoff3  who  assumes  the  formation  of  the  following  car- 
bonium salt: 

Ck      ™» 

>C<  >  :  NOH 

H( 


1  Jour.  Am.  Chem.  Soc.,  35,  1035  (1913). 
2Annalen  der  Chemie,  376,  217  (1910). 
3  Annalen  der  Chemie,  373,  219  (1910). 


THE  BASIC  PROPERTIES  OF  OXYGEN  567 

The  principal  opposition  to  the  extension  of  the  class  of  carbonium 
salts  by  Gomberg  has  come  from  Kehrmann,1  although  recently  Kendall 
has  also  given  support  to  the  oxonium  theory.2  The  latter  has  investi- 
gated the  action  of  organic  acids  on  dimethyl  pyrone  and  considers 
that  these  addition  products  are  true  oxonium  compounds  and  that 
they  are  to  be  expressed  structurally  as  follows: 

O 


H— O— Cl 

Hydrochloride  of  dimethyl  pyrone 

He  considers  that  their  formation  is  due  to  the  basic  properties  of  the 
unsaturated  C=O  group  and  that  oxygen  functions  as  a  quadrivalent 
element.  Kendall3  disagrees  with  Gomberg  and  Cone  and  concludes 
from  the  results  obtained  in  his  experiments  that  organic  acids,  alde- 
hydes and  ketones  all  interact  with  acids  in  an  analogous  manner  giving 
oxonium  compounds: 

~D  T)  TT 

\C=0  +  HC1    ->        '\C=O</ 

w  w        xci 

R.C=0  +  HC1      ->       R.C=0/ 

I  I      xa 

OH  OH 

He  also  concludes  that  the  compounds  formed  by  the  addition  of  sul- 
phuric acid  to  organic  acids  are  true  oxonium  salts  and  represents  them 
structurally  as  follows: 

!=O  +  H2S04      ->    R-C=0/ 

X)-S02.OH 
OH 
and 

TT  TT 

RC— C/  \0=C— R 

X)-SO2-0/ 
)H  OH 


J. 


1Annalen  der  Chemie,  372,  287  (1910). 

2  Jour.  Am.  Chem.  Soc.,  36,  1242  (1914). 

3  Jour.  Am.  Chem.  Soc.,  36,  1722  (1914);  37,  160  (1915). 


568  THEORIES  OF  ORGANIC  CHEMISTRY 

Ismailskii,1  who  has  made  a  critical  examination  of  the  proposed 
hypotheses  in  connection  with  the  relation  between  color  and  structure, 
considers  that  both  the  carbonium  and  the  quinone-carbonium  hypoth- 
eses are  inadequate  to  account  for  the  various  phenomena  of  halo- 
chromism.  For  other  speculations  dealing  with  the  question  of  basic 
properties  of  oxygen  the  reader  is  referred  to  publications  by  Mclntosh  2 
and  Dehn.3 

1  Jour.  Russ.  Chem.  Soc.,  47,  63  (1915);  Chem.  Abstr.,  9,  1471  (1915). 

2  Jour.  Chem.  Soc.,  86,  919,  1098  (1904);  Jour.    Am.  Chem.  Soc.,  27,  26,  1013 
(1905),  28,  588  (1906);  30,  1097  (1908);  32,  542  (1910);  33,  71  (1911);  34,  1273  (1912). 

3  Jour.  Am.  Chem.  Soc.,  39,  2646  (1917). 


CHAPTER  XXI 
THE  THEORETICAL  SPECULATIONS  OF  ARTHUR  MICHAEL 

As  early  as  1878  van't  Hoff  had  pointed  out  in  his  "  Ansichten 
iiber  die  organische  Chemie  "  l  that  many  organic  reactions,  which  were 
at  that  time  regarded  as  processes  of  substitution,  could  be  considered 
as  transformations  involving  addition  reactions.  Arthur  Michael 
arrived  at  the  same  conclusion  as  the  result  of  systematic  experimental 
research.  His  investigations  led  him  to  conclude  that,  in  general, 
addition  reactions  play  a  much  more  important  part  in  chemical  proc- 
esses than  had  previously  been  assumed,  and  that  they  frequently 
serve  as  preliminary  or  intermediate  stages  in  other  types  of  trans- 
formations. Kekule  more  than  fifty  years  earlier  had  given  definite 
expression  to  the  conception  of  molecular-addition  as  forming  an  inter- 
mediate process  in  chemical  change.  This  is  diagramed  below,  although 
it  has  already  been  referred  to  in  a  previous  chapter  in  this  book. 


8 


Before  chemical  action  Addition  product  After  chemical  action 

Intermediate  products,  corresponding  to  expression  2  in  the  diagram, 
must  undoubtedly  have  been  obtained  frequently  in  the  course  of  investi- 
gations in  the  past,  but  for  many  years  it  was  impossible  'to  establish 
their  existence  in  a  sufficient  number  of  cases  to  meet  all  objections, 
and  only  in  recent  times  have  investigators  produced  enough  experi- 
mental evidence  to  establish  their  very  general  occurrence.  The  study 
of  chemical  phenomena  gradually  forced  chemists  to  the  assumption 
of  intermediate  mobile  combinations  in  organic  reactions,  and  in  1907 
Emil  Fischer  gave  expression  to  a  feeling  universally  prevalent  at  that 
time  in  the  following  words:  "  the  conviction  is  gaining  ground  more 
and  more  that,  very  generally  and  even  in  the  ordinary  processes  of 
substitution,  transitional  addition  reactions  take  place  in  the  sense 

'Vol.  I,  225,  244. 
569 


570  THEORIES  OF  ORGANIC  CHEMISTRY 

that  Kekule  and  others  have  regarded  as  probable."  l  Emil  Fischer 
has  since  greatly  extended  this  conception  as  has  already  been  noted 
in  a  previous  chapter  of  this  book.  Even  in  catalysis  there  is  the  assump- 
tion that  an  intermediate  product,  consisting  of  catalyst  and  sub- 
strata, is  formed  since  this  in  most  cases  offers  the  best  explanation  of 
the  facts.2  Recently  methods  have  been  devised  by  which  the  existence 
of  such  intermediate  products  can  be  demonstrated.  In  addition  to  the 
optical  methods  which  have  already  been  referred  to,  the  so-called 
thermal  analysis3  may  be  mentioned.  This  has  been  made  practical 
for  organic  chemists  through  the  work  of  R.  Kremann,4  Ph.  Guye  and 
his  students,  Holleman  5  and  J.  Schmidlin.6 

A.  Michael,  convinced  of  the  general  applicability  of  Kekule's 
conception,  endeavored  to  determine  what  forces  induced  and  regulated 
this  type  of  chemical  reaction,  and  he  came  to  the  conclusion  that 
changes  in  energy  were  the  important  controlling  factors;  for  it  was 
to  be  noted  that  problems  dealing  with  chemical  affinities  found  their 
best  solution  within  the  sphere  of  energy  relations.  The  experiments 
by  means  of  which  he  endeavored  to  trace  valence  and  the  course  of 
chemical  processes  back  to  the  energy  relationships  of  the  atoms,  have 
been  published  in  a  series  of  articles.7  An  outline  of  what  is  funda- 
mental in  Michael's  system  is  all  that  can  be  given  here,  but  it  is  hoped 
that  this  may  serve  as  an  introduction  to  a  more  thorough  study  of  his 
work.  The  author's  own  words  will  be  quoted  frequently  in  an  effort 
to  avoid  any  misunderstanding  of  his  conceptions  and  deductions 
since  these  are  often  extremely  subtle. 

Every  atom  represents  a  definite  quantity  of  potential  chemical 
energy.  Every  unsaturated  (free)  atom  has  a  more  or  less  definite 
tendency  toward  a  condition  of  greater  stability  and  if  it  finds  no  other 
atoms  of  a  different  element  with  which  to  combine,  it  will  combine 
eventually  with  one  or  more  atoms  of  its  own  kind.  When  chlorine 
atoms,  for  example,  unite  to  form  a  molecule,  their  free  atomic  energy 

1  Ber.,  40,  495  (1907);  Walden,  Ber.,  32,  1850  (1899). - 

2Bredig,  Ber.,  41,  754  (1908). 

8  S.  R.  Kremann,  "Uber  die  Anwendung  der  thermischen  Analyse  zum  Nachweis 
chemischer  Verbindungen,"  Bd.  14,  der  Sammlung  chem.  u.  tech-chem.  Vortrage, 
p.  213  (1909). 

4Monatsh.  Chemie,  25,  1215  (1904);  27,  125  (1906);  28  (1907). 

5  "  Die  direkte  Einfiihrung  von  Substituenten  in  den  Benzolkern,"  p.  26,  Leipzig, 
1910. 

c  Schmidlin  and  Lang,  Ber.,  43,  2806  (1910);  45,  899  (1912). 

7  Jour,  prakt.  Chem.,  37,  523  (1888);  40,  178  (1889);  60,  286,  409  (1899);  68,  487 
(1903);  75,  105  (1907);  Ber.,  33,  34,  36,  38,  39,  203,  2138,  2143,  2149,  2157,  2569 
(1906);  "Stereoisomerism  and  the  Law  of  Entropy,"  Am.  Chem.  Jour.,  39,  1  (1908). 


THEORETICAL  SPECULATIONS  OF  ARTHUR  MICHAEL        571 

is  partly  used  up  as  heat,  partly  changed  into  bound  energy,  and  partly 
persists  as  free  energy  within  the  molecule.  Bromine  atoms  and  bromine 
molecules  show  analogous,  though  relatively  smaller  amounts  of  free 
energy,  as  do  similarly  all  atoms  and  all  molecules  whether  made  up 
of  atoms  of  the  same  kind  or  of  different  kinds.  The  tendency  of  atoms 
to  combine  with  each  other  Michael  calls  polymerization,  and  he  repre- 
sents it  as  a  periodic  function  of  the  atomic  weight.  This  very  interest- 
ing development  can  only  be  alluded  to  in  this  brief  review.1 

When  sodium  reacts  with  chlorine  the  process  in  terms  of  the 
Kekule-Michael  conception  is  to  be  conceived  as  follows:  "  when  a 
molecule  of  sodium  comes  within  the  sphere  of  attraction  of  a  molecule 
of  chlorine,  the  two  molecules  are  drawn  together  by  the  action  of  their 
free  energies  and  merge  to  form  a  double  molecule.  Since,  however, 
sodium  and  chlorine  mutually  attract  each  other,  the  bound  energy 
between  atoms  of  the  same  kind  no  longer  suffices  to  hold  these  atoms 
together,  with  the  result  that  the  double  molecules  suffer  a  rearrange- 
ment and  finally  a  decomposition  into  two  molecules  of  sodium  chloride. 
If  the  free  energy  be  represented  by  dotted  and  the  bound  energy  by 
solid  lines,  this  process  may  be  depicted  by  means  of  the  following 
diagram : 

Na        Cl -Na— Cl- 

|      +     |  _>[]_»     2NaCl 
Na         Cl -Na— CI- 
As a  result  of  this  change  the  free  energy  of  the  sodium  and  the  chlorine 
is  changed  into  bound  energy  and  heat  and  simultaneously  the  bound 
energy  between  atoms  of  the  same  kind  disappears,  being  converted  in 
part  into  bound  energy  between  atoms  of  sodium  and  chlorine  (unlike 
atoms)  and  in  part  into  heat. 

In  other  changes,  dependent  upon  the  nature  of  the  elements  in- 
volved, the  interchange  of  the  energy  relations  is  not  so  complete. 
When,  for  example,  a  chlorine  molecule  reacts  with  a  magnesium  mole- 
cule, the  bond  between  the  two  magnesium  atoms  is  broken,  while  on 
the  other  hand,  the  energy  of  the  relatively  weaker  magnesium  atoms 
is  not  sufficient  to  completely  sever  the  bond  between  the  two  chlorine 
atoms.  According  to  Michael  a  new  molecule  is  formed  containing 
only  a  part  of  the  original  bound  energy  uniting  the  two  chlorine  atoms, 
and  in  which  there  is  decidedly  more  free  energy  for  each  chlorine  atom 
than  is  to  be  found  in  the  molecule  of  sodium  chloride.  Thus: 

.  .  .  Mg  Cl  .  ,C1 .  .  . 

|       +  2   |  ->     2Mg<  | 

.  .  .  Mg  Cl XC1 .  .  . 

1  Jour,  prakt.  Chemie,  60,  293  (1899). 


572  THEORIES  OF  ORGANIC  CHEMISTRY 

On  the  other  hand,  atoms  of  magnesium  and  oxygen  may  be  regarded 
as  almost  completely  neutralizing  each  other  and  this  accounts  for 
the  fact  that  magnesium  oxide  is  such  an  indifferent  chemical  compound. 
Again,  oxygen  does  not  fully  neutralize  the  bound  energy  of  the  alkali 
metals  and  thus  oxides  arise  in  which  the  metallic  atoms  are  combined 
not  only  with  oxygen,  but  are  also  in  direct  union  with  each  other. 

While  it  has  been  assumed  previously  that  valency  was  the  measure 
of  the  total  affinity  of  the  atoms,  in  terms  of  Michael's  conception, 
valency  becomes  merely  a  rough  measure  of  the  resultant  of  the  chemical 
attractions,  operating  directly  or  indirectly  between  atoms.1  This  con- 
ception supposes  that  the  processes  in  operation  during  an  exchange  of 
atoms  are  analogous  to  the  processes  at  work  during  neutralization  as 
in  the  case  of  acids  and  bases.  The  well-known  phrase — "  every 
system  tends  to  a  condition  representing  the  maximum  of  entropy  " 
may  be  correctly  applied  in  determining  the  course  of  organic  proc- 
esses if  the  term  "  chemical  neutralization  "  is  substituted  for  the  term 
"  entropy."  This  phrase  may  be  applied  most  advantageously  to 
reactions  involving  unsaturated  atoms,  since  in  such  cases  it  is  often 
possible  to  predict  which  will  be  the  more  favorable  of  all  conceivable 
neutralization  phases.  This  so-called  neutralization  principle  may 
be  briefly  formulated  in  the  following  words:  "Every  chemical  system 
tends  to  so  arrange  itself  that  the  maximum  of  chemical  neutralization  is 
attained."  2 

Michael  regards  the  atoms  themselves  as  the  fundamental  units  in- 
volved in  any  process  of  chemical  neutralization,  and  makes  a  sharp  dis- 
tinction between  positive  and  negative  units.  According  to  him  the 
more  strongly  negative  elements  are  the  halogens,  especially  chlorine, 
while  elements  like  oxygen  and  nitrogen  are  more  weakly  negative. 
Potassium  and  sodium  are  to  be  regarded  as  strongly  positive  while 
magnesium  and  silver  are  weakly  positive,  and  mercury  is  very  weakly 
positive.  Furthermore,  he  believes  that  every  chemical  compound, 
even  if  from  its  valence  formula  it  appears  to  be  saturated,  contains 
more  or  less  free  chemical  energy.  This  follows  from  the  fact  that  the 
free  energies  of  its  component  atoms  never  completely  neutralize  each 
other  during  combination.  The  free  chemical  energy,  considered 
together  with  the  affinity  relationships  of  the  atoms  involved,  determines 
the  chemical  potential  of  any  system.  The  meaning  of  this  conception 
is  expressed  in  the  following  sentences:  "a  sufficient  chemical  potential 
is  necessary  in  order  that  a  given  reaction  may  take  place.  This  is 
dependent  upon  two  factors:  first,  free  chemical  energy  and  second, 

1  Jour,  prakt.  Chemie,  68,  489  (1903). 

2  Jour,  prakt.  Chemie,  60,  292,  300  (1899). 


THEORETICAL  SPECULATIONS  OF  ARTHUR  MICHAEL        573 

the  affinity  relations  between  the  reacting  atoms;  neither  factor  alone 
is  capable  of  inducing  a  reaction.1" 

The  properties  of  the  element  carbon  are  in  complete  harmony 
with  its  position  in  the  periodic  system.  This  has  already  been  pointed 
out  by  van't  Hoff,2  who  has  emphasized  the  fact  that  carbon  occupies 
a  transitional  position  between  the  most  widely  different  pairs  of  ele- 
ments and  that  to  its  double  nature  is  due  its  ability  to  enter  into 
combination  with  so  many  other  elements.  Since,  moreover,  the 
chemical  behavior  of  an  element  depends  upon  the  nature  of  the  atoms 
which  are  in  union  with  it,  it  follows  that  carbon,  since  it  is  itself  rela- 
tively neutral  in  character,  must  be  exceptionally  susceptible  to  out- 
side influences  and  that  its  chemical  properties  should  therefore  vary 
in  the  directions  determined  by  such  influences.  Each  and  every 
element  may  thus  impart  its  properties  to  a  greater  or  less  degree  to 
the  element  carbon.  This  extremely  important  property  of  carbon — 
the  realization  of  which  is  fundamental  to  an  understanding  of  organic 
chemistry — has  been  called  by  Michael  its  "  chemical  plasticity  "  3 
Hydrogen  is  the  only  other  element  which  compares  with  carbon  in 
the  degree  to  which  it  possesses  this  property.  The  fact  that  carbon 
is  essentially  a  weakly  negative  element  and  that  the  positive-negative 
difference  between  carbon  and  hydrogen  is  slight,  serves  to  explain 
why  even  minor  changes  in  the  constitution  of  organic  compounds 
materially  influence  the  properties  of  these  atoms.4 

Michael  also  developed  special  conceptions  in  regard  to  the  condi- 
tion of  unsaturation.  The  fact  that  free  methyl  does  not  exist  tends 
to  show  that  the  presence  of  three  hydrogen  atoms  bound  to  carbon 
does  not  interfere  with  the  ability  of  the  carbon  atom  to  undergo  polym- 
erization. When  a  methyl  group  unites  with  a  second  like  group  the 
two  carbon  atoms,  which  were  previously  unsaturated,  enter  into 
chemical  combination  and  simultaneously  a  change  in  the  chemical 
properties  of  the  hydrogen  and  of  the  carbon  atoms  takes  place.  Again, 
"  if  one  hydrogen  atom  is  removed  from  each  of  the  carbon  atoms  of 
ethane  the  energy,  which  was  previously  exerted  in  holding  these 
hydrogen  atoms,  may  be  used  in  part  to  strengthen  the  mutual  satura- 
tion of  the  carbon  atoms  or  in  part  in  changing  the  relative  relation 
existing  between  these  carbon  atoms  and  their  respective  remaining 
hydrogen  atoms."  The  increased  self -saturation  of  the  carbon  atoms 
in  such  compounds  offers  a  sufficient  explanation  for  the  fact  that  the 

iAnnalen  der  Chemie,  363,  21  (1908). 

2  "Ansichten  uber  die  organische  Chemie,"  I,  280;  II,  242  (1878-1881). 

3  Jour,  prakt.  Chemie,  60,  325  (1899). 

4  Jour,  prakt.  Chemie,  37,  522  (1888). 


574  THEORIES  OF  ORGANIC  CHEMISTRY 

unsaturated  carbon  atoms  of  the  ethylene  series  show  no  tendency 
toward  polymerization.  "It  is  obvious  that  if  several  carbon  atoms 
in  a  molecule  are  simultaneously  unsaturated,  relations  of  multiple  self- 
saturation  between  the  adjoining  or  directly  bound  atoms  must  result, 
and  that,  further,  if  the  number  of  such  unsaturated  atoms  is  uneven, 
the  odd  atoms  of  two  molecules  must  enter  into  a  simple  chemical  com- 
bination with  each  other,  except  in  such  special  cases  as  when  this  car- 
bon atom  is  subject  to  strongly  negative  influences.  According  to  this 
conception  of  unsaturation  a  two-membered  system  of  such  atoms 
represents  not  merely  a  store-house  of  potential  chemical  energy,  but 
also  a  relatively  increased  self  saturation  of  its  unsaturated  atoms."  l 

Just  as  the  elements  of  inorganic  chemistry  are  able  to  combine 
with  one  another  in  a  great  variety  of  ways,  it  happens  that  organic 
compounds  are  frequently  formed  which  contain  atomic  groupings 
endowed  with  pronounced  positive  or  negative  properties  as,  for 
example,  ethyl  acetoacetate,  which  combines  with  sodium  to  form  a 
sodium  derivative: 

ONa 

CH3C=CH.COOC2H5 

+ 

Other  examples  of  this  type  may  in  general  be  found  in  compounds 
with  the  atomic  groupings 

OM  OM  OM 

CH3C— C— ;  CH3C— NH;      — N— O 

+     -  +     -  +     - 

When  the  metallic  derivative  of  such  a  substance  interacts  with  an 
alkyl  halide,  as,  for  example,  methyl  iodide,  several  atomic  forces  are 
involved  in  bringing  about  a  separation  of  the  iodine  from  the  methyl 
group.  According  to  Michael,  the  dissociation  of  the  halide  will  depend 
principally  upon  the  attraction  of  the  metal  for  iodine  as  compared 
with  the  relative  attraction  of  the  metal  for  oxygen,  and  of  one  of  the 
unsaturated  atoms  in  the  molecule  for  methyl.  The  more  positive 
the  metal,  the  more  readily  will  the  methyl  tend  to  combine  with  that 
one  of  the  unsaturated  atoms  which  is  most  negative  in  character. 
This  is  due  to  the  fact  that  the  attraction  of  oxygen  for  methyl  is  less 
than  that  of  the  unsaturated  carbon  atoms,  since  the  former  is  directly 
bound  to  the  strongly  positive  metal  while  the  latter  are  more  remote 
from  this  influence.  In  other  words  the  positive  character  of  the  metal 
1  Jour,  prakt.  Chemie,  60,  298  (1899). 


THEORETICAL  SPECULATIONS  OF  ARTHUR  MICHAEL        575 

usually  operates  in  such  a  manner  as  to  direct  the  course  of  the  reaction. 
It  may  happen,  however,  that  the  properties  of  the  unsaturated  atoms 
have  been  so  changed  by  substitutions  in  the  molecule,  as  to  make  their 
attraction  for  methyl  less  than  that  of  oxygen,  in  which  case  the  course 
of  the  reaction  will  be  altered  and  methyl  will  combine  directly  with 
oxygen.  In  the  case  of  ethyl  acetoacetate  and  methyl  iodide  the  reac- 
tion may  be  considered  as  proceeding  by  addition  with  the  formation 
of  a  carbon  homologue.  Thus  ethyl  methylacetoacetate  will  be  the 
product  of  the  reaction  and  may  be  regarded  as  resulting  from  the 
decomposition  of  the  addition  product  (I)  into  sodium  iodide  and  the 
/3-ketone  ester  (II) : 

ONa 

CH3C  :  CHCOOC2H5  +  CH3I     -* 

+  +     - 

ONa  0 

CH3C— CHCOOC2H5    ->     CH3C— CHCOOC2H5+NaI l 

I     CH3  CH3 

I  II 

It  is  also  possible  that  the  attraction  of  sodium  for  iodine  and  of  methyl 
for  the  negative  unsaturated  carbon  atom  may  in  themselves  be  suf- 
ficiently powerful  to  bring  about  a  direct  reaction  without  the  formation 
of  an  intermediate  addition  product. 

If,  however,  the  more  negative  carbethoxy  radical  replaces  the  methyl 
group  of  ethyl  acetoacetate,  as  for  example  in 

ONa 


C2H5OOC  •  C=CH  -  COOC2H5 
III 

the  course  of  the  reaction  with  methyl  iodide  is  changed  and  the  produc- 
tion of  carbon  homologues  becomes  so  difficult  that  at  best  only  poor 
yields  of  these  substances  are  obtained.  Under  these  conditions  oxygen 
substitution  products  are  formed  and  this  may  be  explained  as  result- 
ing from  the  relatively  increased  attraction  of  the  oxygen  for  methyl, 
which  in  turn  is  due  to  the  influence  of  the  neighboring  carbethoxy 
group.  At  the  same  time  the  tendency  of  the  unsaturated  methine 

1  Michael  believed  at  one  time  that  he  had  succeeded  in  isolating  such  an  addition 
product  [see  Ber.,  38,  3220  (1905)]  corresponding  to  formula  I,  but  this  was  contested 
by  C.  Paal  [see  Ber.,  39,  1436  (1906)]. 


576  THEORIES  OF  ORGANIC  CHEMISTRY 

grouping  to  undergo  polymerization  is  depressed.  Moreover,  since  the 
tendency  toward  carbon  homology  is  slight  in  the  case  of  such  com- 
pounds the  formation  of  oxygen-homologues  ought  to  be  still  further 
favored  by  the  substitution  of  weakly  positive  silver  in  place  of  the 
strongly  positive  sodium  or  potassium.  As  a  matter  of  fact  it  has  been 
shown  that  esters  of  methoxy-fumaric  acid  may  be  obtained  in  this  way. 
If  the  conception  of  the  plasticity  of  carbon  be  applied  to  the  inter- 
pretation of  the  ethyl  acetoacetate  synthesis  the  difference  observed 
in  the  reactions  of  the  substances  just  cited  is  still  more  readily  expli- 
cable. In  the  sodium  salt  of  ethyl  acetoacetate  the  oxygen  bound  to 
the  metal,  and  the  carbon  of  the  me  thine  group  occupy  the  same  relative 
position  in  the  molecule  with  respect  to  the  methyl  group 


CH3 


•C=CH— 


By  substitution  of  a  negative  carbethoxy  for  methyl  not  only  is  the 
character  of  the  whole  molecule  changed,  but  the  influence  of  this  group 
upon  the  carbon  of  the  methine  radical  is  greater  than  upon  the  oxygen 
because  of  the  plasticity  of  the  former  element.  The  result  is  that  in 
the  reaction  with  methyl  iodide  the  possibility  of  addition  of  methyl  to 
carbon  is  decreased.  Thus  Michael  introduced  new  conceptions  based 
upon  the  principle  of  neutralization  in  place  of  the  old  idea  of  double 
decomposition  and  formulated  the  following  general  rule:  In  any  reac- 
tion the  most  favorable  neutralization  is  realized  when  combination 
takes  place  between  those  atoms  or  radicals  which  are  endowed  with 
pronounced  chemical  properties.  In  cases  where  the  reacting  bodies 
possess  neither  unsaturated  atoms  nor  systems  of  atoms  with  so-called 
double  bonds,  thus  offering  simultaneously  the  possibility  of  mutual 
saturation,  combination  takes  place  between  the  remaining  residues. 
In  any  case  the  further  course  of  the  reaction  depends  upon  the  relative 
attractions  of  the  atoms  which  take  part  in  the  change.1 

van't  Hoff  has  already  pointed  out 2  that,  in  considering  the  total 
attraction  which  two  atoms  in  a  molecule  exercise  toward  each  other, 
it  is  necessary  to  consider  not  only  the  nature  of  the  atoms  (i.e.,  their 
relative  positive  or  negative  character),  but  also  their  spacial  relations 
in  the  molecule.  It  is  possible  in  this  way  to  subdivide  the  total  attrac- 
tion of  the  atoms  into  forces  which  operate  directly  in  space  and  forces 
which  operate  indirectly  through  chemical  combination  or  through 

1  Jour,  prakt.  Chemie,  60,  324  (1899). 

2  "Ansichten  liber  die  organische  Chemie,"  1,  284;  Vol.  2,  252. 


THEORETICAL  SPECULATIONS  OF  ARTHUR  MICHAEL        577 

other  atoms.  On  the  basis  of  this  conception  Michael  has  worked  out 
a  scheme  showing  the  relative  sequences  of  the  space  relationships  for 
normal  paraffines.1  In  conformity  with  the  nomenclature  agreed  upon 
at  Geneva  a  chain  of  atoms  may  be  provided  with  numerals  in  such  a 
way  that  starting  with  a  given  carbon  atom,  not  only  the  succeeding 
carbon  atoms  but  also  the  other  atoms  in  the  system  are  numbered. 
For  example  : 


H3  H2  H2  H2  H2  H3 

I       I      I       I 

c—  c—  c—  c 

1        2        3         4         5         6 


I      I 

c—  c— 


According  to  our  present  knowledge,  then,  the  sum  of  all  the  attractions 
which  a  given  atom  exercises  for  every  other  atom  in  the  molecule  may 
be  represented  by  the  following  scheme  showing  the  relative  scale  of 
the  positions  of  the  atoms: 

1—  2—  3—  5—  6—  4—  7—  (9—10—  1  1)—  8 

Atoms  in  direct  combination  are  represented  by  1  —  2.  The  degree  of 
attraction  decreases  as  we  pass  down  in  the  series. 

If  we  imagine,  for  example,  that  one  of  the  positive  hydrogen  atoms 
in  CH4  has  been  replaced  by  C  it  follows  that  the  methane  carbon  atom, 

C,  will  be  essentially  more  negative  in  CHs  —  C  than  it  was  in  methane. 

1 

If,  however,  three  hydrogen  atoms  are  in  union  with  C,  it  follows  that 

C  will  be  less  negative  than  it  was  originally  in  the  methane.  The 
truth  of  these  deductions  is  obvious  from  a  consideration  of  the  chem- 

1  23 

istry  of  CHa-CHs,  and   may  be  accounted  for  by  supposing  that  the 

3 

sum  of  the  positive  influences  of  Hs  is  greater  than  the  single  negative 

2 

influence  of  C.  "  While  the  positions  2  and  3  are  of  the  greatest  impor- 
tance, position  4  is  subordinate  and  plays  an  even  less  important  role 
than  positions  5  and  6.  The  reasons  for  this  are  to  be  found  in  the 
fact  that  the  indirect  influences  of  4  are  less  than  3  and  much  less  than  2. 
The  indirect  influence  of  position  4  is  so  insignificant  that  usually  it 
may  be  regarded  as  equivalent  to  5  —  6  —  7  —  8.  The  direct  attraction 
between  atoms,  which  depends  upon  the  relative  distance  of  the  atoms 
in  space,  obviously  plays  a  much  more  important  role  than  the  indirect 
attraction  in  the  case  of  positions  4  —  5  —  6,  etc.,  and  from  this  it  follows 
that  the  sum  of  the  influences  for  position  5,  or  for  position  6,  is  greater 
1  Jour.  Am.  Chem.  Soc.,  32,  999  (1910). 


578  THEORIES  OF  ORGANIC  CHEMISTRY 

than  for  position  4  with  respect  to  position  1."  The  relative  influence 
of  positions  9,  10  and  11  is  uncertain  and  in  general  it  may  be  said  that 
the  scale  represents  only  the  present  state  of  our  knowledge. 

1234 

Formic  acid,  HOCH,  is  by  far  the  strongest  acid  of  the  series  of 

O 

acids  represented  by  the  general  formula  CnH2n+i  •  COOH.  If  the 
hydrogen  atom  in  position  4  is  replaced  by  a  methyl  group,  acetic  acid 

12345 

is  formed,  HOCCHs,  and  in  this  new  compound  one   carbon  atom 


is  to  be  found  in  the  subordinate  position  4,  and  three  hydrogen  atoms 
in  the  relatively  more  influential  position  5.  The  derived  acid,  there- 
fore, should  be  noticeably  weaker  than  the  original,  and  this  reasoning 
is  in  harmony  with  the  facts,  the  affinity  constant,  K,  of  acetic  acid 
being  roughly  one-twelfth  that  of  formic  acid.  Such  disparity  in  the 
strength  of  the  acids  following  each  other  in  this  series  does  not  occur 
again,  since  in  the  derivation  of  propionic  acid  from  acetic,  for  example, 
a  negative  carbon  atom  replaces  hydrogen  in  the  relatively  important 
position  5,  while  the  three  new  hydrogen  atoms  are  added  in  the  rela- 
tively less  important  position  6.  For  other  illustrations  see  Jour, 
prakt.  Chemie,  60,  333  (1899). 

Recently  these  ideas  of  Michael  have  been  partially  verified  and 
put  upon  a  quantitative  basis  by  C.  G.  Derick.1  Derick  holds  that 
the  best  quantitative  measure  of  chemical  polarity  is  to  be  found  in 
the  free  energy  of  ionization  wrhich  is  expressed  in  the  affinity  constant, 
K,  (ionization  constant)  for  acids  and  bases.  He  has  demonstrated 
experimentally  that  the  free  energy  of  ionization  in  the  case  of  all  nega- 
tively substituted  acids  of  the  fatty  series,  in  aqueous  solution  and  at 
25°,  may  be  compounded  additively  from  the  individual  effect  of  every 
atom  in  the  molecule.  It  follows  that  the  structural  position  of  a  neg- 
ative substituent  in  a  fatty  acid  molecule  may  be  determined  with 
certainty  if  the  influence  of  the  substituent  in  question  is  known. 

For  reasons,  which  are  to  be  found  in  the  original  papers  on  the 
subject,  the  measure  of  the  scale  of  influence  of  a  given  substituent 
upon  a  given  ionizable  group  may  be  expressed  thus: 

_         log.  K  (of  the  unsubstituted  acid)          _ . 
"log.  K  (of  the  same  acid  after  substitution) 

If,  for  example,  the  influence  of  the  chlorine  atom  upon  the  carboxyl 

group  in  a-chlorbutyric  acid  is  to  be  determined   accurately,  we  must 

*  Jour.  Am.  Chem.  Soc.,  33,  1152,  1162,  1167,  1181  (1911);  34,  74  (1912). 


THEORETICAL  SPECULATIONS  OF  ARTHUR  MICHAEL        579 

also  take  into  consideration  the  influence  of  the  other  atoms  joined 
to  carbon.     The  influence  of  the  atoms  in  the  radical 

CH3-CH2-CH-COOH 


upon  the  ionization  of  the  COOH  must  be  the  same  as  in  the  unsub- 
stituted  butyric  acid  if  the  influence  of  a  single  hydrogen  atom  be  dis- 
regarded. In  the  case  of  butyric  acid  the  value  of  K  has  been  deter- 
mined and  K  =  1.56  X  10~5,  log.  K  =  -  4.807  ;  in  the  case  of  a-chlorbutyric 
acid 

K=1.39X10-3  log.  K=  -2.857 

and  from  this  it  follows  that  the  place  factor  =  _  o  057  ~  ^  =  0-6826. 

Similarly  the  influence  of  all  substituting  groups  may  be  expressed 
numerically  provided  that  the  ionization  constants  of  the  acids  and 
bases  in  which  substitution  takes  place  are  known.  Thus,  the  place 
factors  of  the  combined  and  indirect  influences  of  chlorine  on  COOH 
in  a,  j8,  7,  and  X-chlorbutyric  acids  have  been  determined  and  found  to 
to  be  respectively  0.6825,  0.1873,  0.0627,  0.0229.  If  the  factor  for  the 
a-position  is  made  equal  to  unity,  the  relation 


is  established.  That  is  to  say,  the  effect  of  substituting  a  chlorine 
atom  in  the  /3-position  is  only  -J  that  of  substitution  in  the  a-position, 
while  the  effect  of  substitution  in  the  7  and  X  positions  is  ^  and  -£? 
respectively.  The  same  rule  has  been  found  to  hold  for  Br,  I,  OH  and 
C6H5. 

A  great  many  phenomena  in  organic  chemistry  may  be  interpreted 
in  terms  of  the  principle  of  neutralization  if  the  same  laws  which  govern 
the  neutralization  of  acids  by  bases  be  applied  to  processes  taking  place 
as  the  result  of  an  exchange  of  atomic  affinities.  "  Just  as  the  weakest 
acid  will  combine  with  a  small  quantity  of  a  given  base  even  though 
the  amount  of  base  be  infinitesimal  as  compared  with  the  total  quantity 
which  would  be  necessary  to  neutralize  completely  all  of  the  acid,  so 
also  will  the  weakest  atom  of  a  molecule  compete  with  the  strongest  in 
an  interchange  of  chemical  affinities  provided  only  that  the  trans- 
formation involves  the  formation  of  a  new  substance  which  is  capable 
of  existing,  or  a  spontaneous  dissociation  involving  an  increase  in  the 
total  entropy." 

The  more  closely  the  two  atoms  of  a  double  molecule  resemble  each 
other  in  their  chemical  affinity  for  a  third  atom,  with  which  they  both 


580  THEORIES  OF  ORGANIC  CHEMISTRY 

can  enter  into  combination,  the  more  nearly  equal  will  be  the  relative 
number  of  molecules  of  the  two  compounds  which  theoretically  can 
be  formed.1  For  example,  let  us  assume  that  in  a  given  reaction  the 
atoms  A  and  B  of  a  molecule  AB  possess  unequal  attractions  for  the 
atoms  C  and  D  of  the  molecule  CD.  If  now  A  has  a  greater  affinity 
than  B  for  C,  a  reaction  of  this  order  will  take  place  provided  that  the 
affinities  represented  by  the  expression  BD  are  greater  than  those  of 
CD.  The  ease  and  completeness  of  the  reaction  will  increase  in  pro- 
portion as  the  former  value  exceeds  the  latter.  In  addition  reactions, 
however,  other  forces  have  to  be  taken  into  consideration.  The  course 
of  the  above  reaction  will  be  affected  not  only  by  the  affinity  of  A  for  C 
and  of  B  for  D,  but  also  by  those  of  A  for  D  and  of  B  for  C,  and  thus 
the  possibility  arises  that  combinations  may  take  place  not  only  in  the 
sense  AC — BD,  but  also  in  the  sense  AD — BC.  In  other  words  the 
latter  reaction  will  be  favored  in  direct  proportion  as  the  value 
AC-\-BD>AD-{-BC  becomes  less.  If  now,  these  relationships  are 
altered  in  such  a  way  that  the  attraction  of  A  for  C  becomes  relatively 
greater  than  B  for  C,  it  follows  AC — BD  will  be  formed  at  the  expense 
of  AD — BC,  while  if  B  has  a  greater  affinity  than  A  for  D  it  may  happen 
that  the  quantity  of  AD — BC  formed  may  be  relatively  so  small  as  to 
be  negligible.2 

Inorganic  chemistry  deals  with  the  mutual  action  of  two  electrolytes 
while  organic  chemistry  treats  with  only  one  electrolyte.  Whether  the 
difference  existing  between  these  two  branches  of  chemistry  is  to  be 
found  here  or  not  is  questionable.  It  is  undoubtedly  true,  however,  in 
organic  reactions  involving,  for  example,  the  affinities  of  A  and  B  for 
C  and  D,  that  the  formation  of  AC — BD  will  take  place  if  A  has  a 
more  pronounced  attraction  for  C  than  B  has  for  D.  The  same  change 
is  also  favored  when  A  and  B  are  not  changed  but  CD  is  so  altered 
that  C,  the  more  energetic  of  the  two  constituents,  becomes  relatively 
more  reactive  than  B  toward  A.3  Michael  calls  the  relation  which  is 
derived  in  this  way  from  the  law  of  neutralization,  the  "  distribution 
principle."  It  affords  an  explanation  for  the  facts  so  frequently 
observed  that  rearrangements  proceed  at  times  with  the  formation 
of  a  single  derivative  and  at  other  times  with  the  formation  of  several. 

Reactions  involving  molecular  addition  4  and  polymerization  5  may 
be  readily  interpreted  from  the  standpoint  of  the  principle  of  distribu- 

1  Jour,  prakt.,  Chemie,  60,  324  (1899). 

2  Jour,  prakt.  Chemie,  60,  339  (1899). 

3  Jour,  prakt.  Chemie,  60,  341  (1899). 

4  Jour,  prakt.  Chemie,  60,  341  (1899);  Ber.,  39,  2140  (1906). 

5  Jour,  prakt.  Chemie,  60,  437,  443  (1899). 


THEORETICAL  SPECULATIONS  OF  ARTHUR  MICHAEL        581 

tion,  but  the  relations  become  very  involved  when  processes  of  substi- 
tution and  cleavage  are  taken  into  consideration.  In  the  case  of  the 
latter  two  different  forces  must  be  considered,  namely,  the  energy  which 
is  needed  to  separate  the  atoms  which  constitute  the  organic  molecule, 
and  the  relative  attraction  which  the  particular  reagent  has  for  the  atoms 
in  question.  Such  changes  may,  however,  be  interpreted  in  terms  of 
the  distribution  principle  if  the  following  statement  of  Michael  is 
valid:  "  the  substitution  of  a  hydrogen  atom,  attached  to  carbon,  by 
an  organic  radical  results  in  a  relative  increase  in  the  positive  or  negative 
energy  of  the  other  atoms  in  the  molecule.  Whether  the  effect  which 
is  produced  will  be  positive  or  negative  in  character  depends  upon 
whether  the  substituting  group  is  relatively  positive  or  negative  as 
compared  with  hydrogen.  If  the  effect  is  positive,  it  follows  that  the 
attraction  of  the  carbon  atom  for  such  atoms  as  are  relatively  negative 
to  carbon  is  increased,  but  that  at  the  same  time  it  is  decreased  to  a 
corresponding  degree  for  the  relatively  positive  atom  of  hydrogen. 
The  substitution  of  a  negative  radical  decreases  the  power  of  the  carbon 
to  hold  other  atoms  in  the  molecule."  1 

A.  Michael  and  H.  Leupold  2  have  recently  studied  the  course  of 
intramolecular  rearrangements  in  the  case  of  alkyl  bromides,  and  have 
brought  forward  experimental  evidence  which  is  of  value  in  solving  the 
problem  as  to  the  causes  which  operate  to  bring  about  a  condition  of 
equilibrium  in  reversible  reactions.  The  rearrangements  of  propyl 
bromide,  butyl  bromide,  etc.,  were  investigated  and  it  was  found  that 
both  isobutyl  bromide  and  tertiary  butyl  bromide  rearrange,  albeit  at 
different  rates,  according  to  the  following  expression : 

(CH3)2CH-CH2Br    <=»     (CH3)3CBr 

The  reaction  proceeds  far  more  readily  in  the  direction  of  the  formation 
of  the  tertiary  bromide  than  in  the  reverse  direction.  For  example 
fifteen  hours  heating  of  the  isobutyl  bromide  at  140°  results  in  the 
transformation  of  about  74  per  cent  of  the  halide  into  the  tertiary 
bromide.  At  this  temperature  the  tertiary  bromide  shows  no  tendency 
to  isomerize  into  the  secondary  bromide.  When  heated  at  a  temper- 
ature of  184°  for  two  or  three  hours  isobutyl  bromide  is  also  converted 
into  the  tertiary  compound  to  the  extent  of  74  per  cent  while  the  terti- 
ary bromide  yields  a  mixture  containing  1  to  2  per  cent  of  the  second- 
ary compound  when  heated  at  the  same  temperature  for  the  same 
length  of  time.  Again  at  262°  the  secondary  bromide  undergoes 

1  Jour,  prakt.  Chemie,  76,  105  (1907). 

2  Annalen  der  Chemie,  379,  263  (1911). 


582  THEORIES  OF  ORGANIC  CHEMISTRY 

rearrangement  giving  a  mixture  containing  74  per  cent  of  the  tertiary 
modification.  In  other  words,  this  percentage  of  tertiary  bromide 
represents  a  condition  of  equilibrium,  and  the  slowness  with  which  the 
reverse  rearrangement  takes  place  is  due  to  the  fact  that  the  tertiary 
compound  possesses  relatively  less  free  chemical  energy  than  the  sec- 
ondary bromide.  A  primary  decomposition  in  the  sense  of  a  dis- 
sociation apparently  does  not  take  place  to'  any  appreciable  extent  at 
the  temperature  of  rearrangement.  It  is,  therefore,  impossible  to 
explain  the  rearrangement  by  assuming  that  hydrogen  bromide  is  first 
split  off  and  then  adds  in  a  different  sense  so  as  to  give  an  isomeric 
product. 

The  transformations  in  the  case  of  the  propyl  bromides  are  analogous 
although  not  quite  so  easily  studied.  Michael  and  Leupold  have 
explained  the  rearrangements  of  these  substances  from  the  standpoint 
of  the  law  of  entropy  as  follows:  if  hydrogen  bromide  adds  to  propy- 
lene  the  reaction  must  proceed  in  such  a  way  that  the  maximum  entropy 
possible  under  given  conditions  is  obtained,  with  the  result  that  the  free 
chemical  energy  of  the  reacting  molecules  is  changed  as  completely 
as  possible  into  bound  chemical  energy.  Molecular  resistance  to  chem- 
ical change  must,  however,  first  be  overcome  in  order  that  the  reaction 
may  proceed.  In  other  words,  a  certain  amount  of  energy  must  be 
expended  in  order  to  sever  or  at  least  to  weaken  the  atomic  unions 
or  forces  already  existing,  as,  for  example,  between  hydrogen  and  bro- 
mine, and  represented  by  the  bound  chemical  energy  of  the  hydrogen 
bromide  molecule.  Since  in  the  case  of  propylene  addition  the  resist- 
ance to  be  overcome  is  the  same  whatever  product  is  formed,  it  follows 
that  addition  in  this  case  must  necessarily  result  in  the  production  of 
that  one  of  all  possible  isomers  which  possesses  the  greater  heat  of 
formation.  The  molecular  distribution  of  all  opposing  chemical  forces 
is  relatively  more  uniform  in  the  isopropyl  bromide  molecule  than  in 
normal  propyl  bromide.  The  former  substance  has,  therefore,  a  dis- 
tinctly greater  heat  of  formation  than  the  latter  1  and,  in  fact,  is  prac- 
tically the  sole  product  of  the  addition  of  hydrobromic  acid  to  propylene. 

It  has  been  observed  very  generally  that  the  attraction  of  elements 
or  compounds  for  each  other  increases  with  rise  of  temperature  up  to 
a  given  point  and  that  from  this  point  on  it  decreases  until  a  second 
point  is  reached  at  which  no  combination  takes  place  or  at  which,  in 
other  words,  the  compounds  formed  in  the  reaction  are  completely 
dissociated.  The  behavior  of  propylene  toward  hydrobromic  acid  may 
be  considered  in  illustration.  These  substances  unite  at  relatively  low 
temperatures  and  their  reactivity  is  increased  with  increase  in  temper- 
1  Jour,  prakt.  Chemie,  68,  499  (1903);  79,  418  (1909);  Ber.,  39,  2140  (1906). 


THEORETICAL  SPECULATIONS  OF  ARTHUR   MICHAEL        583 

ature  until  finally  a  temperature  is  reached  at  which  isopropyl  bromide 
begins  to  dissociate.  From  this  temperature  on  the  tendency  to  addi- 
tion becomes  less  until  finally  a  point  is  reached  at  which  propylene 
and  hydrobromic  acid  no  longer  combine.  Normal  propyl  bromide 
shows  a  similar  tendency  to  dissociate  at  high  temperatures.  Since 
the  secondary  bromide  represents  a  molecular  arrangement  of  maximum 
entropy,  the  primary  compound  should  show  a  tendency  to  pass  over 
into  this  form.  The  reason  why  this  rearrangement  does  not  take 
place  under  ordinary  conditions  is  due  to  the  fact  that  the  bound 
chemical  energy  in  the  molecule  of  normal  propyl  bromide  is  greater 
than  the  free  chemical  energy  striving  to  bring  about  a  rearrangement  of 
the  molecule.  This  bound  energy  can  be  conceived  as  operating  between 
hydrogen  and  bromine  on  the  one  hand  and  between  the  other  atoms 
of  the  molecule  on  the  other  hand.  Upon  heating,  normal  propyl  bro- 
mide absorbs  energy  until  finally  at  a  given  temperature  there  is  a  simul- 
taneous loosening  of  hydrogen  and  bromine  atoms  from  the  rest  of  the 
molecule,  and  when  sufficient  energy  has  been  absorbed  to  overcome 
the  total  chemical  resistance  to  this  change  a  rearrangement  of  the 
primary  into  the  secondary  bromide  will  take  place.  In  the  case  of 
isopropyl  bromide,  which  possesses  much  less  free  chemical  energy 
than  the  normal  bromide,  isomerization  takes  place  much  less  readily 
or,  in  other  words,  a  greater  amount  of  energy  is  necessary  in  order  to 
overcome  the  resistance.  It  is  to  be  noted,  further,  that  isopropyl 
bromide  has  a  greater  specific  heat  than  the  normal  bromide  and  is 
thus  better  able  to  absorb  energy.  From  this  it  follows  that  the  energy 
relations  between  the  two  isomers  must  necessarily  change  with  rise  in 
temperature  and  that  while  at  ordinary  temperatures  the  difference 
between  the  relative  energies  of  the  two  substances  is  great,  at  high 
temperatures  this  difference  becomes  less.  A  similar  relationship  exists 
between  the  iso-  and  tertiary  butyl  bromides  and  accounts  for  the  fact 
that  the  zso-bromide  does  not  isomerize  completely  at  high  temperatures 
but  gives  a  mixture  consisting  of  74  per  cent  of  the  tertiary  and  26  per 
cent  of  the  secondary  bromide.  This  constant  mixture  represents 
apparently  the  maximum  of  entropy  at  high  temperatures. 

The  rearrangement  of  isobutyl  into  tertiary  butyl  bromide  is  rever- 
sible. Now  it  is  usually  assumed  that  as  a  condition  of  chemical  equi- 
librium is  approached,  the  relative  quantities  of  the  substances  formed 
during  any  instant  bear  the  same  relation  as  the  relative  velocities  of 
the  opposing  reactions.  A  condition  of  equilibrium  is  reached  when 
the  velocities  in  opposite  directions  become  equal.  In  other  words 
such  equilibrium  is  considered  dynamic  in  that  rearrangements  take 
place  in  both  senses  although  their  resultant  is  equal  to  zero.  The  cor- 


584  THEORIES  OF  ORGANIC  CHEMISTRY 

rectness  of  this  conception  has  been  put  to  experimental  proof  in  the 
case  of  butyl  bromide  by  Michael  and  Leupold.  They  determined 
the  rates  for  both  rearrangements  and  obtained  results  which  were  in 
direct  contradiction  to  this  fundamental  law.  This  led  Michael  to  the 
conclusion:1  "  in  the  experiments  with  butyl  bromides  the  question 
as  to  the  cause  of  the  resulting  condition  of  equilibrium  in  reversible 
reactions  has  been  investigated  experimentally  for  the  first  time  and  as 
a  result  it  may  be  said  that  in  this  case  the  general  assumption  that 
such  a  process  may  be  represented  as  dynamic  in  character  has  been 
found  to  be  untenable.  Equilibrium  is  here  conditioned  not  by  the 
velocity  constant  but  by  the  more  presence  of  the  two  bromides  in  the 
proportion  of  74  per  cent  tertiary  to  26  per  cent  secondary.  This 
mixture  probably  represents  the  maximum  of  entropy  possible  under 
the  conditions  and  when  attained  all  further  change  ceases.  In  other 
words,  the  equilibrium  is  apparently  static  in  character.  It  is,  of 
course,  possible  to  conceive  a  condition  of  equilibrium  of  this  sort  as 
static  merely  with  reference  to  chemical  reactivity  in  one  direction  or 
the  other  and  at  the  same  time  to  regard  it  as  dynamic  with  reference 
to  the  energy  relations,  in  that  it  may  be  assumed  that  a  constant  inter- 
change of  energy  takes  place  between  the  two  isomeric  bromides."  2 

Michael  distinctly  avoids  explanations  of  chemical  processes  which 
involve  the  use  of  mechanical  representations.  How  he  is  able  to  main- 
tain this  attitude  in  dealing  with  the  problems  of  stereochemistry  may 
be  learned  by  reading  his  treatise,  "  Die  van't  Hoff-Wisclicermssche 
Konfigurationslehre."  3  His  theories  conceive  the  problems  of  organic 
chemistry  from  a  large  point  of  view  and  it  is  possible  by  means  of  them 
to  follow  the  course  of  organic  rearrangements  in  much  greater  detail 
than  ever  before.  Real  progress  is  to  be  anticipated  in  the  develop- 
ment of  Michael's  conceptions  and  it  is  to  be  expected  confidently  that 
not  only  will  the  quantitative  applications  of  his  theories  be  perfected 
but  that  their  use  will  also  be  simplified. 

1  Annalen  der  Chemie,  379,  285  (1911). 

2  For  a  further  review  of  Michael's  views  the  reader  is  referred  to  the  following 
papers:   Das  Chinon  vom  Standpunkt  des  Entropiegesetzes  und  der  Partialvalenz- 
hypothese,  Jour,  prakt.  Chemie,  79,  418  (1909);  iJber  Desmotropie  und  Merotropie 
Annalen  der  Chemie,  363,  20,  36,  64,  94  (1908);  Ber.,  41,  1080  (1908);  Annalen  der 
Chemie,  364,  64,  129  (1909). 

3  Jour,  prakt.  Chemie,  75,  105  (1907);  Am.  Chem.  Jour.,  39,  1  (1908);  Ber.,  41, 
2907  (1908);  Ber.,  42,  310,  317,  3157  (1909). 


CHAPTER  XXII 


RECENT  ELECTROCHEMICAL  THEORIES 

THE  mechanism  of  the  process  of  reduction  in  organic  chemistry 
was  first  investigated  systematically  in  connection  with  a  study  of  the 
nitro-derivatives  of  the  aromatic  hydrocarbons.  Thus  W.  Lob,1  taking 
as  a  basis  the  pioneer  work  of  K.  Elbs,2  Haussermann,3  and  Gatter- 
mann,4  interpreted  the  mechanism  of  the  reduction  of  nitro-compounds 
in  alkaline  solution  by  supposing  that  the  first  step  in  such  a  process 
is  the  formation  of  intermediate  orZ/io-hydrate  combinations: 

C6H5N02    ->    C6H5N(OH)4 

It  was  assumed  that  in  the  course  of  alkaline  reduction  the  radicals 
RN(OH)3— ,RN(OH)2=,  RN(OH)==,  R-  N=,  were  successively  formed, 
and  that  azoxy-,  azo-,  and  other  reduction  products  were  produced  as  a 
result  of  the  subsequent  coupling  of  these  radicals.  In  1898,  however, 

H5  •  N02 


C6H5-N N-C6H5 

X0X 


H.NH 


C6H5.NH 


NH   C6 


OH  (1) 
NH2(4) 


F.  Haber  5  was  able  to  demonstrate  that  nitrosobenzene  and  phenyl- 
hydroxylamine  were  formed  in  both  alkaline  and  acid  solution  as  pri- 
mary reduction  products  of  nitrobenzene,  and  that  in  alkaline  solutions 
they  combine  to  give  azoxy-compounds.  In  strongly  acid  solutions 
CeHsNHOH  rearranges  to  give  p-amidophenol,  and  in  the  presence 
of  weak  acids  it  is  reduced  to  aniline.  The  so-called  "  Haber  scheme 
of  reduction  "  has  been  developed  as  a  result  of  this  investigation: 

^eitschr.  Elektrochemie,  2,  529  (1896);    3,  471  (1897). 

2  Jour,  prakt.  Chemie,  43,  39  (1891). 

3  Chem.  Zeitung.  (1893),  129,  209. 
4Ber.,  26,  1844  (1893);   27,  1927  (1894). 
6Zeitschr.  Elektrochemie,  4,  506  (1898). 

585 


586  THEORIES  OF  ORGANIC  CHEMISTRY 

In  this  scheme  the  directly  descending  arrows  on  the  right  represent 
the  process  of  electrolytic  reduction,  while  all  others  represent  purely 
chemical  transformations  and  rearrangements.1 

Recent  developments  in  the  field  of  electro-chemistry  and  related 
subjects  might  be  expected  to  throw  new  light  upon  the  atomic  rela- 
tions of  matter.  It  is,  therefore,  necessary  to  pause  for  a  brief  histor- 
ical review  of  this  subject  in  order  to  ascertain  whether  the  new  interpre- 
tations which  were  evolved  in  this  way  really  escape  the  charge  of  one- 
sidedness,  which  the  application  of  Kekule's  structural  theories  brought 
with  them. 

The  fundamental  defect  in  the  theories  of  Berzelius  was  that  they 
failed  to  establish  any  numerical  relationship  between  electrical  attrac- 
tion and  chemical  affinity.  The  first  fundamental  law  governing  such 
a  relationship  was  discovered  by  Faraday  in  1833,  and  may  be  stated 
as  follows:  "  the  amount  of  substance  which  is  decomposed,  as  the 
result  of  the  passage  of  the  electric  current,  is  proportional  to  the 
quantity  of  current,  and  the  amounts  of  different  materials  formed  by 
decomposition  in  a  given  interval  of  time,  bear  the  same  relation  to 
each  other  numerically  as  their  respective  chemical  equivalents." 
This  law  of  Faraday's  introduced  the  quantitative  period  of  electro- 
chemistry. 

Faraday's  law  exercised  no  influence  upon  the  theories  of  Berzelius, 
and  was  indeed  in  such  apparent  contradication  to  the  fundamental 
assumptions  upon  which  these  theories  were  based  that  the  great 
Swedish  chemist  even  doubted  Faraday's  results.  Berzelius  had  assumed 
that  different  atoms  possess  different  charges  of  electricity,  and  that 
the  greater  the  positive  and  negative  charges  of  two  reacting  atoms 
the  stronger  will  be  their  attraction  for  each  other.  Faraday's  investi- 
gations proved  conclusively  that  all  atoms  possess  equally  great  positive 
and  negative  charges,  but  failed  to  answer  the  question  as  to  what  con- 
stituted the  difference  in  chemical  attraction  between  different  atoms. 

Faraday  had  distinguished  quite  definitely  between  the  strength 
of  current  and  the  intensity  of  the  current  (electromotive  force),  but 
Berzelius  used  the  two  terms  interchangeably,  and  therefore  assumed 
that  equal  quantities  of  electricity  must  also  represent  an  equal  degree 
of  chemical  attraction.  It  thus  happened  that  Faraday's  law  was  not 
embodied  in  the  leading  theory  of  his  day,  and  remained  comparatively 

1 K.  Elbs,  "  Ubungsbeispiele  fur  die  elektrolytische  Darstellung  chemischer 
Praparate,"  2.  Aufl.,  Halle,  1911;  K.  Brand,  "Die  elektrochemische  Reduktion 
organischer  Nitrokorper, "  Stuttgart,  Enke,  1908;  Loeb,  "  Elektrochemie  organischer 
Verbindungen " ;  A.  Moser,  "Die  elektrolytische  Prozesse  der  organischer  Chemie," 
Halle,  Knapp,  1910. 


RECENT  ELECTROCHEMICAL  THEORIES  587 

unnoticed  until  1881,  when  H.  von  Helmholtz  incorporated  it  in 
the  valence  theory,  which  had  in  the  meantime  come  into  promi- 
nence. 

The  electrochemical  terms  in  common  use  at  the  present  time  such 
as,  for  example,  electrode,  electrolyte,  cation,  and  anion,  etc.,  owe 
their  origin  to  Faraday.  Substances  whose  movements  were  directed 
towards  the  two  electrodes  were  called  ions,  and  the  study  of  these 
ions  brought  about  step  by  step  the  most  important  developments  in 
the  theory  of  electrochemistry.  It  took  decades  to  complete  the  chain 
of  these  developments  but  this  was  partly  Faraday's  fault  since  hin- 
drances as  well  as  guides  to  further  important  investigations  are  to  be 
met  with  side  by  side  in  his  work.  For  example,  he  had  observed  that 
weak  currents  may  pass  through  electrolytes  without  effecting  any 
apparent  decomposition  and  he,  therefore,  assumed  that  this  was  due 
to  the  existence  of  a  second  type  of  electrolytic  conductivity  which 
resembled  the  conductivity  of  the  metals.  Only  the  later  investi- 
gations of  Buff  and  others  served  to  dissipate  these  ideas  and  to  estab- 
lish the  fact  that  the  passage  of  an  electric  current  through  solutions 
is  always  accompanied  by  chemical  decomposition.  It  has  been  cal- 
culated that  the  quantity  of  electricity  necessary  to  decompose  one 
gram  equivalent  of  any  substance  is  equal  to  96540  coulombs.  Since 
the  equivalent  weight  of  an  element  is  equal  to  its  atomic  weight 
divided  by  its  valency,  Helmholtz  was  able,  in  1881,  to  point  out  the 
relationship  existing  between  Faraday's  law  and  the  theory  of  valency. 
In  his  famous  "  Faraday  Lectures  "  Helmholtz  gave  the  following  expres- 
sion to  the  law  of  electrolysis  as  applied  to  solid  bodies:  "  the  same 
quantity  of  current  in  equal  periods  of  time  sets  free  an  equal  number 
of  valencies  in  different  electrolytes,  and  causes  them  to  assume  other 
forms  of  combination."  The  fact  that  not  only  acids,  bases,  and  salts, 
but  that  all  substances  may  be  regarded  as  electrolytes  led  Helmholtz 
to  the  further  development  of  this  conception  which  he  expressed  as 
follows:  "  if  every  unit  of  affinity  is  charged  with  an  equivalent  amount 
of  electricity,  whether  positive  or  negative,  it  follows  that  the  electrically 
neutral  bodies  must  arise  as  a  result  of  the  mutual  combination  under 
the  influence  of  the  powerful  electrical  attraction  of  units  which  are 
respectively  positively  and  negatively  charged.  It  is  obvious  that  in 
the  compounds  which  arise  in  this  way,  every  unit  of  affinity  possessed 
by  an  atom  must  be  in  union  with  one  and  only  one  other  unit  of  affinity 
possessed  by  another  atom.  This  must  be  recognized  at  once  as  the 
modern  theory  of  valency  and  includes  all  saturated  compounds." 
"  Unsaturated  compounds  which  possess  an  even  number  of  free  units 
of  affinity  furnish  no  ground  for  objection  to  this  hypothesis,  since  they 


588  THEORIES  OF  ORGANIC  CHEMISTRY 

may  be  imagined  as  charged  with  equal  amounts  of  positive  and  nega- 
tive electricity." 

If  Faraday  delayed  progress  in  the  development  of  electrochemical 
theory  by  his  false  assumption  that  electrolytes  were  capable  of  metallic 
as  well  as  of  electrolytic  conduction  of  electricity,  he  certainly  gave 
the  impulse  for  extremely  important  discoveries  in  the  field  of  affinity 
relationships  by  a  remark  in  one  of  his  later  papers.  In  1840  he  pointed 
out  that  the  electric  current  can  do  work.  Now  if  electrical  energy 
could  result,  by  mere  contact,  it  would  follow  that  energy  (work)  could 
be  created  out  of  nothing.  This  idea,  soon  after  the  discovery  of  the 
law  of  conservation  of  energy  led  W.  Thomson  and  Helmholtz  to 
apply  systematically  the  law  of  the  mechanical  theory  of  heat  to  the 
phenomena  of  electrolysis. 

When  a  compound  is  decomposed  by  the  electric  current,  the  work 
done  must  be  sufficient  to  overcome  the  sum  of  all  the  forces  which 
hold  together  the  various  parts  of  the  molecule.  This  would  be  repre- 
sented by  the  product  of  (a)  the  quantity  of  electricity  necessary  for 
decomposition,  and  (6)  the  electromotive  force.  Since,  moreover,  the 
same  quantity  of  electricity  (96540  coulombs)  is  necessary  for  the  decom- 
position of  each  gram  equivalent,  irrespective  of  the  kind  of  matter, 
it  follows  that  the  work  is,  therefore,  proportional  to  the  electromotive 
force.  Since  the  work  necessary  to  decompose  a  given  chemical  com- 
pound must,  moreover,  be  the  same  as  that  necessary  for  its  formation, 
the  electromotive  force  may  be  regarded  as  the  measure  of  chemical 
affinity.  It  is  not  possible  to  discuss  fully  at  this  time  how  these 
facts  led  to  the  discovery  that  actually  only  a  relatively  small  fraction 
of  the  total  affinity,  which  Helmholtz  called  the  free  energy  of  the  atoms, 
is  proportional  to  the  heat  of  formation  of  a  given  molecule,  but  it  is 
obvious  that  affinities  can  be  measured  in  the  case  of  reversible  proc- 
esses by  the  determination  of  the  electromotive  forces.  Calculations 
by  Helmholtz,  Richarz,  and  Ebert  show  that  the  electrical  forces  which 
hold  the  atoms  together  are  of  such  magnitude  that  they  must  be  the 
principal  if  not  the  only  forces  which  are  at  work  in  the  molecule. 

The  so-called  ionic  theory,  which  has  played  such  an  important 
part  in  the  development  of  inorganic  and  analytical  chemistry,  offers 
a  very  probable  explanation  for  the  phenomena  which  have  just  been 
described.  Energy  conceptions  have  led  to  the  assumption  that  in 
solutions  which  conduct  electricity  the  molecules  are  already  dissociated 
into  ions,  so  that  this  work  does  not  need  to  be  done  by  the  electric 
current.  In  the  sense  of  this  theory  the  ions  may  be  either  single  atoms 
or  atomic  complexes  charged  with  one  or  more  units  of  positive  or 
negative  electricity.  They  are  entirely  different  from  the  elements 


RECENT  ELECTROCHEMICAL  THEORIES  589 

in  their  properties  and  are  regarded  by  Nernst  as  saturated  chemical 
compounds.  The  different  ions  are  to  be  distinguished  from  one  another 
by  the  fact  that  they  are  able  to  bind  their  electrons  more  or  less  securely, 
or,  in  other  words  to  hold  their  electric  charges  more  or  less  firmly. 
Nernst  succeeded  in  finding  in  the  so-called  dissociation  tension  a 
definite  measure  for  the  energy  which  binds  an  electron  to  a  given  atom 
or  group  of  atoms  to  form  an  ion  and  he  was  able  to  do  this  by  a  very 
ingenious  combination  of  the  laws  of  osmosis  and  the  ionic  theory. 
It  has  been  observed  that  a  definite  amount  of  electromotive  power  is 
necessary  for  the  electrical  discharge  of  an  ion,  and  that  this  value  is 
constant  for  ions  of  the  same  kind  but  is  different  for  different  ions. 
In  this  way  it  is  possible  to  obtain  numbers  which  express  the  relative 
strengths  of  union  of  a  unit  of  electricity  with  the  different  atoms  and 
groups  of  atoms. 

A  system  which  is  based  upon  the  electron  theory  has  been  developed 
by  R.  Abegg  and  G.  Bodlander.  This  has  been  of  the  greatest  value 
in  systematizing  inorganic  compounds  but  since  it  is  also  applicable 
to  organic  compounds,  it  may  be  briefly  outlined  at  this  point.  These 
investigators  consider  the  relation  of  the  atom  to  the  unit  charge,  i.e., 
the  electron,  rather  than  the  relation  of  one  atom  to  another  atom. 
This  value,  which  is  measurable,  is  called  the  electro-affinity  and  is 
regarded  as  inherent  in  the  atoms.  It  is,  however,  capable  of  holding 
only  a  limited  and  relatively  small  number  of  other  atoms  no  matter 
how  great  the  strength  of  affinity  of  any  particular  atom.  A  new  con- 
ception of  valency  was  based  upon  these  considerations.  If  atomic 
and  molecular  compounds  are  to  be  interpreted  from  a  common  point 
of  view  it  is  not  possible  to  assume  a  constant  valency  for  the  atoms, 
but  a  maximum  valency  which  is  not  always  fully  attained  in  all  the 
compounds  of  the  elements.  The  number  of  valencies  which  a  given 
atom  may  exercise  will  vary  according  to  the  nature  of  the  element 
combining  with  it.  This  variation  of  valency  takes  place  in  a  regular 
manner.  Thus  the  greater  the  difference  between  one  element  and 
another,  the  greater  will  be  the  valency  shown  by  one  for  the  other. 
A  measure  of  the  difference  between  elements  is  to  be  found  in  their 
electro-affinity,  and  this  may  be  readily  estimated  and  tabulated 
according  to  the  periodic  classification,  the  greatest  extremes  of  electro- 
affinity  being  found  in  elements  at  the  ends  of  the  horizontal  series. 

Electro-valence  is  the  name  given  by  Abegg  to  that  valence  which 
is  exercised  in  holding  the  ionic  charge,  and  he  seeks  to  reduce  the 
expression  of  all  affinity  to  the  mutual  action  of  electro-valencies.  He 
further  assumes  that  all  elements  possess  positive  as  well  as  negative 
maximum  valencies,  and  that  in  every  case  the  sum  of  these  is  eight. 


590 


THEORIES  OF  ORGANIC  CHEMISTRY 


The  electro-valency  of  the  element  is  therefore  polar  in  character. 
Normal  valencies  is  the  name  given  to  those  positive  or  negative  va- 
lencies which  while  fewer  in  number  are  stronger  in  effect,  while  contra- 
valencies  is  the  name  given  to  the  other  type  of  valencies.  Thus  the 
positive  maximum  valencies  of  an  atom  correspond  in  number  to  the 
group  number  of  a  given  element  in  the  periodic  system,  as  will  be 
seen  by  reference  to  the  following  table : 


I 

II 

III 

IV 

V 

VI 

VII 

Groups  of  the  Periodic  System 

+1 

+2 

+3 

+4 

-3 

-2 

-1 

Normal-valencies 

-7 

-6 

-5 

-4 

+5 

+6 

+7 

Contra-valencies 

The  old  idea  of  Berzelius  is  thus  resurrected,  clothed  in  modern  phrase- 
ology, and  given  a  more  definite  quantitative  expression.1 

In  the  groups  represented  at  the  extreme  right  and  left  of  the  table 
the  polar  character  of  the  element  is  strongly  marked,  and  is  decidedly 
positive  or  negative,  since  the  value  of  the  normal  valence  is  in  each 
case  equal  to  1.  Near  the  middle  of  a  horizontal  series  the  normal 
and  contra-valencies  become  more  nearly  equal  in  number  and  strength. 
Because  of  this  the  atoms  become  less  pronounced  (i.e,  decidedly  posi- 
tive or  negative)  in  their  chemical  affinities.  The  carbon  atom  is  especi- 
ally conspicuous  as  belonging  to  a  group  where  the  normal  and  contra- 
valencies  are  equal  in  number.  According  to  Abegg  this  accounts  for 
the  ability  of  carbon  to  combine  not  only  with  other  carbon  atoms  but 
with  both  positive  H,  Zn,  etc.),  and  negative  (Cl,  O,  etc.)  elements. 
Thus  it  happens  that  the  methyl  group  may  function  as  a  negative 
group  as  in  Zn(CHs)2,  and  as  a  positive  group,  as  in  (CHaCl). 

In  order  to  answer  the  question  as  to  why  only  four  valencies  of 
the  possible  eight  are  commonly  exercised  by  carbon,  Abegg  assumes 
that  the  exercise  of  one  kind  of  valence  prevents  the  functioning  of  the 
other.  Thus  if  a  positive  electro-valence  is  saturated,  the  correspond- 
ing negative  valence  becomes  inactive.  If  the  mutual  union  between 
carbon  atoms  depends  upon  their  opposite  polarity  it  follows  that  in 
CHa  •  CHs  the  two  carbon  atoms  have  different  functions.  It  has  been 
noted  that  in  other  compounds  CHs  may  function  as  a  positive  or  as 
negative  group,  so  that  this  idea  need  not  seem  impossible  when  applied 
to  ethane. 

In  C2H5C1  the  carbon  atom  exercises  a  positive  valence  for  the 
chlorine.  It  might  be  supposed  that,  as  a  result  of  this,  it  would  be 
more  inclined  to  develop  a  negative  valence  for  the  carbon  atom  adjacent 
1  D.  Mendelejeff,  "  Grundlagen  der  Chemie,"  5th  Edition,  1889,  p.  304. 


RECENT  ELECTROCHEMICAL  THEORIES  591 

to  it.  Experience  shows  that  further  chlorination  results  in  the  sub- 
stitution of  a  second  chlorine  atom  on  the  carbon  which  already  holds 
one,  and  it  may  be  supposed  that  the  exercise  of  a  second  positive  va- 
lency strengthens  the  original  inclination  of  this  carbon  atom  to  exhibit 
a  negative  valency  for  the  carbon  of  the  adjacent  CH3  group.  It  may 
even  be  that  this  propensity  on  the  part  of  two  carbon  atoms  to  combine 
mutually  by  means  of  polar  valencies  accounts  for  the  tendency  of 
chlorine  to  combine  with  carbon  already  holding  chlorine  (thus  increas- 
ing the  polarity  of  the  carbon),  and  also  for  the  following  rearrangements 
and  additions: 

CH3-CH2-CH2Br       ->     CH3.CHBr-CH3 
CH3CH=CH2+HI    ->    CH3CHI-CH3 

It  may  also  account  for  the  ease  with  which  C02  is  split  off  from  acids 
of  the  formulas  >C(COOH)2  and  NO2CH2COOH  and,  further,  for 
the  fact  that  in  aromatic  compounds  the  substitution  of  a  negative 
group  takes  place  in  the  meta-  rather  than  in  the  or^/io-position  with 
reference  to  a  negative  group  already  substituting  in  the  ring.  By  this 
arrangement  it  is  obviously  possible  for  the  two  carbon  atoms  bearing 
negative  groups  to  exercise  negative  polar  valencies  for  the  carbon 
atom  which  occupies  an  intermediate  position  between  them. 

Although  the  CH3  group  may  exercise  a  positive  or  a  negative 
valence,  facts  go  to  show  that  its  preference  is  for  the  former.  The 
CH2  group,  in  which  only  two  negative  valence-electrons  of  carbon 
are  exercised  in  holding  hydrogen,  reacts  negatively  toward  other  atoms 
more  readily  than  the  methyl  group  and,  in  general,  the  less  hydrogen 
there  is  in  union  with  carbon  the  greater  the  ease  with  which  the  latter 
will  function  negatively  toward  other  atoms.  The  negative  character 
of  unsaturated  radicals  tends  to  confirm  this  view. 

Abegg  gives  the  following  interpretation  to  the  constitution  of 
organo-ammonium  salts.  In  terms  of  the  present  theory  it  must  be 
assumed  that  the  nitrogen  atom  in  NH^Cl  exercises  more  than  one 
kind  of  valency.  Since  only  three  of  the  positive  hydrogen  atoms  can 
unite  with  the  three  negative  valencies  of  nitrogen  (three  being  the 
maximum  number  of  normal  valencies  for  this  element)  the  fourth 
hydrogen  atom  is  called  upon  to  exercise  its  own  negative  contra- valence 
toward  a  positive  contra- valence  of  nitrogen.  The  instability  of  this 
union  is  clearly  shown  by  the  ease  with  which  ammonia  is  split  off 
from  ammonium  salts  and  is  evidence  of  the  weak  affinity  of  positive 
nitrogen  valencies.  If  alkyl  groups  replace  the  hydrogen  in  an  ammo- 


592  THEORIES  OF  ORGANIC  CHEMISTRY 

nium  salt,  it  follows  that  nitrogen  valencies  are  now  exercised  toward 
carbon.  That  the  negative  valencies  of  carbon  are  much  stronger  than 
those  of  hydrogen  is  shown  by  the  existence  of  alkyl  metallic  hydroxides 
while  the  corresponding  compounds  with  hydrogen  do  not  exist.  Thus 
of  the  four  substituting  groups  three  combine  with  negative  valencies 
of  nitrogen,  while  the  fourth  combines  perforce  with  a  positive  nitrogen 
valence.  And  it  is  here  that  a  striking  increase  in  the  stability  of  the 
molecule  is  to  be  observed.  The  instability  of  substituted  ammonium 
salts  containing  less  than  four  alkyl  groups  may  be  assumed  to  be  due 
to  the  dissociation  of  the  N' — H'  union,  and  from  this  it  follows  that 
the  first  three  alkyl  groups  must  combine  with  nitrogen  by  means  of 
its  three  negative  valencies,  for  if  any  one  of  them  had  combined  with 
a  positive  valence  of  nitrogen  the  union  would  be  stable. 

There  can  be  no  doubt  but  that  the  introduction  of  elements  and 
radicals  of  marked  polarity  (metals,  oxygen,  halogen,  etc.)  into  organic 
compounds  increases  enormously  the  chemical  reactivity  of  these  sub- 
stances. This  may  be  explained  by  supposing  that  the  particular 
atom,  or  group  of  atoms,  gives  chemically  neutral  or  indifferent  com- 
pounds when  in  union  with  polar  partners  which  are  not  essentially  dif- 
ferent from  it  (as,  for  example,  C  and  H)  and  that,  on  the  other  hand, 
it  becomes  chemically  active  in  the  presence  of  substituting  groups  of 
marked  polarity,  through  the  necessity  of  developing  an  opposite 
polarity  for  itself.  For  example,  CHa-H  does  not  react  with  HC1 
because  CHa  and  H  are  so  much  alike  that  neither  develops  polarity 
in  the  other  while,  on  the  other  hand,  CHa -OH  does  react  with  HC1 
because  the  radical  CHa  is  forced  into  playing  a  definitely  positive  role 
through  the  presence  of  the  negative  OH  group.  In  the  latter  case 
CHaCl  and  H20  are  formed  as  a  result  of  the  double  decomposition 
of  the  two  molecules.  CHa  reacts  with  HC1  in  quite  a  different  sense 
when  playing  a  definitely  negative  role  as  in  (CHa^Zn.  In  this  case 
CHaH  is  formed  along  with  CHaCl  as  a  result  of  the  reaction  with  HC1. 
That  the  same  group  reacts  differently  with  a  given  reagent  under 
different  circumstances,  is  important  only  in  so  far  as  it  shows  that 
reactivity  goes  hand  in  hand  with  a  marked  difference  in  polarity  of 
the  parts  within  a  molecule. 

Abegg  1  has  recently  given  particular  attention  to  the  interpretation 
of  those  chemical  changes  which  are  included  under  the  head  of  the 
Grignard  reactions  and  which  involve  combinations  of  magnesium 
alkyl  halides  with  aldehydes,  ketones,  esters,  and  other  unsaturated 
molecules.  It  has  been  noted  that  alkyl  radicals  may  function  in 
either  a  positive  or  negative  capacity.  In  alkyl  magnesium  halides  the 
iBer.,  38,  4112  (1905). 


RECENT  ELECTROCHEMICAL  THEORIES  593 

alkyl  group  must  be  imagined  as  compelled  to  play  a  negative  role  for 
which  it  has  no  liking.  Thus  there  is  a  tendency  in  the  molecule  for 
the  alkyl  to  abandon  this  role  for  one  more  to  its  general  satisfaction, 
and,  simultaneously,  a  tendency  for  the  magnesium  halide  complex 
to  seek  for  itself  a  more  comfortable  companion.  These  two  tendencies 
represent  the  impelling  forces  at  work  in  magnesium  halogen  com- 
pounds. They  are  neutralized  during  the  process  of  addition  to  ketones 
since  the  influence  of  doubly  bound  oxygen  asserts  itself  and  affords 
the  magnesium  halogen  residue  an  opportunity  to  enter  into  combi- 
nation with  the  stronger  of  the  two  negative  valencies  of  the  oxygen, 
while  at  the  same  time  the  alkyl  radical  attaches  itself  to  the  free  va- 
lency of  carbon.  That  this  carbon  valence  functions  in  a  positive 
sense  is  obvious  from  the  fact  of  its  previous  union  with  a  negative 
oxygen  atom.  It  is  equally  obvious  that  the  new  adjustment  of 
valencies  makes  for  a  more  stable  condition  in  the  molecule. 

The  reaction  of  organo-magnesium  halides  with  acid  esters  leads 
to  interesting  deductions  in  regard  to  the  polar  character  of  the  com- 
ponents of  the  esters.  These  bodies  react  in  either  of  two  ways  depend- 
ing upon  the  reagent  employed : 

(R -COO) -  Alkyl+        or        (RCO)  +  (O— Alkyl) - 

With  a  Grignard  reagent  the  reaction  takes  place  in  the  latter  sense 
and  the  magnesium  halogen  residue  combines  with  ( — O  Alkyl). 
A  second  molecule  of  the  alkyl  magnesium  halide  then  reacts  with  a 
carbonyl  group  and  at  the  same  time  the  alkyl  group  of  the  first  mole- 
cule of  halide  combines  with  the  carbon  to  give  (Alkyla — C-O)~- 
(Mg-Halogen)+.  The  addition  of  CO2,  COS,  and  S02  to  the  organic 
part  of  the  organo-magnesium  compounds  bears  evidence  to  the  need 
of  such  radicals  to  become  better  fitted  for  the  negative  function  forced 
upon  them  by  the  magnesium. 

Abegg  also  discussed,  from  the  standpoint  of  the  present  theory, 
the  conditions  governing  the  relative  stability  and  the  rearrangements 
of  the  stereoisomeric  oximes,  the  discussion  being  essentially  the  same 
as  that  given  in  an  earlier  paper.1  Oximes  frequently  give  salts  with 
acids  as  well  as  with  bases.  If  these  salts  are  dissolved  in  water,  the 
ion  of  the  oxime  sometimes  functions  as  the  cation  (acid  solution)  and 
sometimes  as  an  anion  (alkaline  solution)  receiving  thus  in  one  case  a 
positive,  and  in  the  other  case,  a  negative  charge.  This  is  most  readily 
interpreted  by  supposing  that  the  charge  is  located  in  that  part  of  the 

1Ber.,32,  291  (1899). 


594  THEORIES  OF  ORGANIC  CHEMISTRY 

molecule  from  which  the  dissociation  of  the  other  ion  takes  place,  viz., 
on  the  oxygen  of  the  oxime  group.     For  example: 

Riv  /Cl  Rlx         /+ 

\C=N^-H       ->          >C=NH       +  Cl-  (in  acid  solution) 
R2/  \OH  R/         \QH 

Riv  Ri\  + 

>C=NONa       ->          >C=NO  +  Na  (in  alkaline  solution) 


It  may  be  assumed,  further,  that  Ri  and  R2  also  possess  charges  of 
electricity  and  that  those  oximes  would  be  most  stable  in  which  the 
arrangement  of  parts  is  such  that  the  positively  charged  radical  is  placed 
immediately  opposite  the  hydroxyl  group  of  the  oxime.  Thus  in  acid 
solution  the  stable  and  labile  forms  are  respectively  : 

•f  + 

-Cx-i  —  O  —  R2  R!  —  C  —  -tv2 

II       +  and  +    || 

NH(OH)  (HO)NH 

Stable  Labile 


and  analogously  in  alkaline  solution  there  would  be  present 
Ri-C-Rj  Ri-C-Ra 

oli         -         lo 

Stable  Labile 

This  offers  a  basis  for  determining  the  relative  stability  of  oximes, 
since  the  dissociation  constants  of  acids  and  bases,  which  contain 
Ri  and  R2  in  suitable  forms  of  combination,  make  it  possible  to  con- 
struct a  table  showing  the  relative  influence  of  these  respective  radicals. 
According  to  Abegg,  the  series  runs  as  follows: 

—  COOH,  Cl,  C6H5,  C6H5X,  H,  Alkyl,  CH3 

and  agrees  in  all  essentials  with  the  series  arranged  by  A.  Hantzsch  l 
to  show  the  relative  chemical  attraction  of  radicals  for  the  oxime  group. 
In  conclusion  it  may  be  said  that  it  is  exceedingly  difficult  to  formu- 
late a  composite  picture  of  the  theories  of  organic  chemistry.  The 
present  treatise  has,  therefore,  confined  itself  to  a  consideration  of  cer- 
tain of  the  more  fundamental  conceptions  which  are  of  general  applica- 
tion, and  has  endeavored  to  point  out  in  each  case  the  strength  and  the 
weakness  of  the  individual  views.  In  this  way  a  certain  unity  in  the 

lBer,,  25,2164(1892). 


RECENT  ELECTROCHEMICAL  THEORIES  595 

development  of  the  theory  of  organic  chemistry  has,  however,  become 
apparent.  Thus  the  earlier  ideas  in  regard  to  unit  valencies  have  been 
observed  gradually  to  give  place  to  more  detailed  and  complex  con- 
ceptions in  regard  to  the  existence  and  function  of  so-called  partial 
valencies.  The  questions  about  which  the  greatest  interest  now  centers 
have  come  to  be  those  which  involve  the  distribution  of  affinity  among 
the  different  parts  of  the  molecule  under  different  conditions  and  which, 
therefore,  require  for  their  solution  not  merely  the  application  of  chem- 
ical methods  but  frequently  of  physical  methods  as  well.  Since  it  has 
often  been  difficult  to  harmonize  the  data  which  have  been  obtained 
from  these  different  sources,  a  definite  effort  has  been,  and  is  still 
being  made  to  bring  about  a  more  satisfactory  correlation  of  physical 
and  chemical  methods,  and  it  is  to  be  hoped  that  the  future  will  see 
material  progress  in  this  direction.  It  may  be  added  that  because  at 
the  present  time  a  correct  understanding  of  the  constitution  of  organic 
compounds  involves  a  knowledge  of  the  constitution  of  the  atoms 
themselves,  future  important  developments  in  the  field  of  organic 
chemistry  must  necessarily  depend  upon  the  ability  of  the  chemist  to 
interpret  the  data  which  may  become  available  in  the  future  as  the 
result  of  the  exact  application  of  physical  methods. 


AUTHOR'S  INDEX 


ABEGG,  R.,  589 

AKERMANN,  A.,  324 

ALLEN,  142 

ANSCHUTZ,  R.,  84 

ANGELI,  A.,  29 

ANTRICK,  238 

ARAGO,  291 

ARMSTRONG,  H.,  27,  36,  58,  68,  192 

AUTENRIETH,  E.,  149 

VON  AUWERS,  K.,  69,  276,  293,  296 
AVERBECK,  H.,  271 

VON  BAEYER,  A.,  19,  25,  56,  145,  237, 

526,  556 
BAMBERGER,  E.,  28,  66,  67,  147,  148, 193, 

501,  557 

BAUDISCH,  O.,  281 
BAUMANN,  E.,  557 
BECKENKAMP,  J.,  105 
BECKMANN,  E.,  368,  524,  530 
BERLE,  147 
BERNTHSEN,  A.,  443 
BERZELIUS,  J.  J.,  2,  3,  6,  15 
BIELECKI,  J.,  391 
BIILMANN,  E.,  469 
BILTZ,  H.,  159 
BIOT,  291 

BLANKSMA,  J.  J.,  535 
BLOCK,  E.,  77,  80,  81 
BLOMSTRAND,  C.  W.,  10,  17,  143,  552 

B6DLANDER,  G.,  589 

BOHR,  142 
BORSCHE,  W.,  52 

BOTTCHER,  517 

BOTTLER,  T.,  429 
BRAGG,  W.  H.,  472 
—  W.  T.,  472 


BRANCH,  G.  E.  K.,  128 
VON  BRAUN,  J.,  21,  22 
BRAUNS,  M.,  236 
BRAY,  W  C.,  128 
BROWN,  C.,  192 
BULOW,  502 
BRUHL,  J.  W.,  295 
BRUNI,  173 
BUSCH,  M.,  512 
BUTLEROW,  A.,  14,  17 
VON  BUTTLAR,  101 

CANNIZZARO,  14 

CHATTAWAY,  F.  D.,  194 

CIAMICIAN,  G.,  29,  31 

CLAAS,  M.,  473 

CLAISEN,  L.,  145,  241,  244,>49 

GLAUS,  A.,  75 

COLLIE.  J.,  552 

COMSTOCK,  W.  J.,  238 

COUPER,  13 

CURTIUS,  T.,  149 

GUY,  140 

DALE,  291 

DAVY,  SIR  HUMPHREY,  2 
DEBYE,  P.,  175 
DECKER,  H.,  283,  561 
DERICK,  C.  G.,  578 

DlECKMANN,  N.,   150 

— ,  W.,  148 

DlLTHEY,  W.,  563 

DIMROTH,  O.,  69,  148,  230,  234,  251,  252, 

256,  263,  524 
DUMAS,  J.  B.,  5,  6 


EISENLOHR,  F.,  292,  296 


597 


598 


AUTHOR'S  INDEX 


ELBS,  K,  585 

EMMERT,  B.,  378 

ENGLER,  C.,  56 

ERLENMEYER,  E.,  JR.,  47,  67,  73,  147, 

543 

— ,  E.,  SR.,  12,  14,  15,  66,  530 
EUWES,  107 

FALK,  G.,  109,  129 

FlNKELSTEIN,  H.,  49 

FISCHER,  E.,  159,  185,  417 
-,  O.,  417 
FITTIG,  R.,  34 
FRANKLIN,  E.  C.,  134 
FRANKLAND,  P.,  11,  12 
FLURSCHEIM,  B.,  195 
FRY,  H.  S.,  116 

GARNER,  142 
GERHARDT,  K.,  8 
GHOSH,  B.  N.,  165 
GIBSON,  182 
GIEBE,  G.,  159 
GLADSTONE,  291 
GOLDSCHMIDT,  H.,  267,  565 
GOMBERG,  M.,  362,  421 
GORKE,  153 
GRAEBE,  C.,  163 
GRAUL,  O.,  462 
GUYE,  PH.,  169 

HAAKH,  447 

HALLER,  A.,  318 

HAMMELMAYR,  123 

HANKE,  140 

HANTZSCH,  A.,  149,  238,  240,  247,  271, 

278,  286,  400,  462,  481 
HARDTMANN,  A.,  392 
HARRIES.  C.,  20,  41,  46,  177 
HARTLEY,  W.  N.,  385 
HARTMANN,  230 
HEINTSCHEL,  420 
HENLE,  173 
HENRI,  391 
HENRICH,  F.,  31,  144,  145,  155,  164,  488, 

547 

HEROLD,  P.,  276 
HERRMANN,  F.,  240 
HIBBERT,  H.,  257 
HILDITCH,  318 


HlNRICHSEN,  W.,  45,  46 

HINSBERG,  O.,  92,  155,  159 

VAN'T  HOFF,  J.  H.   17,  18,  253 

VON  HOFMANN,  A.  W.,  11,  21,  529 

HOFMANN,  K.  A.,  557 

HOLLEMAN,  A.  F.,   127,   169,   194,  195, 

197,  278,  279 
HOMOLKA,  285 

HUBNER,  \92 

JACOBSON,  P.,  33,  161,  166,  240,  420 
JOHNSON,  T.  B.,  519,  522,  537 
JONES,  L.,  131,  134 

KAGI,  H.,  324 

KAPPELMEIER,  P.,  262 

KARRER,  P.,  233 

KATZ,  A.,  149 

KAUFMANN,  A.,  283 

— ,  H.,  90,  209,  408,  435,  492 

KEHRMANN,  E.,  553 

— ,  F.,  163,  423 

KEKULE,  A.,  11,  12,  14 

KENDALL,  J.,  558 

KLINGER,  H.,  39 

KNORR,  L.,  238,  245,  249,  267,  271 

KNOEVENAGEL,  E.,  73 

KOESSLER,  140 

KOLBE,  H.,  12,  13 

KON,  N.,  54,  172,  416 

KONOWALOW,  D.,  278 

KOPP,  H.,  289 

KOSTANECKI,  475 

KRAUS,  E.,  134 

KREMANN,  R.,  169 

LAAR,  C.,  239,  260 
LACHMAN,  A.,  550 
LABHARDT,  160 
LANDOLT,  291 
LAPLACE,  291 
LAP  WORTH,  A.,  539 
VON  LAUE,  M.,  175,  472 
LAURENT,  5 
LAVOISIER,  1 
LECHER,  H.,  379 
LENHARDT,  164,  232 
LEWIS,  G.  N.,  129 
LEY,  H.,  492 

LlEBERMANN,  C.,  69 


AUTHOR'S  INDEX 


599 


LlEBIG,  J.,  4,  7, 

LIFSCHITZ,  J.,  408,  474 
LTMPACH,  O.,  512 

LlNDEMANN,  69 

LORENTZ,  H.  A.,  292 
LORENZ,  L.,  292 
LOURIE,  A.,  474 

MARKWALD,  W.,  31,  145 
MEERWEIN,  533 
MEISENHEIMER,  J.,  65,  69,  70 
METZ,  160 
MEYER,  H.,  481 

—  K.  H.,  164,  232,  249,  251,  252,  264, 
276,  367,  446,  481 

— ,  R.,  262,  481,  486,  492 

—  V.,  33,  144,  149,  277 

MICHAEL,  A.,   149,  244,  246,  251,  277, 

550,  569 

MOHLAU,  R.,  475 
MONTAGNE,  P.  J.,  532,  533 

MULLIKAN,  267 

NEF,  J.  U.,  36,  143,  270,  277,  335,  549 

NELSON,  J.  W.,  109,  129 

NERNST,  W.,  288 

NICOLET,  B.,  550 

NIETZKI,  R.,  418 

NOLTE,  O.,  154 

NOLTING,  E.,  192 

NORRIS,  J.  F.,  423 

NOTES,  W.  A.,  118,  142 

OBERMILLER,  J.,  197 
OCHS;  R.,  428 
OEKONOMIDES,  237 
OFFENBACHER,  M.,  377 
ORTON,  194 
OSSWALD,  286 
OSTWALD,  W.,  484 

PAAL,  C.,  408 

PARSONS,  142 

PASTEUR,  18 

PAUL,  T.,  368 

PAULY,  H.,  101,  102,  175,  208,  214 

VON  PECHMANN,  H.,  238,  248 

PERKIN,  W.  H.,  69 

PETERSON,  134 

PFEIFFER,  P.,  81,  169,  429,  438,  447,  476 


PICCARD,  J.,  364,  376,  388,  422 
PICTET,  A.,  509 
POSNER,  T.,  63,  65 
PUMMERER,  R.,  247 

RAMSAY,  142 

REDDELIEN,  G.,  169,  170,  173,  179,  180 

REICH,  S.,  36,  173 

ROTH,  K.,  377 

ROTHE,  O.,  271 

RUPE,  H.,  34,  318,  324 

SAKALLARIOS,  E.,  182 
SANDMEYER,  T.,  247 
SCHAEFER,  K.,  392 

SCHAUM,  K.,  Ill 
SCHETBER,  J.,  276 
SCHERRER,  P.,   175 

SCHLENK,  W.,  100,  236,  364,  373,  422, 

426,  428 
SCHMIDLIN,  J.,  169,  373 

SCHREIBER,   165 

SCHROETER,  G.,  544,  550 

SCHULTZE,  O.  W.,  247,  278 

SKRABAL,  A.,  234 

SKRAMP,  S.,  85 

SNELL,  291 

STAGNER,  B.  A.,  131 

STARK,  J.,  92,  101,  102,  209,  385,  490 

-,  O.,  236 

STAUDINGER,   H.,   22,   54,   56,   57,   172, 

409,  416 

STEIMMIG,  F.,  475 
STELZNER,  R.,  166 

STIEGLITZ,  J.,  118,  131,  133,  479,  525,  544 
STOBBE,  H.,  262,  271,  407,  470 
STOHMANN,  25 
STRAUS,  45 

THIELE,  J.,  33,  35,  145,  173 
THOMSON,  J.  J.,  36,  118,  142 
TICKLE,  552 
TIFFENEAU,  M.,  532,  550 

TORNANI,   173 
TSCHITSCHIBABIN,   A.,  428 

TSCHUGAEFF,  L.,  318,  476 

VILLIGER,  57,  556 

VORLANDER,  D.,  46,  150,  155,  192,  225 


600 


AUTHOR'S  INDEX 


WALDEN,  P.,  186,  267,  318,  426 

WASER,  E.,  176 

WEIGERT,  F.,  391 

VON  WEINBERG,  A.,  105,  229 

WERNER,  A.,  75,  80,  81,  476 

WHEELER,  H.  L.,  519,  522,  537 

WTEDEMANN,  E.,  486 

WIELAND,  H.,  45,   169,   182,  364,  367, 

375,  380,  526 
WILLIAMSON,  9,  13 
WILLSTATTER,  R.,  176,  283,  419 


WlSLICENUS,  J.,  17 

— ,  W.,  145,  241,  246,  249,  537 

WITT,  O.  N.,  402 

WOHLER,  F.,  4 

WOLFF,  149 

WULLNER,  291 

VON  WUNDERLICH,  33 

WURSTER,  C ,  443 

ZANETTI,  31 
ZINCKE,  T.,  50 


SUBJECT  INDEX 


Absorption  spectra  phenomena,  384 
Acetylangelica  lactone,  247,  259 
Acid  chlorides,  reduction  of,  39 
Activating  groups,  166 

—  influence,  166 

Addition  reactions,  59,  351,  569 

-  theory  of  Kekule,  169 
Allelotropic  mixtures,  259 
Allelotropism,  259 
Allylphenol,  504 
Allylphenyl  ether,  504 
Aminity,  155 
Ammono  acids,  134 

—  bases,  134 
Angstrom  units,  384 
Anlagerungsverbindung,  86 
Anthraquinone  monoxime,  70 
Atomic  compounds,  13 
Atomicity.  11 

Atomic  theory,  1 
Auxoflore  groups,  493 
Auxiliary  valencies,  82 
Auxochromes,  403 
Auxochrome  theory,  410 
—  of  Kauffmann,  91 

Basicity  of  atoms,  11 

Bathochrome  groups,  384 

Bathoflore  groups,  493 

Beckmann  rearrangement,  131,  524 

Beer's  Law,  388 

Benzene,  electronic  formula  of,  127 

-  formula,  Kekul6-Thiele,  180 
,  Obermiller,  198 

,  Pauly-Stark  electron,  209 

,  Thiele,  180 

,  Werner,  80 

,  valence  electron,  218 

—  oxidation  in  body,  178 


Benzidine  rearrangement,  162,  510 
Benzil,  reduction  of,  39 
Benzilic  acid  rearrangement,  500 
Binary  or  dualistic  theory,  6 
Bivalent  carbon,  143 
— ,  theory  of  Nef,  335 

Carbinol  bases,  283 
Carbonium  compounds,  87,  565 

—  valence,  425 
Cationic  valency,  433 

Centric  formula  for  benzene,  27,  58 

—  benzene     formula     of     Baeyer    and 
Armstrong,  176 

Chromenes,  314 
Chromogens,  403 
Chromoisomerism,  455 

—  of  nitro  compounds,  461 
Chromophore  theory,  403 
Chromophores,  403 
Chromotropism,  456 
Color  lakes,  475 
Commuting  radicals,  157 
Conjugate  systems,  37 
Conjugated  double  bonds,  43 
Conjunction  formulas,  431 
Continuous  absorption,  384 
Coordination  number,  83 
Crossed  double  bonds,  50 
Cumarilic  acid,  314 
Cyclo-octatetraene,  176 
Cyclostatic  chromophores,  406 

Diatomic  chromophores,  405 
Dimethyl  pyrone,  553 
Diminoflore  groups,  493 
Dinitrodiphenyl  ethylene,  159 
Diphenylketone,  54 
Diphenylnitride,  364 


601 


602 


SUBJECT  INDEX 


Desmotropism,  237 
Dual  radicals,  158 

Einlagerungsverbindung,  87 
Electroatomic  theory  of  Stark  and  Pauly, 

223 

Electrochemical  theory  of  Berzelius,  2,  4 
Electro-dual  atoms,  95 
Electromers,  123 
Electron  theory  of  Stark,.  209 
Electronic  formulas,  109 
• —  isomers,  123 

—  oxidation,  111 

—  reduction,  111 

—  valency,  111 
Electrons,  92 
Electro  valency,  105 
Elements,  electro-positive,  4 
— ,  electro-negative,  4 
Ethyl  acetoacetate,  269 

— ,  tautomeric  forms,  264 

—  benzoylacetate.  276 

—  formylacetate,  245 

—  nitrolic  acid,  462 

—  succinyl-succinate,  464 

Fluorgens,  486 
Fluorophore  group,  489 
Fluorophores,  486 
Free  organic  radicals,  361,  373 
Functional  tautomerism,  248 

Halochromism,  170,  426 
-  theory  of  Pfeiffer,  170 
Heat  of  combustion,  327 

—  formation,  328 
Helmholtz-Drude  equation,  399 
Henri-Bielecki  exponential  formula,  399 
Hofmann  rearrangement,  529 
Homochromoisomerism,  468 
Hydrocarbo  bases,  134 
Hydronium  compounds,  88 
Hydroxyformamidine.  511 
Hypsochrome  groups.  384 
Hypsoflore  group,  493 

Imidoacid  anhydrides,  519 
Imidoester  rearrangements,  519,  522,  537 
Index  of  refraction,  291 
Indigo  formulas,  473 


lonogens  (first  and  second  order),  156 
Isatin,  tautomeric  formula  of,  237 
Isatinoxime,  454 
Isorropesis,  446 

Kauffmann's  theory  of  color,  433 
Kehrmann-Gomberg,  hypothesis,  427 
Kinetic  theory  of  von  Weinberg,  105 
Kineto-electro  magnetic  theory,  105 

Law    of    simple    and   multiple   propor- 
tions, 1 

Lorentz-Lorenz  formula,  292 
Luminophore  groups,  489 
Luminophores,  411 

Magneton  theory,  142 
Mercury  fulminate,  345 
Metal-ketyls,  369 
Methylene  chemistry,  337 

—  dissociation,  335 
Methylphenyl-picramide,  469 
Molecular  compounds,  13 

—  refraction,  294 

—  rotatory  power,  318 
Monatomic  chromophores,  405 
Muconic  acid,  34 

—  from  benzene,  178 
N-Chloracetanilides,  507 
Naphthalene  formula,  66 
Negative  and  positive  nature  of  radicals, 

145 

Nicotine  synthesis,  509 
Nitration  and  addition  reactions,  183 
Nitric  acid,  addition  products  of,  170 
Nitroanthracene,  69 
Nitrogen  dioxide  in  addition  reactions,  45 

—  radicals,  375 
Nitroform,  453 
Nitroparaffins,  144,  453 
Nitromethane,  279 
Non-polar  compounds,  129 

Octet  theory  of  Langmuir,  128 

Onium  compounds,  112 

Optical  rotation,  318 

Oscillation  hypothesis  of  Kekule,  24 

Otto-Fischer-Hepp   rearrangement,    506 

Oxalyl  dibenzylketone,  526 


SUBJECT  INDEX 


603 


Oxonium  hydroxide,  553 

-  salts,  553 
Oxydihydro  bases,  283 

Parafuchsine,  419 
Pararosaniline,  419 
Pfeiffer's  theory  of  color,  436 
Phenolphthalein  tautomerism,  481 
Phenol,  valence  electron  formula  of,  218 
Phenoquinones,  446 
Phenylnitromethane,  278 
Physical  isomerism,  17 
Piccard's  colorimetric  law,  389 
Pinacone-pinacoline  rearrangement,  531 

—  rearrangement,  498 
Polar  bond,  128 

—  compounds,  129 

—  number,  128 

—  valency,  128 
Polarity  of  atoms,  96 
Polychromism,  459 
Potential  valency,  67 
Principal  valencies,  82 
Pseudo  acids,  281 

-  bases,  281 

—  compounds,  239 
Pseudomerism,  248,  260 
Pyrrol  rearrangements,  509 

Quanti  valence,  11 

Quaternary   salt    containing    sulphonyl 

group,  154 
Quinhydrone,  443 
Quinoles,  501 
Quinone  addition,  62 
Radical  of  benzoic  acid,  4 
Radicals,  polyatomic,  9 
Reactive  group,  166 
Red  helianthine;  483 
Residual  affinity,  43 

Selective  absorption,  384 
Semidine  rearrangement,  162,  510 
Stark's  theory  of  absorption  phenomena, 
477 


Streptostatic  chromophores,  406 
Substitution  in  benzene,  192 

—  ring,  laws  regulating,  201 
Sulphur  radicals,  379 

Tautomerism,  237 
— ,  definition  of,  260 
Tetravalent  oxygen,  553 
Thermochemistry,  327 
Theory  of  indicators,  480 

—  maximum  saturation  capacity,  14 
partial  valencies,  35 

—  radicals,  7 

-  J.  Stark,  92 
tension,  19,  22 

-  types,  5 

—  valency  isomerism,  460 
Thiocyanacetanilide  rearrangements,  513 
Thiolcarbanilates,  522 
Thiolcarbazinates,  523 
Thioncarbanilates,  522 
Thioncarbazinates,  523 

Triphenyl  methyl,  362,  421 

—  peroxide,  526 
Trouton's  rule,  327 

Type  theory  of  Gerhardt,  8 

Unitary  theory,  6 

Valence  electrons,  94,  477 
-  field,  90,  100 

—  number,  128 

Valency,  electrochemical  conception,  90 
— ,  recent  theories,  90 
Variochromism,  456 
Violuric  acid,  287 

-  salts,  455 
Virtual  tautomerism,  248 

Walden  rearrangement,  185 

Werner's  theory,  75 

Wiedermann's  theory  of  fluorescence,  491 

Yellow  helianthine,  483 

—  methyl  orange,  483 


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